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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Feb. 23, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	SRPT
Entity Registrant Name	Sarepta Therapeutics, Inc.
Entity Central Index Key	0000873303
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		64,978,369
Entity Public Float			$ 1,865,548,919

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 599,691	$ 122,420
Short-term investments	479,369	195,425
Accounts receivable	29,468	5,228
Inventory	83,605	12,813
Restricted investment		10,695
Other current assets	36,511	26,895
Total Current Assets	1,228,644	373,476
Property and equipment, net of accumulated depreciation of $18,022 and $30,346 as of December 31, 2017 and 2016, respectively	43,156	37,801
Intangible assets, net of accumulated amortization of $4,145 and $3,134 as of December 31, 2017 and 2016, respectively	14,355	8,076
Investments and other assets	21,809	4,751
Total Assets	1,307,964	424,104
Current Liabilities:
Accounts payable	8,467	29,690
Accrued expenses	68,982	31,016
Current portion of long-term debt	6,175	10,108
Deferred revenue	3,316	3,303
Other current liabilities	1,392	1,305
Total Current Liabilities	88,332	75,422
Long-term debt	424,876	6,042
Deferred rent and other	5,539	5,949
Total Liabilities	518,747	87,413
Commitments and contingencies (Note 19)
Stockholders’ Equity:
Preferred stock, $.0001 par value, 3,333,333 shares authorized; none issued and outstanding
Common stock, $.0001 par value, 99,000,000 shares authorized; 64,791,670 and 54,759,234 issued and outstanding at December 31, 2017 and 2016, respectively	6	5
Additional paid-in capital	2,006,598	1,503,126
Accumulated other comprehensive loss	(379)	(120)
Accumulated deficit	(1,217,008)	(1,166,320)
Total Stockholders’ Equity	789,217	336,691
Total Liabilities and Stockholders’ Equity	$ 1,307,964	$ 424,104

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Property and equipment, accumulated depreciation	$ 18,022	$ 30,346
Intangible assets, accumulated amortization	$ 4,145	$ 3,134
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	3,333,333	3,333,333
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	99,000,000	99,000,000
Common stock, issued	64,791,670	54,759,234
Common stock, outstanding	64,791,670	54,759,234

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Product, net	$ 154,584	$ 5,421
Revenue from research contracts and other grants			$ 1,253
Total revenues	154,584	5,421	1,253
Cost and expenses:
Cost of sales (excluding amortization of in-licensed rights)	7,353	101
Research and development	166,707	188,272	146,394
Selling, general and administrative	122,682	83,749	75,043
Settlement and license charges	28,427
Amortization of in-licensed rights	1,053	29
Total cost and expenses	326,222	272,151	221,437
Operating loss	(171,638)	(266,730)	(220,184)
Other income (loss) :
Interest (expense) income and other, net	(1,990)	(535)	154
Gain from sale of Priority Review Voucher	125,000
Total other income (loss)	123,010	(535)	154
Loss before income tax expense	(48,628)	(267,265)	(220,030)
Income tax expense	2,060
Net loss	(50,688)	(267,265)	(220,030)
Other comprehensive loss:
Unrealized loss on short-term securities - available-for-sale	(259)	(9)	(16)
Total other comprehensive loss	(259)	(9)	(16)
Comprehensive loss	$ (50,947)	$ (267,274)	$ (220,046)
Net loss per share — basic and diluted	$ (0.86)	$ (5.49)	$ (5.20)
Weighted average number of shares of common stock outstanding for computing basic and diluted net loss per share	58,818	48,697	42,290

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
BALANCE at Dec. 31, 2014	$ 247,653	$ 4	$ 926,769	$ (95)	$ (679,025)
BALANCE (in shares) at Dec. 31, 2014		41,312,000
Exercise of options for common stock	$ 10,010		10,010
Exercise of options for common stock, shares	816,696	817,000
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		181,000
Shares withheld for taxes	$ (182)		(182)
Shares withheld for taxes, shares		(6,000)
Issuance of common stock for cash, net of offering costs	119,916	$ 1	119,915
Issuance of common stock for cash, net of offering costs, shares		3,250,000
Issuance of common stock under employee stock purchase plan	879		879
Issuance of common stock under employee stock purchase plan, shares		76,000
Stock-based compensation	32,117		32,117
Unrealized loss from available-for-sale securities	(16)			(16)
Net loss	(220,030)				(220,030)
BALANCE at Dec. 31, 2015	190,347	$ 5	1,089,508	(111)	(899,055)
BALANCE (in shares) at Dec. 31, 2015		45,630,000
Exercise of options for common stock	$ 19,353		19,353
Exercise of options for common stock, shares	1,056,821	1,113,000
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		50,000
Shares withheld for taxes	$ (2,168)		(2,168)
Shares withheld for taxes, shares		(47,000)
Issuance of common stock for cash, net of offering costs	364,749		364,749
Issuance of common stock for cash, net of offering costs, shares		7,876,000
Issuance of common stock under employee stock purchase plan	1,577		1,577
Issuance of common stock under employee stock purchase plan, shares		137,000
Stock-based compensation	30,107		30,107
Unrealized loss from available-for-sale securities	(9)			(9)
Net loss	(267,265)				(267,265)
BALANCE at Dec. 31, 2016	336,691	$ 5	1,503,126	(120)	(1,166,320)
BALANCE (in shares) at Dec. 31, 2016		54,759,000
Exercise of options for common stock	$ 13,799		13,799
Exercise of options for common stock, shares	792,845	793,000
Grant of restricted stock awards and vest of restricted stock units, net of cancellations, shares		400,000
Shares withheld for taxes	$ (2,227)		(2,227)
Shares withheld for taxes, shares		(60,000)
Issuance of common stock for cash, net of offering costs	353,959	$ 1	353,958
Issuance of common stock for cash, net of offering costs, shares		8,798,000
Issuance of common stock under employee stock purchase plan	1,425		1,425
Issuance of common stock under employee stock purchase plan, shares		102,000
Equity component of convertible notes	156,953		156,953
Purchase of capped call share options	(50,901)		(50,901)
Stock-based compensation	30,465		30,465
Unrealized loss from available-for-sale securities	(259)			(259)
Net loss	(50,688)				(50,688)
BALANCE at Dec. 31, 2017	$ 789,217	$ 6	$ 2,006,598	$ (379)	$ (1,217,008)
BALANCE (in shares) at Dec. 31, 2017		64,792,000

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (50,688)	$ (267,265)	$ (220,030)
Adjustments to reconcile net loss to cash flows in operating activities:
Gain from sale of Priority Review Voucher	(125,000)
Depreciation and amortization	8,092	5,611	5,247
(Accretion of discount) amortization of premium on available-for-sale securities	(888)	80	652
Non-cash interest expense	2,679	355	367
Loss on disposal of assets	805	293	197
Stock-based compensation	30,465	29,962	32,117
Non-cash restructuring expense		911
Changes in operating assets and liabilities, net:
Increase in accounts receivable	(24,240)	(1,251)	(1,561)
Increase in inventory	(70,792)	(12,813)
(Increase) decrease in other assets	(15,354)	(9,012)	15,249
Increase in accounts payable, accrued expenses, deferred revenue and other liabilities	12,925	7,309	18,297
Net cash used in operations	(231,996)	(245,820)	(149,465)
Cash flows from investing activities:
Purchase of property and equipment	(12,000)	(5,341)	(3,401)
Purchase of intangible assets	(9,215)	(1,525)	(1,432)
Purchase of available-for-sale securities	(589,520)	(195,427)	(162,001)
Proceeds from sale of Priority Review Voucher	125,000
Purchase of restricted investment			(10,695)
Maturity of restricted investment	10,695
Maturity and sales of available-for-sale securities	296,225	112,100	185,939
Net cash (used in) provided by investing activities	(178,815)	(90,193)	8,410
Cash flows from financing activities:
Proceeds from term loan	30,000		20,000
Proceeds from revolving line of credit	39,708
Payments of term loan and notes payable	(15,109)	(7,603)	(2,598)
Payment of revolving line of credit	(39,645)
Proceeds from sales of common stock, net of offering costs	353,959	364,802	119,916
Proceeds from convertible debt offering	570,000
Debt issuance costs	(15,154)		(399)
Purchase of capped call options	(50,901)
Proceeds from exercise of options and warrants and employee stock purchase program	15,224	20,930	10,889
Net cash provided by financing activities	888,082	378,129	147,808
Increase in cash, cash equivalents and restricted cash	477,271	42,116	6,753
Cash, cash equivalents and restricted cash:
Beginning of period	122,556	80,440	73,687
End of period	599,827	122,556	80,440
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	1,912	1,562	769
Cash paid during the period for income taxes	5,336
Supplemental schedule of non-cash investing activities and financing activities:
Accrued exit and legal fees for debts	625	400	400
Reclassification of software licenses	204
Property and equipment reclassified to asset held for sale	1,529
Property and equipment included in accrued expenses	2,525	1,186	318
Intangible assets included in accrued expenses	343	1,163	335
Shares withheld for taxes	$ 2,227	2,168	$ 182
Accrual for offering costs related to the equity offerings		$ 53

ORGANIZATION AND NATURE OF BUSINESS	12 Months Ended
ORGANIZATION AND NATURE OF BUSINESS	Dec. 31, 2017
Accounting Policies [Abstract]
ORGANIZATION AND NATURE OF BUSINESS	1. ORGANIZATION AND NATURE OF BUSINESS Sarepta Therapeutics, Inc. (together with its wholly-owned subsidiaries, “Sarepta” or the “Company”) is a commercial-stage biopharmaceutical company focused on the discovery and development of unique RNA-targeted therapeutics, gene therapy and other genetic medicine approaches for the treatment of rare neuromuscular diseases. Applying its proprietary, highly-differentiated and innovative platform technologies, the Company is able to target a broad range of diseases and disorders. Its first commercial product in the U.S., EXONDYS 51^® (eteplirsen) Injection (“EXONDYS 51”), was granted accelerated approval by the U.S. Food and Drug Administration (“FDA”) on September 19, 2016. EXONDYS 51 is indicated for the treatment of DMD in patients who have a confirmed mutation of the Duchenne muscular dystrophy (“DMD”) gene that is amenable to exon 51 skipping. Continued approval for this indication may be contingent upon verification of a clinical benefit in confirmatory trials. In addition to advancing its exon-skipping product candidates for DMD, including eteplirsen, golodirsen, casimersen and SRP-5051, the Company is working with several strategic partners under various agreements to research and develop multiple treatment approaches to DMD, which include Nationwide Children’s Hospital, Genethon, Duke University and Summit (Oxford) Ltd. (“Summit”). As of December 31, 2017, the Company had approximately $1,089.9 million of cash, cash equivalents and investments, consisting of $599.7 million of cash and cash equivalents, $479.4 million of short-term investments, $10.0 million of long-term investments and $0.8 million of long-term restricted cash and investments. The Company believes that its balance of cash, cash equivalents and investments as of December 31, 2017 is sufficient to fund its current operational plan for at least the next twelve months, though it may pursue additional cash resources through public or private debt and equity financings, seek additional government contracts and establish collaborations with or license its technology to other companies.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS

2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS

Basis of Presentation

The accompanying consolidated financial statements, which have been prepared in accordance with accounting principles generally accepted in the United States (“U.S. GAAP”), reflect the accounts of Sarepta Therapeutics, Inc. and its wholly-owned subsidiaries. All intercompany transactions between and among its consolidated subsidiaries have been eliminated. Management has determined that the Company operates in one segment: discovering, developing, manufacturing and delivering therapies to patients with DMD. The Company’s CEO, as the chief operating decision-maker, manages and allocates resources to the operations of the Company on a total company basis. The Company’s research and development organization is responsible for the research and discovery of new product candidates and supports development and registration efforts for potential future products. The Company’s supply chain organization manages the development of the manufacturing processes, clinical trial supply and commercial product supply. The Company’s commercial organization is responsible for commercialization of EXONDYS 51 in the U.S. and internationally. The Company is supported by other back-office general and administration functions. Consistent with this decision-making process, the Company’s CEO uses consolidated, single-segment financial information for purposes of evaluating performance, forecasting future period financial results, allocating resources and setting incentive targets.

Estimates and Uncertainties

The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, revenue, expenses and the disclosure of contingent assets and liabilities. Actual results could differ from those estimates. Significant items subject to such estimates and assumptions include revenue recognition, inventory, convertible debt, valuation of stock-based awards, research and development expenses and income tax.

Reclassification

The Company has revised the presentation as well as the caption of certain items within the consolidated balance sheets to conform to the current period presentation. “Restricted cash and investments” of $0.8 million as December 31, 2016 is grouped into “investments and other assets”. This revision had no impact on total assets.

Additionally, the Company has revised the presentation as well as caption of certain items within the consolidated statements of operations and comprehensive loss to conform to current period presentation. “Amortization of in-licensed rights” of less than $0.1 million was reclassified from “cost of sales” and presented separately in the consolidated statements of operations and comprehensive loss. The reclassification had no impact on operating loss or net loss.

The Company also has revised the presentation as well as the caption of certain items within the consolidated statements of cash flows for 2015 to conform to current period presentation “Debt issuance costs” of $0.4 million has been reclassified from “proceeds from term loan” and presented separately in the statements of cash flows. This reclassification had no impact on net cash provided by financing activities or cash, cash equivalents and restricted cash at end of period.

Further, the Company has revised the presentation as well as the caption of certain other current assets in Note 8, Other Assets to the consolidated financial statements to conform to the current period presentation. “Prepaid maintenance services” of approximately $0.1 million, “prepaid clinical and preclinical expenses” of $1.2 million and “prepaid commercial expenses” of less than $0.1 million as of December 31, 2016 have been reclassified from “other prepaids” and presented separately in the other current assets table. These reclassifications had no impact on total current assets or total assets.

The Company also has revised the presentation as well as the caption of certain accrued expenses in Note 10, Accrued Expenses to the consolidated financial statements to conform to the current period presentation. “Accrued interest expenses” of $0.1 million, “Product revenue related reserves of $0.3 million and “accrued property and equipment” of $1.2 million as of December 31, 2016 have been reclassified from “Other” and presented separately in the accrued expenses table. The reclassification had no impact on total current liabilities or total liabilities.

Fair Value Measurements

The Company has certain financial assets that are recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements:

•

Level 1—quoted prices for identical instruments in active markets;

•

Level 2—quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-derived valuations in which all significant inputs and significant value drivers are observable in active markets; and

•

Level 3—valuations derived from valuation techniques in which one or more significant value drivers are unobservable.

The fair value of some of the Company’s financial assets is categorized as Level 2 within the fair value hierarchy. These financial assets have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, through income-based approaches utilizing observable market data. For additional information related to fair value measurements, please read Note 5, Fair Value Measurements to the consolidated financial statements.

Cash and Cash Equivalents

Only investments that are highly liquid and readily convertible to cash and have original maturities of three months or less are considered cash equivalents.

Available-For-Sale Debt Securities

Available-for-sale debt securities are recorded at fair market value and unrealized gains and losses are included in accumulated other comprehensive loss in stockholder’s equity. Realized gains and losses are reported in interest (expense) income and other, net, on a specific identification basis.

Accounts Receivable

The Company’s accounts receivable arise from product sales, government research contracts and other grants. They are generally stated at the invoiced amount and do not bear interest.

The accounts receivable from product sales represents receivables due from the Company’s specialty distributor and specialty pharmacies in the U.S. as well as distributors outside of the U.S. The Company monitors the financial performance and creditworthiness of its customers so that it can properly assess and respond to changes in the customers’ credit profile. The Company provides reserves against trade receivables for estimated losses that may result from a customer’s inability to pay. Amounts determined to be uncollectible are written-off against the established reserve. As of December 31, 2017, the credit profile for the Company’s customers is deemed to be in good standing and reserves against accounts receivable are not considered necessary. Historically, no accounts receivable amounts related to government research contracts and other grants have been written off and, thus, an allowance for doubtful accounts receivable related to government research contracts and other grants is not considered necessary.

Concentration of Credit Risk

Financial instruments which potentially subject the Company to concentrations of credit risk consist of accounts receivable from customers and cash, cash equivalent and investments held at financial institutions.

For the year ended December 31, 2017, the majority of the Company’s accounts receivable arose from product sales in the U.S. and all customers have standard payment terms which generally require payment within 30 to 60 days. Outside of the U.S., the payment terms range between 60 and 120 days. Three individual customers accounted for 47%, 34% and 19% of net product revenues and 56%, 27% and 16% of accounts receivable from product sales, respectively. The Company monitors the financial performance and creditworthiness of its customers so that it can properly assess and respond to changes in the customers’ credit profile. As of December 31, 2017, the Company believes that such customers are of high credit quality.

As of December 31, 2017, the Company’s cash equivalents and investments were concentrated at a single financial institution, which potentially exposes the Company to credit risks. However, the Company does not believe that there is significant risk of non-performance by the financial institution.

Inventory

Inventories are stated at the lower of cost and net realizable value with cost determined on a first-in, first-out basis. The Company capitalizes inventory costs associated with products following regulatory approval when future commercialization is considered probable and the future economic benefit is expected to be realized. EXONDYS 51 which may be used in clinical development programs are included in inventory and charged to research and development expense when the product enters the research and development process and no longer can be used for commercial purposes.

The following table summarizes the components of the Company’s inventory for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Raw materials	$	53,875	$	9,531
Work in progress		27,442		3,175
Finished goods		2,288		107
Total inventory	$	83,605	$	12,813

The Company periodically reviews its inventories for excess amounts or obsolescence and writes down obsolete or otherwise unmarketable inventory to its estimated net realizable value. Additionally, though the Company’s product is subject to strict quality control and monitoring which it performs throughout the manufacturing processes, certain batches or units of product may not meet quality specifications resulting in a charge to cost of sales.

Property and Equipment

Property and equipment are initially recorded at cost, including the acquisition cost and all costs necessarily incurred to bring the asset to the location and working condition necessary for its intended use. The cost of normal, recurring or periodic repairs and maintenance activities related to property and equipment are expensed as incurred. The cost for planned major maintenance activities, including the related acquisition or construction of assets, is capitalized if the repair will result in future economic benefits. Interest costs incurred during the construction period of major capital projects are capitalized until the asset is ready for its intended use, at which point the interest costs are amortized as depreciation expense over the life of the underlying asset.

The Company generally depreciates the cost of its property and equipment using the straight-line method over the estimated useful lives of the respective assets, which are summarized as follows:

Asset Category		Useful lives
Lab equipment		5 years
Office equipment		5 years
Software and computer equipment		3 - 5 years
Leasehold improvements		Lesser of the useful life or the term of the respective lease
Land		Not depreciated
Building		30 years
Construction in Progress		Not depreciated until put into service

Intangible assets

The Company’s intangible assets consist of in-licensed rights and patent costs, which are stated in the Company’s consolidated balance sheets net of accumulated amortization and impairments, if applicable.

The in-licensed rights relates to agreements with BioMarin (defined in Note 3) and the University of Western Australia (“UWA”). Following the execution of the settlement and license agreements with BioMarin in July 2017, The Company recorded a $6.6 million intangible asset related to EXONDYS 51 in the U.S. Additionally, as a result of the FDA approval and the subsequent commercial sale of EXONDYS 51, as defined in the Amended and Restated UWA License Agreement (also defined in Note 3), the Company was obligated to pay a $1.0 million sales milestone to UWA and, accordingly, recorded an in-licensed right. The in-licensed rights are being amortized on a straight-line basis over the remaining life of the related patent because the life of the related patent reflects the expected time period that the Company will benefit from the in-licensed right. For the years ended December 31, 2017 and 2016, the Company recorded $1.1 million and less than $0.1 million, respectively, of amortization related to the in-license rights.

Patent costs consist primarily of external legal costs, filing fees incurred to file patent applications and renewal fees on proprietary technology developed or licensed by the Company. Patent costs associated with applying for a patent, being issued a patent and annual renewal fees are capitalized. Costs to defend a patent and costs to invalidate a competitor’s patent or patent application are expensed as incurred. Patent costs are amortized on a straight-line basis over the shorter of the estimated economic lives or the initial term of the patents, which is generally 20 years. Patent amortization expense was $0.6 million, $0.6 million and $0.5 million for the years ended December 31, 2017, 2016 and 2015, respectively. The Company also expensed the remaining net book value of previously capitalized patents that were later abandoned of $0.6 million, $0.3 million and $0.2 million for the years ended December 31, 2017, 2016 and 2015, respectively, which were included in research and development expenses on the consolidated statements of operations and comprehensive loss.

The following table summarizes the estimated future amortization for intangible assets for the next five years:

	As of December 31, 2017 (in thousands)
2018		1,681
2019		1,507
2020		1,465
2021		1,453
2022		1,446
Total	$	7,552

Impairment of Long-Lived Assets

Long-lived assets held and used by the Company and intangible assets with definite lives are reviewed for impairment whenever events or circumstances indicate that the carrying amount of assets may not be recoverable. The Company evaluates recoverability of assets to be held and used by comparing the carrying amount of an asset to future net undiscounted cash flows to be generated by the asset. If such assets are considered to be impaired, the impairment to be recognized is measured as the amount by which the carrying amount of the assets exceeds the fair value of the assets. Such reviews assess the fair value of the assets based upon estimates of future cash flows that the assets are expected to generate.

Convertible Debt Transactions

The Company separately accounts for the liability and equity components of convertible debt instruments that can be settled in cash by allocating the proceeds from issuance between the liability component and the embedded conversion option in accordance with accounting for convertible debt instruments that may be settled in cash (including partial cash settlement) upon conversion. The value of the equity component is calculated by first measuring the fair value of the liability component, using the interest rate of a similar liability that does not have a conversion feature, as of the issuance date. The difference between the proceeds from the convertible debt issuance and the amount measured as the liability component is recorded as the equity component with a corresponding discount recorded on the debt. The Company recognizes the amortization of the resulting discount using the effective interest method as interest expense in its consolidated statements of operations and comprehensive loss. Simultaneously, the Company bought capped call options from certain counterparties to minimize the impact of potential dilution upon conversion. The premium for the capped call options was recorded as additional paid-in capital in its consolidated balance sheets. For additional information related to the convertible debt transactions, please read Note 12, Indebtedness to the consolidated financial statements.

Revenue Recognition

The Company recognizes revenue when all of the following criteria are met:

persuasive evidence of an arrangement exists;

delivery has occurred or services have been rendered;

price to the customer is fixed or determinable; and

collectability is reasonably assured.

Product revenues

Revenue from product sales is recognized when title and risk of loss have passed to the customer and is recorded net of applicable reserves for rebates, discounts and allowances.

Reserves for rebates, discounts and allowances

The Company establishes reserves for various government rebate and chargeback programs, prompt payment discounts and co-pay assistance. Reserves established for these discounts and allowances are classified as either reductions of accounts receivable (if the amount is payable to the Company’s customers) or a liability (if the amount is payable to a party other than the Company’s customers). These reserves are based on estimates of the amounts earned or to be claimed on the related sales. Product revenue reserves represent the Company's estimates of outstanding claims for end-user rebate-eligible sales that have occurred, but for which related claim submissions have not been received. They are categorized as follows:

•

Medicaid rebates relate to the Company’s estimated obligations to states under established reimbursement arrangements. Rebate accruals are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a liability which is included in accrued expenses.

•

Governmental chargebacks, including Public Health Service (“PHS”) chargebacks, represent the Company’s estimated obligations resulting from contractual commitments to sell products to qualified healthcare providers at prices lower than the list prices that the Company charges to wholesalers. The wholesaler charges the Company for the difference between what the wholesaler pays for the products and the ultimate selling price to the qualified healthcare providers. Chargeback reserves are established in the same period as the related revenue is recognized, resulting in a reduction in product revenue and accounts receivable. Chargeback amounts are generally determined at the time of resale to the qualified healthcare provider from the wholesaler, and the Company generally issues credits for such amounts within a few weeks of receiving notification of resale from the wholesaler.

•

Prompt payment discounts relate to estimated obligations for credits to be granted to a specialty pharmacy for remitting payment on their purchases within established incentive periods. The reserve for the prompt payment discounts are recorded in the same period as the related revenue is recognized, resulting in a reduction in product revenue and accounts receivable.

•

Co-pay represents financial assistance to qualified patients, assisting them with prescription drug co-payments required by insurance. The copay reserves are recorded in the same period as the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a liability which is included in accrued expenses.

The Company also maintains certain customer service contracts with distributors and other customers in the distribution channel that will provide inventory management, data and distribution services, which generally will be reflected as a reduction of revenue. To the extent the Company can demonstrate a separable benefit and fair value for these services, the Company will classify these payments as selling, general and administrative expenses.

Revenue from research contracts and other grants

The Company’s contracts with the U.S. government are cost plus contracts providing for reimbursed costs which include overhead and general and administrative costs and a target fee. The Company recognizes revenue from government research contracts during the period in which the related expenses are incurred and presents such revenues and related expenses on a gross basis in the consolidated financial statements. The Company’s government contracts are subject to government audits, which may result in catch-up adjustments. As of December 31, 2014, the Company had completed all development activities under its contracts with the U.S. government. The majority of the revenue under government contracts was recognized as of December 31, 2017 and only revenue for contract finalization, if any, is expected in the future.

Deferred revenue

If a technology, right, product or service is separate and independent of our performance under other elements of an arrangement, the Company defers recognition of non-refundable up-front fees if it has continuing performance obligations when the technology, right, product or service conveyed in conjunction with the non-refundable fee has no utility to the licensee. In addition, if the Company has continuing involvement through research and development services that are required because of its know-how or because the services can only be performed by the Company, such up-front fees are deferred and recognized over the period of continuing involvement. As of December 31, 2017, the Company had deferred revenue of $3.3 million, which primarily represents up-front fees which it may recognize as revenue upon settlement of certain obligations.

Research and Development

Research and development expenses consist of costs associated with research activities as well as those with the Company’s product development efforts, conducting preclinical trials, clinical trials and manufacturing activities. Research and development expenses are expensed as incurred. Up-front fees and milestones paid to third parties in connection with technologies which have not reached technological feasibility and do not have an alternative future use are expensed when incurred. Nonrefundable advance payments for goods or services that will be used or rendered for future research and development activities pursuant to an executory contractual arrangement will be capitalized, and recognized as an expense as the related goods are delivered or the related services are performed. If the Company does not expect the goods to be delivered or services to be rendered, the advance payment capitalized will be charged to expense.

Direct research and development expenses associated with the Company’s programs include clinical trial site costs, clinical manufacturing costs, costs incurred for consultants and other external services, such as data management and statistical analysis support and materials and supplies used in support of clinical programs. Indirect costs of the Company’s clinical programs include salaries, stock-based compensation and an allocation of its facility costs.

When third-party service providers’ billing terms do not coincide with the Company’s period-end, the Company is required to make estimates of its obligations to those third parties, including clinical trial and pharmaceutical development costs, contractual services costs and costs for supply of its drug candidates, incurred in a given accounting period and record accruals at the end of the period. The Company bases its estimates on its knowledge of the research and development programs, services performed for the period, past history for related activities and the expected duration of the third party service contract, where applicable.

Stock-Based Compensation

The Company’s stock-based compensation programs include stock options, restricted stock awards (“RSAs”), restricted stock units (“RSUs”), stock appreciation rights (“SARs”) and employee stock purchase program (“ESPP”). The Company accounts for stock-based compensation using the fair value method.

The fair values of stock options and SARs are estimated on the date of grant using the Black-Scholes-Merton option-pricing model. The fair values of RSAs and RSUs are based on the fair market value of the Company’s common stock on the date of the grant. The fair value of stock awards, with consideration given to estimated forfeitures, is recognized as stock-based compensation expense on a straight-line basis over the vesting period of the grants. For stock awards with performance-vesting conditions, the Company does not recognize compensation expense until it is probable that the performance-vesting condition will be achieved.

Under the Company’s ESPP, participating employees purchase common stock through payroll deductions. The purchase price is equal to 85% of the lower of the closing price of the Company’s common stock on the first business day and the last business day of the relevant purchase period. The fair values of stock purchase rights are estimated using the Black-Scholes-Merton option-pricing model. The fair value of the look-back provision plus the 15% discount is recognized on a graded-vesting basis as stock-based compensation expense over the purchase period.

In addition to stock options with service and performance conditions, the Company also granted its new CEO options with service and market conditions. A market condition relates to the achievement of a specified price of the Company’s common stock, a specified amount of intrinsic value indexed to the Company’s common stock or a specified price of the Company’s common stock in terms of other similar equity shares. The grant date fair value for the options with service and market conditions is determined by a lattice model with Monte Carlo simulations and, with consideration given to estimated forfeitures, is recognized as stock-based compensation expense on a straight-line basis over the service period.

Income Taxes

The Company follows the asset and liability method of accounting for income taxes. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the consolidated financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. It is the intention of the Company to reinvest the earnings of its non-U.S. subsidiaries in those operations and not to repatriate the earnings to the U.S. Accordingly, the Company does not provide for deferred taxes on the excess of the financial reporting over the tax basis in its investments in foreign subsidiaries as they are considered permanent in duration. To date, the Company has not had any earnings in its non-U.S. subsidiaries.

Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered and settled. A valuation allowance is recorded to reduce the net deferred tax asset to zero when it is more likely than not that the net deferred tax asset will not be realized. The Company recognizes the effect of income tax positions only if those positions are more likely than not of being sustained upon an examination.

The Tax Cuts and Jobs Act (“the Act”) was enacted on December 22, 2017. The Act reduces the U.S. federal corporate tax rate from 35% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. On December 22, 2017, the Securities and Exchange Commission (“SEC”) issued guidance under Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”). SAB 118 directs taxpayers to consider the impact of the U.S. legislation as “provisional” when it does not have the necessary information available to prepare or analyze (including computations) in reasonable detail to complete its accounting for the change in tax law.

At December 31, 2017, the Company made reasonable estimates of the effects on its existing deferred tax balances and corresponding valuation allowance. For the year ended December 31, 2017, the Company recognized no transition tax, and has remeasured deferred taxes, its reassessment of permanently reinvested earnings, uncertain tax positions and valuation allowances.

Rent Expense

The Company’s operating leases for its Cambridge, Massachusetts and Corvallis, Oregon facilities provide for scheduled annual rent increases throughout each lease’s term. The Company recognizes the effects of the scheduled rent increases on a straight-line basis over the full term of the leases.

For the years ended December 31, 2017, 2016 and 2015, the Company recognized rent expense and occupancy costs of $4.4 million, $5.6 million and $5.2 million, respectively.

Commitments and Contingencies

The Company records liabilities for legal and other contingencies when information available to the Company indicates that it is probable that a liability has been incurred and the amount of loss can be reasonably estimated. Legal costs in connection with legal and other contingencies are expensed as costs are incurred.

Subsequent Events

The Company has evaluated events from December 31, 2017 through the date of issuance of this report and concluded that no subsequent events have occurred that would require recognition or disclosure in the consolidated financial statements.

Recent Accounting Pronouncements

In May 2017, the FASB issued ASU No. 2017-09, “Compensation – Stock Compensation (Topic 718): Scope of Modification Accounting”. The amendments in this update provide guidance about which changes to the terms or conditions of a stock-based payment award requires an entity to apply modification accounting in Topic 718. ASU No. 2017-09 will be effective for fiscal years beginning after December 15, 2017, with early adoption permitted. The Company elected to early adopt this guidance as of June 30, 2017 and determined that the adoption of this guidance does not have any impact on its consolidated financial statements.

In November 2016, the FASB issued ASU No. 2016-18, “Statement of Cash Flows: Restricted Cash”. The amendments in this update requires amounts generally described as restricted cash and restricted cash equivalents be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU No. 2016-18 will be effective for fiscal years beginning after December 15, 2017, with early adoption permitted. The Company elected to early adopt this guidance as of January 1, 2017. This guidance was applied using a retrospective transition method for each period and, accordingly, the Company included approximately $0.1 million of restricted cash in cash and cash equivalents as of the beginning and ending periods in the consolidated statements of cash flows.

In August 2016, the FASB issued ASU No. 2016-15, “Statement of Cash Flows: Classification of Certain Cash Receipts and Cash Payments”. The amendments in this update clarify how certain cash receipts and cash payments are presented and classified in the statement of cash flows. ASU No. 2016-15 will be effective for fiscal years beginning after December 15, 2017, with early adoption permitted. As of December 31, 2017, the Company has elected to early adopt this guidance and determined that adoption of this guidance does not have any impact on its consolidated financial statements.

In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)”, which supersedes Topic 840, “Leases”. Under the new guidance, a lessee should recognize assets and liabilities that arise from its leases and disclose qualitative and quantitative information about its leasing arrangements. In transition, lessees and lessors are required to recognize and measure leases at the beginning of the earliest period presented using a modified retrospective approach. ASU No. 2016-02 will be effective for fiscal years beginning after December 15, 2018, with early adoption permitted. The adoption of this standard is expected to have an impact on the amount of the Company’s assets and liabilities. As of December 31, 2017, the Company has not elected to early adopt this guidance or determined the effect that the adoption of this guidance will have on its consolidated financial statements.

In May 2014, the FASB issued ASU No. 2014-09, “Revenue from Contracts with Customers (Topic 606)”. This ASU supersedes the revenue recognition requirements in Accounting Standards Codification Topic 605, “Revenue Recognition”. Under the new guidance, a company is required to recognize revenue when it transfers goods or renders services to customers at an amount that it expects to be entitled to in exchange for these goods or services. The new standard allows for either a full retrospective with or without practical expedients or a retrospective with a cumulative catch upon adoption transition method. This guidance was originally intended to be effective for the fiscal years beginning after December 15, 2016, with early adoption not permitted. In August 2015, the FASB issued ASU No. 2015-14, “Deferral of the Effective Date”, which states that the mandatory effective date of this new revenue standard will be delayed by one year, with early adoption only permitted in fiscal year 2017. During the second quarter of 2016, the FASB issued three amendments to the new revenue standard to address some application questions: ASU No. 2016-10, “Identifying Performance Obligations and Licensing”, ASU No. 2016-11, “Rescission of SEC Guidance Because of Accounting Standards Updates 2014-09”, and ASU No. 2016-12, “Narrow-Scope Improvements and Practical Expedients”. In December 2016, the FASB issued ASU No. 2016-20, “Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers”, which amends certain narrow aspects of the guidance issued in ASU 2014-09 including guidance related to the disclosure of remaining performance obligations and prior-period performance obligations, as well as other amendments to the guidance on loan guarantee fees, contract costs, refund liabilities, advertising costs and the clarification of certain examples. These three amendments will be effective upon adoption of Topic 606. The Company has assessed the impact of the new standards as compared to its current accounting policies with respect to its contracts with its customers. As of December 31, 2017, the Company has completed its assessment and determined that it will utilize the full retrospective adoption method and that adoption of this guidance will have an immaterial financial impact on its consolidated financial statements.

LICENSE AND COLLABORATION AGREEMENTS	12 Months Ended
LICENSE AND COLLABORATION AGREEMENTS	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
LICENSE AND COLLABORATION AGREEMENTS	3. LICENSE AND COLLABORATION AGREEMENTS BioMarin Pharmaceutical, Inc. In July 2017, the Company and UWA entered into a settlement agreement with BioMarin Leiden Holding BV, its subsidiaries BioMarin Nederlands BV and BioMarin Technologies BV (collectively, “BioMarin”). On the same day, the Company entered into a license agreement with BioMarin and Academisch Ziekenhuis Leiden (“AZL”) (collectively with the Company, UWA and BioMarin, the “Settlement Parties”). Under these agreements, BioMarin agreed to provide the Company with an exclusive license to certain intellectual property with an option to convert the exclusive license into a co-exclusive license and the Settlement Parties agreed to stop most existing efforts to continue with ongoing litigation and opposition and other administrative proceedings concerning BioMarin’s intellectual property. Under terms of the agreements, the Company agreed to make total up-front payments of $35.0 million upon execution of the agreements, consisting of $20.0 million under the settlement agreement and $15.0 million under the license agreement. Additionally, the Company may be liable for up to approximately $65.0 million in regulatory and sales milestones for eteplirsen as well as exon 45 and exon 53 skipping product candidates. BioMarin will also be eligible to receive royalty payments, ranging from 4% - 8%, for exon 51 skipping products, exon 45 skipping products and exon 53 skipping products. The royalty terms under the license agreement will expire in December 2023 in the U.S. and September 2024 in the EU. In connection with the above agreements, in July 2017, the Company made a cash payment of $35.0 million to BioMarin. Accordingly, the Company has recorded an intangible asset of $6.6 million on its consolidated balance sheets. In addition, for the year ended December 31, 2017, the Company recorded $28.4 million settlement and license charges, in its consolidated statements of operations and comprehensive loss. The intangible asset represents the fair value of the U.S. license to BioMarin’s intellectual property related to EXONDYS 51, which was determined by an income-based approach, and is being amortized on a straight-line basis over the remaining life of the patent. For the year ended December 31, 2017, the Company recognized intangible asset amortization expense and royalties of approximately $1.0 million and $4.7 million, respectively. The royalties are included in cost of sales in the Company’s consolidated statements of operations and comprehensive loss. Summit (Oxford) Ltd. In October 2016, the Company entered into an exclusive Collaboration and License Agreement (the “Collaboration Agreement”) with Summit which grants the Company the exclusive right to commercialize products in Summit’s utrophin modulator pipeline in the EU, Switzerland, Norway, Iceland, Turkey and the Commonwealth of Independent States (the “Licensed Territory”). Under the terms of the Collaboration Agreement, the Company made an up-front payment of $40.0 million to Summit, with additional payments of up to $192.0 million based on achievement of certain development and regulatory milestones for ezutromid, a Summit product candidate in its utrophin modulator pipeline. For each of Summit’s future generation small molecule utrophin modulators, the Company may be required to make up to $290.0 million in development and regulatory milestone payments. Additionally, on a product-by-product basis, the Company may be required to make up to $330.0 million in sales milestone payments. The Collaboration Agreement also grants the Company an option to expand the Licensed Territory (“Option Territory”). If the Company exercises this option, it will be liable for a one-time $10.0 million option fee as well as up to $7.0 million in regulatory milestone payments. For each licensed product in the Option Territory, the Company may be liable for up to $82.5 million in sales milestone payments. Additionally, the Company may be required to make tiered royalty payments ranging from a low to high teens percentage of net sales on a product-by-product basis in the Licensed Territory. Under the Collaboration Agreement, a joint steering committee has been established to plan, monitor and coordinate future development activities for ezutromid and future generation small molecule utrophin modulators. Summit will be solely responsible for all research and development costs for the licensed products until December 31, 2017. Thereafter, Summit will be responsible for 55.0% of the budgeted research and development costs related to the licensed products in the Licensed Territory, and the Company will be responsible for 45.0% of such costs. Any costs in excess of 110.0% of the budgeted amount are borne by the party that incurred such costs. Summit is also obligated to spend a specified minimum amount on the research and development of certain licensed products prior to the end of 2019. For the years ended December 31, 2017 and 2016, the Company recorded a $22.0 million milestone and a $40.0 million up-front payment to Summit, respectively, as research and development expense in its consolidated statement of operations and comprehensive loss. University of Western Australia In April 2013, the Company and UWA entered into an agreement under which an existing exclusive license agreement between the Company and UWA was amended and restated (the “Amended and Restated UWA License Agreement”). The Amended and Restated UWA License Agreement grants the Company specific rights to the treatment of DMD by inducing the skipping of certain exons. EXONDYS 51 falls under the scope of the license agreement. Under the Amended and Restated UWA License Agreement, the Company may be required to make payments of up to $6.0 million in aggregate to UWA based on the successful achievement of certain development and regulatory milestones relating to EXONDYS 51 and up to five additional product candidates. The Company may also be obligated to make payments to UWA of up to $20.0 million upon the achievement of certain sales milestones. Additionally, the Company may be required to pay a low-single-digit percentage royalty on net sales of products covered by issued patents licensed from UWA during the term of the Amended and Restated UWA License Agreement. However, the Company has the option to purchase future royalties up-front. Under this option, prior to the First Amendment (defined below), the Company could elect to make a one-time royalty payment of $30.0 million to UWA. In June 2016, the Company and UWA entered into the first amendment to the Amended and Restated UWA License Agreement (the “First Amendment”). Under the First Amendment, the Company was obligated to make an up-front payment of $7.0 million to UWA upon execution of the amendment. Under the terms of the First Amendment, UWA has waived certain rights and amended the timing of certain payments under the Amended and Restated UWA License Agreement, including lowering the up-front payment that is due by the Company upon exercise of the option to purchase future royalties up-front. Upon exercise of the option to purchase future royalties up-front, the Company will be obligated to make a $23.0 million payment to UWA. Additionally, the Company would still be obligated to make up to $20.0 million in payments to UWA upon achievement of certain sales milestones. For the years ended December 31, 2016 and 2015, the Company recorded $7.6 million and $0.2 million, respectively, relating to the development milestone and up-front payments to UWA as research and development expense in the consolidated statement of operations and comprehensive loss as the Amended and Restated UWA License Agreement and its First Amendment were entered into before the FDA approval of EXONDYS 51. The Company did not incur any milestone expense for the year ended December 31, 2017. Additionally, corresponding to the FDA approval and the subsequent commercial sale of EXONDYS 51, as defined in the Amended and Restated UWA License Agreement, the Company recorded a $1.0 million milestone payment as an in-license right in its consolidated balance sheet. As of December 31, 2017, the in-license right is recorded net of $0.1 million accumulated amortization on its consolidated balance sheets. The amortization of the in-licensed right is recorded as cost of sales in the Company’s consolidated statements of operations and comprehensive loss. As of December 31, 2017, the Company did not make any royalty payments but may be obligated to make these payments in the future.

GAIN FROM SALE OF PRIORITY REVIEW VOUCHER	12 Months Ended
GAIN FROM SALE OF PRIORITY REVIEW VOUCHER	Dec. 31, 2017
Goodwill And Intangible Assets Disclosure [Abstract]
GAIN FROM SALE OF PRIORITY REVIEW VOUCHER	4. GAIN FROM SALE OF PRIORITY REVIEW VOUCHER In February 2017, the Company entered into an agreement with Gilead Sciences, Inc. (“Gilead”) to sell the Company’s Rare Pediatric Disease Priority Review Voucher (“PRV”). The Company received the PRV when EXONDYS 51was approved by the FDA for the treatment of patients with DMD amenable to exon 51 skipping. Following the early termination of the applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, in March 2017, the Company completed its sale of the PRV to a subsidiary of Gilead. Pursuant to the Agreement, the subsidiary of Gilead paid the Company $125.0 million, which was recorded as a gain from sale of the PRV as it did not have a carrying value at the time of the sale.

FAIR VALUE MEASUREMENTS

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

FAIR VALUE MEASUREMENTS

5. FAIR VALUE MEASUREMENTS

The tables below present information about the Company’s financial assets that are measured and carried at fair value and indicate the level within the fair value hierarchy of the valuation techniques it utilizes to determine such fair value:

	Fair Value Measurement as of December 31, 2017
	Total		Level 1		Level 2		Level 3
	(in thousands)
Assets
Cash equivalents	$	352,370	$	352,370	$	—	$	—
Commercial paper		133,368		—		133,368		—
Government and government agency bonds		294,717		284,745		9,972		—
Corporate bonds		127,956		127,956		—		—
Certificates of deposit		648		648		—		—
Total assets	$	909,059	$	765,719	$	143,340	$	—


	Fair Value Measurement as of December 31, 2016
	Total		Level 1		Level 2		Level 3
	(in thousands)
Cash equivalents	$	1,147	$	1,147	$	—	$	—
Commercial paper		69,304		—		69,304		—
Government and government agency bonds		105,287		—		105,287		—
Corporate bonds		20,834		—		20,834		—
Certificates of deposit		11,343		11,343		—		—
Total assets	$	207,915	$	12,490	$	195,425	$	—

The Company’s assets with fair value categorized as Level 1 within the fair value hierarchy include money market funds, government and government agency bonds, corporate bonds and certificates of deposit. Certain of the government and government agency bonds and corporate bonds are publicly traded fixed income securities and are presented as cash equivalents on the consolidated balance sheets as of December 31, 2017

The Company’s assets with fair value categorized as Level 2 within the fair value hierarchy consist of commercial paper, government and government agency bonds and corporate bonds. These assets have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, through income-based approaches utilizing market observable data.

The carrying amounts reported in the consolidated balance sheets for cash and cash equivalents, accounts receivable, accounts payable and revolving line of credit approximate fair value because of the immediate or short-term maturity of these financial instruments. The carrying amounts for the term loan approximate fair value based on market activity for other debt instruments with similar characteristics and comparable risk.

CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES

12 Months Ended

Dec. 31, 2017

Cash And Cash Equivalents [Abstract]

CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES

6. CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES

The following table summarizes the Company’s financial assets with maturities of less than 90 days from the date of purchase included in cash equivalents in the consolidated balance sheets for each of the periods indicated:

	As of December 31
	2017		2016
	(in thousands)
Money market funds		352,370		1,147
Corporate bonds		16,720		—
Government and government agency bonds		49,972		—
Total		419,062		1,147

It is the Company’s policy to mitigate credit risk in its financial assets by maintaining a well-diversified portfolio that limits the amount of exposure as to maturity and investment type. The weighted average maturity of the Company’s available-for-sale securities as of December 31, 2017 and 2016 was approximately seven and four months, respectively. The following tables summarize the Company’s cash, cash equivalents and investments for each of the periods indicated:

	As of December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Market Value
	(in thousands)
Cash and money market funds	$	532,999	$	—	$	—		$	532,999
Commercial paper - current		133,368		—		—			133,368
Government and government agency bonds - current		294,915		2		(200	)		294,717
Corporate bonds
Current		118,121		—		(145	)		117,976
Non-current		10,016		—		(36	)		9,980
Total assets	$	1,089,419	$	2	$	(381	)	$	1,089,040
As reported:
Cash and cash equivalents	$	599,698	$	2	$	(9	)	$	599,691
Short-term investments		479,705		—		(336	)		479,369
Long-term investments		10,016		—		(36	)		9,980
Total assets	$	1,089,419	$	2	$	(381	)	$	1,089,040
	As of December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Market Value
	(in thousands)
Cash and money market funds	$	122,420	$	—	$	—		$	122,420
Commercial paper - current		69,355		—		(51	)		69,304
Government and government agency bonds - current		105,340		—		(53	)		105,287
Corporate bonds - current		20,850		—		(16	)		20,834
Total assets	$	317,965	$	—	$	(120	)	$	317,845
As reported:
Cash and cash equivalents	$	122,420	$	—	$	—		$	122,420
Short-term investments		195,545		—		(120	)		195,425
Total assets	$	317,965	$	—	$	(120	)	$	317,845

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES

12 Months Ended

Dec. 31, 2017

Receivables Net Current [Abstract]

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES

7. ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES

The Company’s accounts receivable arise from product sales, government research contracts and other grants. They are generally stated at the invoiced amount and do not bear interest.

The accounts receivable from product sales represents receivables due from the Company’s specialty distributor and specialty pharmacies in the U.S. as well as certain distributors in Israel and Middle East. The Company monitors the financial performance and creditworthiness of its customers so that it can properly assess and respond to changes in the customers’ credit profiles. The Company provides reserves against trade receivables for estimated losses that may result from a customer’s inability to pay. Amounts determined to be uncollectible are written-off against the established reserve. As of December 31, 2017, the credit profiles for the Company’s customers are deemed to be in good standing and write-offs of accounts receivable are not considered necessary. Historically, no accounts receivable amounts related to government research contracts and other grants have been written off and, thus, an allowance for doubtful accounts receivable related to government research contracts and other grants is not considered necessary.

The following table summarizes the components of the Company’s accounts receivable for the periods indicated:

	As of December 31,
	2017		2016
	(in thousands)
Product sales, net of discounts and allowances	$	28,539	$	4,002
Government contract receivables		929		1,226
Total accounts receivable	$	29,468	$	5,228

The balance for government contract receivables for both periods presented is subject to government audit and will not be collected until the completion of the audit. The decrease in government contract receivables is related to contract finalization and subsequent collection of the European Union SKIP-NMD Agreement related to the Company’s exon 53 product candidate.

The following table summarizes an analysis of the change in reserves for discounts and allowances for the periods indicated:

	Chargebacks			Rebates			Prompt Pay			Other Accruals			Total
	(in thousands)
Balance, as of December 31, 2016	$	1		$	238		$	—		$	67		$	306
Provision		6,455			7,659			469			1,293			15,876
Payments/credits		(5,461	)		(938	)		(300	)		(896	)		(7,595	)
Balance, as of December 31, 2017	$	995		$	6,959		$	169		$	464		$	8,587

The following table summarizes the total reserves above included in the Company’s consolidated balance sheets for the periods indicated:

	As of December 31,
	2017		2016
	(in thousands)
Reduction to accounts receivable	$	1,285	$	1
Component of accrued expenses		7,302		305
Total reserves	$	8,587	$	306

OTHER ASSETS

12 Months Ended

Dec. 31, 2017

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

OTHER ASSETS

8. OTHER ASSETS

The following table summarizes the Company’s other current assets for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Manufacturing-related deposits and prepaids	$	18,650	$	23,604
Prepaid clinical and preclinical expenses		5,175		1,225
Prepaid research expenses		2,896		—
Prepaid maintenance services		1,711		109
Prepaid commercial expenses		1,589		54
Asset held for sale		1,501		—
Other prepaids		2,726		989
Other		2,263		914
Total other current assets	$	36,511	$	26,895

The following table summarizes the Company’s investments and other assets for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Long-term available-for-sale securities	$	9,980	$	—
Prepaid clinical expenses		7,488		3,725
Alternative minimum tax credit		3,315		—
Restricted cash and investments		784		784
Other		242		242
Total investments and other assets	$	21,809	$	4,751

PROPERTY AND EQUIPMENT

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

PROPERTY AND EQUIPMENT

9. PROPERTY AND EQUIPMENT

Property and equipment are recorded at historical cost, net of accumulated depreciation. The following table summarizes components of property and equipment, net for each of the periods indicated:

	As of December 31,
	2017			2016
	(in thousands)
Land	$	4,158		$	4,158
Building and improvements		14,543			12,729
Software and computer equipment		8,965			6,775
Lab equipment		9,860			14,149
Office equipment		2,211			3,028
Leasehold improvements		13,633			22,872
Construction in progress		7,808			4,436
Property and equipment, gross		61,178			68,147
Less: accumulated depreciation		(18,022	)		(30,346	)
Property and equipment, net	$	43,156		$	37,801

For the years ended December 31, 2017, 2016 and 2015, depreciation expense totaled $6.4 million, $5.0 million and $4.7 million, respectively. The decrease in accumulated depreciation related to write-offs of fully deprecated property and equipment that the Company no longer uses as of December 31, 2017. The cost basis of these assets is approximately $17.1 million. No loss was recognized. Included in the $30.3 million accumulated depreciation as of December 31, 2016 was $0.8 million of accelerated depreciation for certain assets whose expected useful lives were shortened due to the Corvallis plan (defined in Note 11).

ACCRUED EXPENSES

12 Months Ended

Dec. 31, 2017

Payables And Accruals [Abstract]

ACCRUED EXPENSES

10. ACCRUED EXPENSES

The following table summarizes the Company’s accrued expenses for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Accrued clinical and preclinical costs	$	15,975	$	10,033
Accrued employee compensation costs		14,402		8,748
Accrued contract manufacturing costs		14,019		4,673
Product revenue related reserves		7,302		305
Accrued professional fees		6,794		2,799
Accrued BioMarin royalties		2,846		—
Accrued property and equipment		2,525		1,186
Accrued interest expenses		1,291		100
Accrued income taxes		943		—
Accrued research costs		401		1,186
Other		2,484		1,986
Total accrued expenses	$	68,982	$	31,016

RESTRUCTURING

12 Months Ended

Dec. 31, 2017

Restructuring And Related Activities [Abstract]

RESTRUCTURING

11. RESTRUCTURING

In March 2016, the Company announced a long-term plan (“Corvallis plan”) to consolidate all of the Company’s operations to Massachusetts and reduce its workforce by approximately 19% as part of a strategic plan to increase operational efficiency. As part of the consolidation, research activities and some employees transitioned to the Company’s facilities in Andover and Cambridge, Massachusetts. As of September 30, 2017, all relocations and terminations under the Corvallis plan were completed.

The second floor and the first floor of the Corvallis facility were vacated and closed and made available for sub-leasing in December 2016 and April 2017, respectively. Using a discounted cash flow methodology and based on monthly rent payments as well as estimated sublease income, the Company recognized a total of approximately $1.6 million and $2.4 million, in restructuring expenses for the second and the first floor, respectively. As of December 31, 2017, the Company continues to be obligated to make $4.9 million of minimum lease payments and certain other contractual maintenance costs for the whole facility.

In August 2016, the Company implemented a restructuring plan in Cambridge, Massachusetts (“Cambridge plan”) and reduced its workforce by approximately 6%. The restructuring costs associated with the Cambridge plan consist of costs associated with workforce reduction totaling $0.6 million. The Cambridge plan was completed as of October 31, 2016.

For the years ended December 31, 2017 and 2016, the Company recognized $3.0 million and $4.6 million as restructuring expenses, respectively, less than $0.1 million and $2.3 of which related to workforce reduction.

The following table summarizes the restructuring costs by function for the period indicated:

	For the Year Ended December 31
	2017						2016
	(in thousands)
	Cash		Non-cash		Total		Cash		Non-cash		Total
Research and development	$	188	$	—	$	188	$	1,631	$	382	$	2,013
Selling, general and administrative		2,832		—		2,832		2,020		529		2,549
Total restructuring expenses	$	3,020	$	—	$	3,020	$	3,651	$	911	$	4,562

The following table summarizes the restructuring reserve for each of the periods indicated:

	For the Year Ended December 31
	2017			2016
	(in thousands)
Restructuring reserve beginning balance	$	1,588		$	—
Restructuring expenses incurred during the period		3,020			3,651
Amounts paid during the period		(1,675	)		(2,063	)
Restructuring reserve ending balance	$	2,933		$	1,588

INDEBTEDNESS

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

INDEBTEDNESS

12. INDEBTEDNESS

2024 Convertible Notes

On November 14, 2017, the Company issued $570.0 million senior notes due on November 15, 2024 (the “2024 Notes”). The 2024 Notes were issued at face value and bear interest at the rate of 1.50% per annum, payable semi-annually in cash on each May 15 and November 15, commencing on May 15, 2018.

Upon conversion, the Company may pay cash, shares of its common stock or a combination of cash and stock, as determined by the Company in its discretion. The 2024 Notes may be convertible into 7,763,552 shares of the Company’s common stock under certain circumstances prior to maturity at a conversion rate of 13.621 shares per $1,000 principal amount of the 2024 Notes, which represents a conversion price of $73.42 per share, subject to adjustment under certain conditions which include:

•

as the result of a spin-off or other distribution of property;

•

if the Company exclusively issues shares of common stock as a dividend or distribution on common stock, or if the Company effects a share split or share combination;

•

cash dividend or distribution is made to all or substantially all holders of the common stock;

•

tender offer or exchange offer

•

if the Company issues to all or substantially all holders of common stock any rights, options or warrants entitling them to subscribe for or purchase shares of the common stock at a price per share that is less than the average of the last reported sales prices of the common stock for the 10 consecutive trading day period ending on, and include, the trading day immediately preceding the date of announcement of such issuance; and

•

at the discretion of the Company’s board of directors if deemed in the best interest of the Company.

In connection with the issuance of the 2024 Notes, the Company incurred $14.8 million of debt issuance costs. These costs are being amortized under an effective interest method and are recorded as additional interest expense over the life of the 2024 Notes. The Company has separately accounted for the liability and equity components of the 2024 Notes by allocating the proceeds from issuance of the Notes between the liability component and the embedded conversion option, or equity component. This allocation was done by first estimating an interest rate at the time of issuance for similar notes that do not include the embedded conversion option. In order to estimate the interest rate, the Company first selected an appropriate implied credit rating for the Company based on its financial metrics and then selected the market yield of the debt issuances within that rating category. The Company allocated $161.1 million to the equity component, net of offering costs of $4.2 million, which was recorded as additional paid-in capital. Additionally, the Company recorded a debt discount which equals the portion allocated to the equity component and debt issuance costs of $10.6 million on the notes, both of which will be amortized under an effective interest method and recorded as additional interest expense over the life of the 2024 Notes. The effective interest rate on the liability component of the Notes for the year ended December 31, 2017 was 6.9%. The Company may incur between 0.25% and 0.5% per annum additional interest expenses should it fail to register the 2024 Notes after 180 days and 380 days after the issuance date of the 2024 Notes, respectively.

Upon the occurrence of a “fundamental change”, which includes (1) change in beneficial ownership of the Company where any person/group possesses more than 50% of the voting power of the Company, (2) consolidation or merger of the Company, (3) shareholder approval of a liquidation plan or (4) the Company is delisted from NYSE or NASDAQ, the holders may require the Company to repurchase all or a portion of the 2024 Notes for cash at 100% of the principal amount of the 2024 Notes being purchased, plus any accrued and unpaid interest. Additionally, upon the occurrence of a “make-whole fundamental change” prior to the maturity date, the Company shall adjust the conversion rate on a sliding scale basis detailed in the agreement

To minimize the impact of potential dilution upon conversion of the 2024 Notes, the Company separately entered into capped call transactions with certain counterparties. The capped calls have a strike price of $73.42 and a cap price of $104.88 and are exercisable when and if the 2024 Notes are converted. If, upon conversion of the 2024 Notes, the price of the Company’s common stock is between the strike price and the cap price of the capped calls, the counterparties will deliver shares of the Company’s common stock and/or cash with an aggregate value equal to the difference between the price of the Company’s common stock at the conversion date and the strike price, multiplied by the number of shares of the Company’s common stock related to the capped calls being exercised. The Company paid $50.9 million for these capped calls transactions, which was recorded as additional paid-in capital.

The following table summarizes the 2024 Notes for the period indicated:

	As of December 31, 2017
	(in thousand)
Par value of the 2024 Notes	$	570,000
Unamortized discounts	$	(158,890	)
Debt issuance expenses		(10,449	)
Net carrying value of convertible debt	$	400,661

Fair value of convertible debt	$	619,641

The fair value of the Company’s 2024 Notes is based on open market trades and is classified as level 1 in the fair value hierarchy.

Term Loans

In July 2017, the Company entered into an amended and restated credit agreement (the “Amended and Restated Credit and Security Agreement”) which provides a term loan (“July 2017 Term Loan”) of $60.0 million with MidCap Financial Trust (“MidCap”). Borrowings under the Amended and Restated Credit and Security Agreement bear interest at a rate per annum equal to 6.25%, plus the one-month London Interbank Offered Rate (“LIBOR”). In addition to paying interest on the outstanding principal under the Amended and Restated Credit and Security Agreement, the Company paid an origination fee equal to 0.50% of the amount of the term loan when advanced under the Amended and Restated Credit and Security Agreement and will be liable for a final payment fee equal to 2.00% of the amount borrowed under the Amended and Restated Credit and Security Agreement when the July 2017 Term Loan is fully repaid. Commencing on July 1, 2018, and continuing for the remaining thirty six months of the facility, the Company will be required to make monthly principal payments of approximately $0.8 million, set forth in the Amended and Restated Credit and Security Agreement, subject to certain adjustments as described therein. The facility matures in July 2021.

The Company may voluntarily prepay outstanding loans under the Amended and Restated Credit and Security Agreement at any time, provided that the Company may not prepay an amount that is less than the total of all of the credit extensions and other related obligations under the Amended and Restated Credit and Security Agreement then outstanding. In the event of a permitted prepayment, the Company is obligated to pay a prepayment fee equal to the following:

•

3.00% of the outstanding principal of such advance, if the prepayment is made within twelve months of the closing date;

•

2.00% of the outstanding principal of such advance, if the prepayment is made on or after the date which is twelve months after the closing date of such advance through the date which is twenty-four months after the closing date of such advance; and

•

1.00% of the outstanding principal of such advance, if the prepayment is made on or after the date which is twenty-four months after the closing date of such advance through the date immediately preceding the maturity date.

The Amended and Restated Credit and Security Agreement contains both affirmative and negative covenants. Affirmative covenants include government compliance, reporting requirements, maintaining property, making tax payments, maintaining insurance, cooperating during litigation, etc. Additionally, the Company is required to maintain an amount of cash and/or cash equivalents equal to not less than 75% of the sum of the outstanding principal amounts under both the Amended and Restated Credit and Security Agreement and the Revolving Credit Agreement (defined below). Negative covenants include restrictions on asset dispositions, mergers or acquisitions, indebtedness, liens, distributions, transactions with affiliates and other restrictions. The Amended and Restated Credit and Security Agreement includes customary events of default, including cross defaults and material adverse changes. Additionally, the Company's failure to be compliant with the affirmative or negative covenants or make payments when they become due will result in an event of default.

After paying off certain debt issuance costs, the Company received net proceeds of $29.1 million related to the July 2017 Term Loan, $9.2 million of which was used to pay off the outstanding balance of the term loan that was taken out in June 2015 (“June 2015 Term Loan”). In connection with the July 2017 Term Loan, the Company recorded $30.0 million as long-term debt in the consolidated balance sheet as of December 31, 2017. In addition, debt issuance costs of $1.1 million related to the July 2017 Term Loan were recorded as a direct deduction to the carrying value of the July 2017 Term Loan in the consolidated balance sheet as of December 31, 2017. These costs are being amortized to interest expense using the effective interest method over the term of the loan. The following table summarizes term loans for each of the periods indicated:

	As of December 31
	2017			2016
	(in thousands)
Principal amount of the 2017 Term Loan	$	30,000		$	—
Principal amount of the 2015 Term Loan		—			15,000
Unamortized debt issuance expense		(918	)		(223	)
Net carrying value of term loan	$	29,082		$	14,777

Revolving Line of Credit

In July 2017, the Company entered into a revolving credit and security agreement (the “Revolving Credit Agreement”) which provides an aggregate revolving loan commitment of $40.0 million (which may be increased by an additional tranche of $20.0 million) with MidCap (“Revolver”). Borrowings under the Revolving Credit Agreement bear interest at a rate of 3.95%, plus the one-month LIBOR. In addition to paying interest on the outstanding principal under the Revolving Credit Agreement, the Company paid $0.2 million of origination fee, which was 0.50% of the amount of the revolving loan. The Company recognized this origination fee as other asset and it is being amortized to interest expense over the term of the line-of-credit. Additionally, the Company is liable for unused line fees, minimum balance fees, collateral fees, deferred revolving loan original fees, etc. This facility matures in July 2021. The Company may voluntarily prepay the outstanding revolving loans under the Revolving Credit Agreement in whole or in part provided that the prepayment shall be in certain amounts as specified therein. As of December 31, 2017, the outstanding balance of the revolving line of credit is less than $0.1 million.

Mortgage Loans

The Company has two mortgage loans outstanding which bear interest at 4.75% with the original maturity date of February 2027 and are collateralized by the Airport Facility in Corvallis, Oregon. At December 31, 2017, these loans had unpaid principal balances of $0.8 million and $0.5 million, for a total indebtedness of $1.3 million, and were presented as current portion of long-term debt on the consolidated balance sheet. In connection with the sale of the Airport Facility in January 2018, the two long-term mortgage loans were paid off.

As of December 31, 2017, the Company recorded approximately $6.2 million as current portion of long-term debt and approximately $424.9 million as long-term debt on the consolidated balance sheets related to the convertible debt, the term loan, the revolver and the mortgage loans. Related to these debt facilities, for the years ended December 31, 2017, 2016 and 2015, the Company recorded $5.8 million, $1.9 million and $1.3 million of interest expenses.

The following table summarizes the total gross payments due under the Company’s debt arrangements:

	As of December 31, 2017
	(in thousands)
2018	$	6,328
2019		10,000
2020		10,000
2021		5,000
2022		—
Thereafter		570,000
Total Payments	$	601,328

EQUITY FINANCING	12 Months Ended
EQUITY FINANCING	Dec. 31, 2017
Equity [Abstract]
EQUITY FINANCING	13. EQUITY FINANCING In July 2017, the Company sold approximately 8.8 million shares of common stock through an underwritten public offering, including 1.2 million shares sold to the underwriters. The offering price was $42.50 per share. The Company received net proceeds of approximately $354.0 million from the offering, net of commission and offering expenses of approximately $20.0 million. In September 2016, the Company sold approximately 5.8 million shares of common stock through an underwritten public offering at a price of $59.75 per share. The Company received aggregate net proceeds of approximately $327.4 million from the offering net of commission and offering expenses of approximately $17.6 million. In June 2016, the Company sold approximately 2.1 million shares of common stock through an underwritten public offering at a price of $17.84 per share. The implied underwriting discount and commission was $1.60 per share. The Company received aggregate net proceeds of approximately $37.3 million from the offering net of offering expense of approximately $0.2 million. In October 2015, the Company sold approximately 3.3 million shares of common stock at an offering price of $39.00 per share. The Company received aggregate net proceeds of approximately $119.9 million, after deducting the underwriting discounts and offering related transaction costs of approximately $6.8 million.

STOCK-BASED COMPENSATION

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

STOCK-BASED COMPENSATION

14. STOCK-BASED COMPENSATION

In June 2011, the Company’s stockholders approved the 2011 Equity Incentive Plan (“2011 Plan”). The 2011 Plan, which authorized 13.0 million shares of common stock to be issued, allows for the grant of stock options, SARs, RSAs, RSUs, performance shares and performance units. In June 2016 and 2015, shareholders authorized the issuance of an additional 1.3 million and 1.7 million shares, respectively, of common stock under the 2011 Plan. As of December 31, 2017, 2.3 million shares of common stock remain available for future grant under the 2011 Plan.

In June 2013, the Company’s stockholders approved the 2013 ESPP with approximately 0.3 million shares of common stock available to be issued. In June 2016, the Company’s stockholders approved an additional approximately 0.3 million shares of common stock available to be issued to be added to the 2013 ESPP. As of December 31, 2017, 0.2 million shares of common stock remain available for future grant under the 2013 ESPP.

In September 2014, the Company initiated the 2014 Employment Commencement Incentive Plan (“2014 Plan”) with approximately 0.6 million shares of common stock available to be issued. In October 2015 and June 2017, the 2014 Plan was increased by 1.0 million 3.8 million shares of common stock available to be issued. As of December 31, 2017, 0.6 million shares of common stock remain available for future grant under the 2014 Plan.

Stock Options

In general, stock options have a ten-year term and vest over a four-year period, with one-fourth of the underlying shares vesting on the first anniversary of the grant and 1/48th of the underlying shares vesting monthly thereafter, such that the underlying shares will be fully vested on the fourth anniversary of the grant, subject to the terms of the applicable plan under which they were granted.

The fair values of stock options granted during the periods presented were measured on the date of grant using the Black-Scholes-Merton option-pricing model, with the following assumptions:

	For the Year Ended December 31,
	2017	2016	2015
Risk-free interest rate (1)	1.6 - 2.1%	1.1 – 1.8%	1.1 – 1.7%
Expected dividend yield (2)	—	—	—
Expected terms (3)	4.2 - 4.8 years	4.2 – 4.8 years	4.7 – 5.0 years
Expected volatility (4)	54.0 - 63.0%	78.2 – 137.1 %	94.3 – 111.1 %

(1)	The risk-free interest rate is estimated using an average of Treasury bill interest rates over a historical period commensurate with the expected term of the option that correlates to the prevailing interest rates at the time of grant.

(2)	The expected dividend yield is zero as the Company has not paid any dividends to date and does not expect to pay dividends in the future.

(3)	The expected terms are estimated using historical exercise behavior.

(4)

Upon commercialization of EXONDYS 51 in the U.S., the Company’s future risk profile changed. As a result, starting January 1, 2017, the Company estimates expected volatility by using only implied volatility in exchange-traded options of the Company’s common stock. Prior to January 1, 2017, the expected volatility was estimated using a blend of calculated volatility of the Company’s common stock over a historical period and implied volatility in exchange-traded options of the Company’s common stock.

The amounts estimated according to the Black-Scholes-Merton option-pricing model may not be indicative of the actual values realized upon the exercise of these options by the holders.

The following tables summarize the Company’s stock option activity for each of the periods indicated:

	For the Year Ended December 31,
	2017							2016					2015
						Weighted					Weighted					Weighted
						Average					Average					Average
						Exercise					Exercise					Exercise
	Shares					Price		Shares			Price		Shares			Price
Grants outstanding at beginning of the period		5,436,951				$	22.70		6,515,976		$	23.91		5,216,203		$	24.45
Granted		4,805,722		(1	)		35.09		1,285,051			14.89		2,830,078			20.28
Exercised		(792,845	)				17.40		(1,056,821	)		18.31		(816,696	)		12.26
Cancelled		(643,624	)				25.44		(1,307,255	)		24.61		(713,609	)		26.78
Grants outstanding at end of the period		8,806,204				$	29.74		5,436,951		$	22.70		6,515,976		$	23.91

Grants exercisable at end of the period		3,288,712				$	24.76		2,942,624		$	24.42		2,617,167		$	23.85
Grants vested and expected to vest at end of the period		6,910,022				$	28.49		5,116,409		$	22.83		5,908,213		$	23.94

(1)

In June 2017, the Company granted its new CEO 3,300,000 options with service and market conditions. These options have a five-year cliff vesting schedule. The fair value of $13.48 for these options was determined by a lattice model with Monte Carlo simulations. The remaining are service-based options which have a four-year vesting schedule with 25% vesting on the first anniversary and 1/48 monthly thereafter.

The weighted-average grant date fair value per share of stock options granted during the years ended December 31, 2017, 2016 and 2015 was $14.78, $12.46 and $14.98, respectively.

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
Options outstanding at December 31, 2017	$	228,081	7.6
Options exercisable at December 31, 2017	$	101,559		5.1
Options vested and expected to vest at December 31, 2017	$	187,582	7.1

The following table summarizes the Company’s stock options vested and exercised for each of the periods indicated:

	For the Year Ended December 31,
	2017		2016		2015
	(in thousands)
Aggregate grant date fair value of stock options vested	$	18,225	$	30,651	$	27,858
Aggregate intrinsic value of stock options exercised	$	20,922	$	30,610	$	18,138

In February 2016, the Company granted to executives approximately 0.3 million stock options with service- and performance-based conditions (“February 2016 performance grants”). Vesting is achieved based upon the FDA approval of EXONDYS 51 and submission of MAA for eteplirsen to the EMA and continuing service over a four-year period. As a result of both milestones achieved in 2016, 50% of the February 2016 performance grants were triggered to be vested immediately and the remaining 50% are subject to the remaining service conditions of the awards.

In June 2013, the Company granted to executives approximately 0.4 million stock options with service- and performance-based conditions (“June 2013 performance grants”). Vesting is achieved based upon various regulatory approval for EXONDYS 51 and filings of investigational new drug (“IND”) submissions for other drug candidates and continuing service over a four-year period. Through the submission of two IND applications during 2014 and the approval of EXONDYS 51 in the U.S. in 2016, cumulatively, 80% of the June 2013 performance grants vested or are eligible to vest subject to the remaining service conditions of the awards. The remaining 20% were forfeited as the performance criteria was not met.

For the years ended December 31, 2017, 2016 and 2015, the Company has recognized approximately $0.9 million, $5.0 million and $0.5 million in stock-based compensation expense related to the options with performance-based criteria, respectively.

As of December 31, 2017, the total stock-based compensation expense related to non-vested awards with only service-vesting conditions not yet recognized is approximately $47.5 million and those with service- and performance-based conditions approximates $0.4 million.

Restricted Stock Awards

The Company grants RSAs to members of its board of directors and certain employees. The following table summarizes the Company’s RSA activity for each of the periods indicated:

	For the Year Ended December 31,
	2017					2016					2015
				Weighted					Weighted					Weighted
				Average					Average					Average
				Exercise					Exercise					Exercise
	Shares			Price		Shares			Price		Shares			Price
Grants outstanding at beginning of the period		153,170		$	34.53		161,320		$	21.50		6,000		$	34.92
Granted		341,500			34.58		117,553			41.22		181,783			20.80
Vested		(63,264	)		14.60		(59,703	)		13.62		(24,463	)		20.21
Forfeited		(19,625	)		43.02		(66,000	)		33.51		(2,000	)		13.90
Grants outstanding at end of the period		411,781		$	37.23		153,170		$	34.53		161,320		$	21.50

In September 2016, the Company granted executives RSAs with certain sales targets. If the sales targets are achieved within the required time frame, the number of RSAs may be increased from 71,925 to 89,906 shares. In December 2017, the Company modified the expiration date of these RSAs from June 30, 2018 to December 31, 2018. As a result of this modification, the grant date fair value was changed to $54.29. As of December 31, 2017, the sales targets related to these RSAs were not deemed probable. If and when deemed probable that such performance milestones may be achieved within the required time frame, the Company may recognize up to $4.9 million of stock-based compensation related to these grants. In September 2015, the Company granted executives 65,000 RSAs with performance conditions. As a result of the performance conditions not being met, these grants were cancelled during 2016.

Restricted Stock Units

The Company also grants RSUs to members of its board of directors and certain employees. The following table summarizes the Company’s RSU activity for the period indicated:

	For the Year Ended December 31, 2017
				Weighted
				Average
				Exercise
	Shares			Price
Granted		181,029		$	33.03
Vested		(78,017	)		32.63
Forfeited		(36,460	)		32.63
Grants outstanding at end of the period		66,552		$	33.72

In March 2017, the Company granted executives 156,029 RSUs with certain sales target and regulatory milestones. In June 2017, one performance condition of these RSUs was achieved. As a result, 50% of these RSUs became immediately vested and, accordingly, the Company recorded $2.5 million of stock-based compensation expenses for the year ended December 31, 2017. As of September 30, 2017, it has become probable that the second performance milestone will be achieved within the required timeline. Accordingly, for the year ended December 31, 2017, the Company recognized approximately $0.4 million of stock-based compensation expenses and there is remaining unrecognized stock-based compensation expense of $0.3 million. The third performance milestone was deemed as not probable of being achieved as of December 31, 2017. If and when deemed probable that the last performance milestone may be achieved within the required time frame, the Company may recognize up to $0.7 million of stock-based compensation related to the third performance milestones of these grants.

Stock Appreciation Rights

The Company issues SARs to employees on the same terms as options granted to employees. The grant date fair value of the SARs is determined using the same valuation assumptions as for stock options described above. Stock-based compensation expense is recognized on a straight-line basis over the vesting period of the SARs.

In August 2012, 70,000 SARs were granted to the Company’s former President and CEO and have an exercise price of $10.08 per share. In November 2012, 100,000 SARs were granted to the Company’s Executive Vice President and CFO and have an exercise price of $23.85 per share. The SARs are classified as equity as the agreements require settlement in shares of stock.

The following table summarizes the Company’s SAR activity for each of the periods indicated:

	For the Year Ended December 31,
	2017				2016					2015
			Weighted					Weighted				Weighted
			Average					Average				Average
			Exercise					Exercise				Exercise
	Shares		Price		Shares			Price		Shares		Price
Grants outstanding at beginning of the period		100,000	$	23.85		170,000		$	18.18		170,000	$	18.18
Exercised		—		—		(67,812	)		10.08		—		—
Forfeited		—		—		(2,188	)		10.08		—		—
Grants outstanding at end of the period		100,000	$	23.85		100,000		$	23.85		170,000	$	18.18
Grants exercisable at end of the period		100,000	$	23.85		100,000		$	23.85		141,249	$	17.59
Grants vested and expected to vest at end of the period		100,000	$	23.85		100,000		$	23.85		167,813	$	18.29

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
SARs outstanding, exercisable and vested at December 31, 2017	$	3,179	4.9

2013 Employee Stock Purchase Plan

Under the Company’s ESPP, participating employees purchase common stock through payroll deductions. The purchase price is equal to 85% of the lower of the closing price of the Company’s common stock on the first business day and the last business day of the relevant purchase period. The 24-month award period will end between February 28, 2018 and August 31, 2019. The following table summarizes the Company’s ESPP activity for each of the periods indicated:

	For the Year Ended December 31,
	2017		2016		2015
Number of shares purchased		102,698		137,113		75,539
Proceeds received (in millions)	$	1.4	$	1.6	$	0.9

Stock-based Compensation Expense

For the years ended December 31, 2017, 2016 and 2015, total stock-based compensation expense was $30.5 million, $30.0 million and $32.1 million, respectively. Included in the amounts for the year ended December 31, 2017 and 2015 are $2.1 million and $8.6 million of stock-based compensation expense incurred in connection with the resignation of the Company’s former CEOs, respectively. The following table summarizes stock-based compensation expense by function included within the consolidated statements of operations and comprehensive loss:

	For the Year Ended December 31,
	2017		2016		2015
	(in thousands)
Research and development	$	8,542	$	9,499	$	10,403
Selling, general and administrative		21,923		20,463		21,714
Total stock-based compensation	$	30,465	$	29,962	$	32,117

The following table summarizes stock-based compensation expense by grant type included within the consolidated statements of operations and comprehensive loss:

	For the Year Ended December 31,
	2017		2016		2015
	(in thousands)
Stock options	$	23,416	$	26,320	$	29,014
Employee stock purchase plan		1,754		2,380		2,165
Restricted stock awards/units		5,295		872		446
Other		—		390		492
Total stock-based compensation	$	30,465	$	29,962	$	32,117

401 (K) PLAN	12 Months Ended
401 (K) PLAN	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
401 (K) PLAN	15. 401 (K) PLAN The Company sponsors a 401 (k) Plan (“the Plan”) which is a defined contribution plan. It is available to all employees who are age 21 or older. Participants may make voluntary contributions and the Company makes matching contributions according to the Plan’s matching formula. For employees who started before January 1, 2016, matching contributions fully vest after one year of service. For employees who started on and after January 1, 2016, matching contributions vest on a straight line basis over three years. The expense related to the Plan primarily consists of the Company’s matching contributions. Expense related to the Plan totaled $1.4 million, $1.2 million and $0.9 million for the years ended December 31, 2017, 2016 and 2015, respectively.

OTHER INCOME AND LOSS

12 Months Ended

Dec. 31, 2017

Other Income And Expenses [Abstract]

OTHER INCOME AND LOSS

16. OTHER INCOME AND LOSS

The following table summarizes other income and loss for the periods indicated:

	For the year ended December 31,
	2017			2016			2015
	(in thousand)
Interest expenses	$	(5,781	)	$	(1,892	)	$	(1,365	)
Interest income		1,809			507			890
Other income		1,982			850			629
Gain from sale of Priority Review Voucher		125,000			—			—
Total other income (loss)	$	123,010		$	(535	)	$	154

INCOME TAXES

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

INCOME TAXES

17. INCOME TAXES

On December 22, 2017, the Tax Cuts and Jobs Act (the “Act") was enacted in the U.S. The Act reduces the U.S. federal corporate tax rate from 35% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. On December 22, 2017, the SEC issued guidance under Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”). SAB 118 directs taxpayers to consider the impact of the U.S. legislation as “provisional” when it does not have the necessary information available to prepare or analyze (including computations) in reasonable detail to complete its accounting for the change in tax law.

At December 31, 2017, the Company made reasonable estimates of the effects of the Act on its existing U.S. deferred tax balances. These estimates may be impacted by the need for further analysis and future clarification and guidance regarding available tax accounting methods and elections, earnings and profits computations, and state tax conformity to federal tax changes.

The Company incurred a $1.8 million state income tax liability in 2017. The state tax liability relates to tax on income of Sarepta U.S. in states where the Company had a profit in excess of net operating losses.

The Company incurred a $0.2 million current federal income tax for alternative minimum tax (“AMT”). Under the Act, corporate AMT is repealed for tax years beginning after December 31, 2017. Any AMT credit carryovers to tax years after that date generally may be utilized to the extent of the taxpayer’s regular tax liability. In addition, under the Act, any excess AMT credits are partially refundable in 2018, 2019, and 2020, and fully refundable in 2021. The Company has recorded a noncurrent receivable for the amount of expected refund of the AMT.

The Company did not incur any federal, state, or foreign income taxes in 2016 and 2015.

For the year ended December 31, 2017, the Company incurred domestic and foreign pre-tax losses of $45.7 million and $2.9 million.

The following table summarizes the reconciliation between the Company’s effective tax rate and the income tax rate for each of the periods indicated:

	For the Year Ended December 31,
	2017				2016				2015
Federal income tax rate		34.0		%		34.0		%		34.0		%
State taxes		(27.7	)			—				—
Research and development and other tax credits		8.5				1.5				0.3
Valuation allowance		(93.2	)			(17.3	)			(19.1	)
Permanent differences		6.4				(3.4	)			(1.7	)
Sarepta International C.V. return to provision		62.1				—				—
Impact of tax reform, net of valuation allowance		5.9				—				—
Foreign rate differential		(0.6	)			(14.8	)			(13.5	)
Other		0.4				—				—
Effective tax rate		(4.2	)	%		—		%		—		%

As a result of the Act, the Company remeasured all deferred tax assets and liabilities based on the rates at which they are anticipated to reverse in the future, which is generally 21%. This reduction to the Company’s deferred tax assets by approximately $87.0 was fully offset by a corresponding decrease in its valuation allowance, resulting in no net tax impact to the Company.

Permanent differences affecting the Company’s effective tax rate primarily include excess tax deductions, net of non-deductible stock-based compensation.

In December 2012, the Company licensed certain intellectual property of Sarepta Therapeutics, Inc. to its wholly owned subsidiary, Sarepta International C.V. The parties also entered into a contract research agreement under which Sarepta Therapeutics, Inc. performs research services for Sarepta International C.V. In January 2016, Sarepta Therapeutics, Inc. entered into a manufacturing and distribution agreement as well as service agreement with Sarepta International C.V. In conjunction with its recent filings, it was determined that Sarepta International C.V. in 2016 and 2017 were effectively connected with the conduct of a trade or business by the entity in the U.S. and, accordingly, the 2016 and 2017 losses are subject to U.S. income taxes. For the years ended December 31, 2017 and 2016, Sarepta International C.V. incurred losses of $172.8 million and $214.3 million, respectively.

The following table summarizes the analysis of the deferred tax assets and liabilities for each of the periods indicated:

	As of December 31,
	2017			2016
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	144,897		$	174,569
Difference in depreciation and amortization		1,210			2,589
Research and development and other tax credits		44,306			37,812
Stock-based compensation		14,392			16,166
Deferred rent		1,210			2,306
Deferred revenue		904			1,315
Capitalized inventory		22,127			36,282
Other		4,071			3,047
Total deferred tax assets		233,117			274,086
Deferred tax liabilities:
Debt discount		(28,548	)		—
Total deferred tax liabilities		(28,548	)		—
Valuation allowance		(204,569	)		(274,086	)
Net deferred tax assets	$	—		$	—

The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of U.S. federal and state net operating loss carryforwards, research and development tax credit carryforwards, stock-based compensation expense, capitalized inventory and intangibles. Under the applicable accounting standards, management has considered the Company’s history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of net federal and state deferred tax assets. Accordingly, a full valuation allowance of the net deferred tax asset had been established at December 31, 2017 and 2016. The net change in the valuation allowance for deferred tax assets was a decrease of $69.5 million and an increase of $54.2 million for the years ended December 31, 2017 and 2016, respectively. This decrease for the year ended December 31, 2017 was primarily due to a reduction in the federal tax rate from 34% to 21% under the Act signed into law on December 22, 2017 and the utilization of federal and state net operating losses.

As of December 31, 2017, the Company had federal and state net operating loss carryforwards of $249.3 million and $174.8 million, respectively, available to reduce future taxable income, which will expire between 2018 and 2037. In addition, the Company had $387.1 million of federal net operating loss carryforwards generated by its wholly-owned subsidiary, Sarepta International C.V., which will expire between 2036 and 2037. Utilization of these net operating losses could be limited under Section 382 of the Internal Revenue Code and similar state laws based on ownership changes and the value of the Company’s stock. Additionally, the Company has $34.3 million and $11.5 million of federal and state research and development credits, respectively, available to offset future taxable income. These federal and state research and development credits begin to expire between 2018 and 2037 and between 2018 and 2032, respectively. The Company also has foreign net operating loss carryforwards of $2.9 million, which will expire in 2024.

The follow table summarizes the reconciliation of the beginning and ending amount of total unrecognized tax benefits for each of the periods indicated:

	For the Year Ended December 31,
	2017			2016		2015
	(in thousands)
Balance at beginning of the period	$	4,644		$	3,706	$	—
Increase related to current year tax positions		735			801		613
Increase related to prior year tax positions		—			137		3,093
Decrease related to prior year tax position		(245	)		—		—
Balance at end of the period	$	5,134		$	4,644	$	3,706

The balance of total unrecognized tax benefits at December 31, 2017, if recognized, would not affect the effective tax rate on income from continuing operations, due to a full valuation allowance against the Company’s deferred tax assets. The Company does not expect that the amount of unrecognized tax benefits to change significantly in the next twelve months. The Company, including its domestic subsidiaries, files consolidated U.S. federal and state income tax returns. The Company’s policy is to recognize interest and/or penalties related to income tax matters in income tax expense. It had no accrual for interest or penalties on its balance sheet at December 31, 2017 or 2016 and has not recognized interest and/or penalties in the statement of operations for years ended December 31, 2017, 2016 or 2015.

NET LOSS PER SHARE

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

NET LOSS PER SHARE

18. NET LOSS PER SHARE

Basic net loss per share is computed by dividing net loss by the weighted-average number of shares of common stock outstanding. Diluted net loss per share is computed by dividing net loss by the weighted-average number of shares of common stock and dilutive common stock equivalents outstanding. Given that the Company recorded a net loss for each of the periods presented, there is no difference between basic and diluted net loss per share since the effect of common stock equivalents would be anti-dilutive and are, therefore, excluded from the diluted net loss per share calculation.

	For the Year Ended December 31,
	2017			2016			2015
	(in thousands, except per share amounts)
Net loss	$	(50,688	)	$	(267,265	)	$	(220,030	)
Weighted-average number of shares of common stock and common stock equivalents outstanding:
Weighted-average number of shares of common stock outstanding for computing basic loss per share		58,818			48,697			42,290
Dilutive effect of outstanding stock awards and stock options after application of the treasury stock method*		—			—			—
Weighted-average number of shares of common stock and dilutive common stock equivalents outstanding for computing diluted loss per share		58,818			48,697			42,290
Net loss per share — basic and diluted	$	(0.86	)	$	(5.49	)	$	(5.20	)

For the year ended December 31, 2017, stock options, RSAs, RSUs and SARs to purchase approximately 9.4 million shares of common stock, respectively, were excluded from the net loss per share calculation as their effect would have been anti-dilutive. The Company accounts for the effect of the 2024 Notes on diluted net loss per share using the if-converted method as they may be settled in cash or shares at the Company’s option. The 2024 Notes have no effect on diluted net loss per share until the Company’s stock price exceeds the conversion price of $73.42 per share. In the period of conversion, the 2024 Notes will have no impact on diluted net loss if they are settled in cash and will have an impact on diluted loss per share if the Notes are settled in shares upon conversion.

For the years ended 2016 and 2015, stock options, RSAs and SARs to purchase approximately 5.7 million and 6.8 million shares of common stock, respectively, were excluded from the net loss per share calculation as their effect would have been anti-dilutive.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

19. COMMITMENTS AND CONTINGENCIES

Lease Obligations

In June 2013, the Company entered into a lease agreement (“Cambridge lease”) for its headquarters located in Cambridge, Massachusetts. As of December 31, 2017, the Company had entered into six amendments to the Cambridge lease, increasing its total rental space for its headquarters to 88,459 square feet. The Cambridge lease and its amendments will expire in January 2021. The agreement calls for a security deposit in the form of a letter of credit totaling $0.6 million. The Company purchased a certificate of deposit (“CD”) to meet the requirement and it is recorded as a long-term restricted investment in the consolidated balance sheets as of December 31, 2017 and 2016.

In June 2014, the Company entered into an agreement to sublease from an unrelated third party 10,939 square feet of office space. The sublease was terminated on December 31, 2016.

In January 2014, the Company entered into an agreement to sublease 15,077 square feet of office space to an unrelated third party. The sublease expired in July 2015. In August 2015, the Company entered into an agreement to sublease this space to another unrelated third party. The sublease expired in March 2017.

In February 2015, the Company entered into an agreement to sublease 7,461 square feet of office space to an unrelated third party. The sublease expired February 2016. In December 2016, the Company entered into an agreement to sublease this space to another unrelated third party. The sublease expired in December 2017.

The Company also leases laboratory and office space in Corvallis, Oregon which will expire in December 2020. The second floor and the first floor of the facility were vacated and closed and made available for sub-leasing in December 2016 and April 2017, respectively.

The following table summarizes the aggregate non-cancelable future minimum payments under the Company’s leases:

	As of December 31, 2017 (in thousands)
2018		4,946
2019		4,986
2020		5,089
2021		332
Total minimum lease payments	$	15,353

Royalty Obligations

The Company is obligated to pay royalties on net sales of certain of its products. The royalty rates are in the low-single-digit to high teens percentages for both inside and outside the U.S. Under the collaboration agreement with Summit signed in October 2016, the Company may be required to make tiered royalty payments ranging from a low to high teens percentage of net sales on a product-by-product basis in the Licensed Territory. Under the license agreement with BioMarin signed in July 2017, the Company is required to make a mid-single-digit percentage of royalty on the net product sales inside the U.S. and maybe required to make a high-single-digit percentage of royalty on net product sales outside the U.S. Under the Under the Amended and Restated License Agreement with UWA signed in April 2013, the Company may be obligated to pay a low-single-digit percentage of royalty on the net sales of products exceeding certain amounts, which includes EXONDYS 51. For the year ended December 31, 2017, the Company recorded $4.7 million royalty payments to BioMarin on its consolidated statements of operations and comprehensive loss.

Milestone Obligations

The Company has collaboration and license agreements for which it could be obligated to pay up-front, development, regulatory and commercial milestones as a product candidate proceeds from the submission of an investigational new drug application through approval for commercial sale and beyond. As of December 31, 2017, the Company may be obligated to make up to $808.5 million of future development, up-front royalty and sales milestone payments associated with its collaboration and license agreements. For the years ended December 31, 2017, 2016 and 2015, the Company recognized approximately $22.0 million, $47.6 million and $0.2 million relating to certain up-front and development milestone payments as research and development expense, respectively, under these agreements. As of December 31, 2017 and 2016, the Company also recorded $6.6 million and $1.0 million as an in-licensed rights, respectively, corresponding to the execution of the settlement and license agreements with BioMarin and the first sale of EXONDYS 51.

Other Funding Commitments

The Company has several on-going clinical trials in various clinical trial stages. Its most significant clinical trial expenditures are to contract research organizations (“CROs”). The CRO contracts are generally cancellable at the Company’s option. As of December 31, 2017, the Company has approximately $52.9 million in cancellable future commitments based on existing CRO contracts. For the years ended December 31, 2017, 2016 and 2015, the Company recognized approximately $13.9 million, $8.1 million and $8.5 million, respectively, for expenditures incurred by CROs.

Litigation

In the normal course of business, the Company may from time to time be named as a party to various legal claims, actions and complaints, including matters involving securities, employment, intellectual property, effects from the use of therapeutics utilizing its technology, or others. For example, purported class action complaints were filed against the Company and certain of its officers in the U.S. District Court for the District of Massachusetts on January 27, 2014 and January 29, 2014. The complaints were consolidated into a single action (Corban v. Sarepta, et. al., No. 14-cv-10201) by order of the court on June 23, 2014. Plaintiffs’ consolidated amended complaint, filed on July 21, 2014, asserted violations of Section 10(b) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) and Securities and Exchange Commission Rule 10b-5 against the Company, and Chris Garabedian, Sandy Mahatme, and Ed Kaye (“Individual Defendants,” and collectively with the Company, the “Corban Defendants”), and violations of Section 20(a) of the Exchange Act against the Individual Defendants. Plaintiffs alleged that the Corban Defendants made material misrepresentations or omissions during the putative class period of July 24, 2013 through November 12, 2013, regarding a data set for a Phase 2b study of eteplirsen and the likelihood of the FDA accepting the Company’s new drug application for eteplirsen for review based on that data set. Plaintiffs sought compensatory damages and fees. On August 18, 2014, the Corban Defendants filed a motion to dismiss, which the Court granted on March 31, 2015. Plaintiffs subsequently sought leave to file a second amended complaint, which the Corban Defendants opposed. On September 2, 2015, the Court denied Plaintiffs’ motion for leave to amend as futile. Plaintiffs filed a notice of appeal on September 29, 2015, seeking review of the Court’s March 31, 2015 order dismissing the case and the Court’s September 2, 2015 order denying leave to amend. On January 27, 2016, Plaintiffs filed in the district court a motion for relief from judgment pursuant to Federal Rule of Civil Procedure 60(b)(2), arguing that the FDA Briefing Document published on or about January 15, 2016, was material and would have changed the Court’s ruling. On February 26, 2016, the First Circuit stayed the appeal pending the district court’s ruling on the 60(b)(2) motion. Defendants opposed the 60(b)(2) motion, and on April 21, 2016, the Court denied Plaintiffs’ motion for relief from judgment. On May 19, 2016, Plaintiffs filed a motion to alter or amend the April 21, 2016 order pursuant to Federal Rule of Civil Procedure 59(e). On May 20, 2016, the Court denied Plaintiffs’ motion, and Plaintiffs filed a notice of appeal of the Court’s April 21, 2016 denial of their 60(b)(2) motion and May 20, 2016 denial of their 59(e) motion. On June 13, 2016, the First Circuit granted Plaintiffs’ motion to consolidate the two appeals. Oral argument took place on March 7, 2017 and the First Circuit affirmed the District Court’s dismissal of this case on August 22, 2017. Plaintiffs filed a Petition for Panel Rehearing and Rehearing En Banc, which the First Circuit denied on October 11, 2017. As such, the risk of loss is not deemed probable.

Another complaint was filed in the U.S. District Court for the District of Massachusetts on December 3, 2014 styled William Kader, Individually and on Behalf of All Others Similarly Situated v. Sarepta Therapeutics Inc., Christopher Garabedian, and Sandesh Mahatme (Kader v. Sarepta et.al 1:14-cv-14318). On March 20, 2015, Plaintiffs filed an amended complaint asserting violations of Section 10(b) of the Exchange Act and Securities and Exchange Commission Rule 10b-5 against the Company, and Chris Garabedian and Sandy Mahatme (“Individual Defendants,” and collectively with the Company, the “Kader Defendants”), and violations of Section 20(a) of the Exchange Act against the Individual Defendants. Plaintiffs alleged that the Kader Defendants made material misrepresentations or omissions during the putative class period of April 21, 2014 through October 27, 2014, regarding the sufficiency of the Company’s data for submission of an NDA for eteplirsen and the likelihood of the FDA accepting the NDA based on that data. Plaintiffs sought compensatory damages and fees. The Kader Defendants moved to dismiss the amended complaint on May 11, 2015. On April 5, 2016, following oral argument on March 29, 2016, the Court granted Defendants’ motion to dismiss. On April 8, 2016, Lead Plaintiffs filed a motion for leave to file an amended complaint, which Defendants opposed. On January 6, 2017, the Court denied Plaintiffs’ motion for leave to amend and dismissed the case. Plaintiffs filed a notice of appeal on February 3, 2017. Appellants’ brief was filed April 24, 2017 and Appellee’s brief was filed May 24, 2017. Oral argument took place on December 4, 2017. A decision has not yet been issued by the First Circuit. As such, the risk of loss is not deemed probable.

On February 5, 2015, a derivative suit was filed in the 215th Judicial District of Harris County, Texas against the Company’s Board of Directors (David Smith, derivatively on behalf of Sarepta Therapeutics, Inc., v. Christopher Garabedian et al., No. 2015-06645). The claims allege that Sarepta’s directors caused Sarepta to disseminate materially false and/or misleading statements in connection with disclosures concerning the Company’s submission of the NDA for eteplirsen. Plaintiff seeks unspecified compensatory damages, actions to reform and improve corporate governance and internal procedures, disgorgement of profits, benefits and other compensation obtained by the directors, and attorneys’ fees. The parties have agreed to stay the case pending resolution of the Corban and Kader cases. As such, the risk of loss is not deemed probable.

On March 16, 2016, a derivative suit was filed in the U.S. District Court for the District of Massachusetts against the Company’s Board of Directors (Dawn Cherry, on behalf of nominal defendant Sarepta Therapeutics, Inc., v. Behrens et al., No. 16-cv-10531). The claims allege that the defendants authorized the Company to make materially false and misleading statements about the Company’s business prospects in connection with its development of eteplirsen from July 10, 2013 through the date of the complaint. Plaintiffs seek unspecified damages, actions to reform and improve corporate governance and internal procedures, and attorneys’ fees. The parties have agreed to stay the case pending resolution of the Corban and Kader cases. As such, the risk of loss is not deemed probable.

Additionally, on September 23, 2014, a derivative suit was filed against the Company’s Board of Directors with the Court of Chancery of the State of Delaware (Terry McDonald, derivatively on behalf of Sarepta Therapeutics, Inc., et al. v. Goolsbee et al., No. 10157). The claims allege, among other things, that (i) the Company’s non-employee directors paid themselves excessive compensation fees for 2013, (ii) that the compensation for the Company’s former Chief Executive Officer, Christopher Garabedian, was also excessive and such fees were the basis for Mr. Garabedian’s not objecting to or stopping the excessive fees for the non-employee directors and (iii) that the disclosure in the 2013 proxy statement was deficient. The relief sought, among others, includes disgorgement and rescindment of allegedly excessive or unfair payments and equity grants to Mr. Garabedian and the directors, unspecified damages plus interest, a declaration that the Company’s Amended and Restated 2011 Equity Plan at the 2013 annual meeting was ineffective and a revote for approved amendments, correction of misleading disclosures and plaintiff’s attorney fees. The parties have agreed to a Memorandum of Understanding concerning the settlement terms and do not believe that disposition of the McDonald suit will have a material financial impact on the Company. The parties have now completed the confirmatory discovery process that will allow plaintiffs’ counsel to represent to the court that the terms of the settlement are fair.

Purchase Commitments

The Company has entered into long-term contractual arrangements from time to time for the provision of goods and services.

The following table presents non-cancelable contractual obligations arising from these arrangements:

	As of December 31, 2017 (in thousands)
2018		81,677
2019		13,939
Total purchase commitments	$	95,616

In connection with an amendment to a supply agreement, in September 2015, the Company issued an irrevocable standby letter of credit totaling $10.7 million to a contract manufacturing vendor. The obligation secured by the letter of credit will be fulfilled upon full payment of all deposits and purchase payments by February 2017. To meet the requirement of the letter of credit, the Company purchased $10.7 million in a certificate of deposit with a September 2017 maturity date. The Company recorded this $10.7 million as a restricted investment on the consolidated balance sheet as of December 31, 2016, which matured in September 2017.

FINANCIAL INFORMATION BY QUARTER (UNAUDITED)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

FINANCIAL INFORMATION BY QUARTER (UNAUDITED)

20. FINANCIAL INFORMATION BY QUARTER (UNAUDITED)

	2017 for Quarter Ended
	December 31			September 30			June 30			March 31
	(in thousands)
Revenues:
Product, net	$	57,277		$	45,954		$	35,011		$	16,342
Total revenues		57,277			45,954			35,011			16,342

Operating expenses:
Cost of sales (excluding amortization of in-licensed rights)		3,546			3,078			505			224
Research and development		44,441			34,239			58,908			29,119
Selling, general and administrative		32,221			28,176			36,069			26,216
Settlement and license charges		—			25,588			2,839			—
Amortization of in-licensed rights		216			780			29			28
Total cost and operating expenses		80,424			91,861			98,350			55,587
Operating loss		(23,147	)		(45,907	)		(63,339	)		(39,245	)
Other (loss) income:
Interest (expense) income and other, net		(2,693	)		184			184			335
Gain from sale of Priority Review Voucher		—			—			—			125,000
Total other (loss) income		(2,693	)		184			184			125,335
(Loss) income before income tax (benefit) expense		(25,840	)		(45,723	)		(63,155	)		86,090
Income tax (benefit) expense		(1,842	)		2,011			(109	)		2,000
Net (loss) income	$	(23,998	)	$	(47,734	)	$	(63,046	)	$	84,090

Net income (loss) per share:
Basic (loss) earnings per share	$	(0.37	)	$	(0.78	)	$	(1.15	)	$	1.53
Diluted (loss) earnings per share	$	(0.37	)	$	(0.78	)	$	(1.15	)	$	1.50
Weighted average number of shares of common stock used in calculating:
Basic (loss) earnings per share		64,277			61,528			54,976			54,850
Diluted (loss) earnings per share		64,277			61,258			54,976			56,012

	2016 for Quarter Ended
	December 31			September 30			June 30			March 31
	(in thousands)
Revenues:
Product, net	$	5,421		$	—		$	—		$	—
Total revenues		5,421			—			—			—

Cost and operating expenses:
Cost of sales (excluding amortization of in-licensed rights)		101			—			—			—
Amortization of in-licensed rights		29			—			—			—
Research and development		70,749			34,349			44,348			38,826
Selling, general and administrative		22,937			22,184			17,752			20,876
Total operating expenses		93,816			56,533			62,100			59,702
Operating loss		(88,395	)		(56,533	)		(62,100	)		(59,702	)
Other (loss) income:
Interest expense and other, net		(57	)		(209	)		(201	)		(68	)
Total other loss		(57	)		(209	)		(201	)		(68	)
Net loss	$	(88,452	)	$	(56,742	)	$	(62,301	)	$	(59,770	)

Net loss per share—basic and diluted	$	(1.62	)	$	(1.18	)	$	(1.35	)	$	(1.31	)
Shares used in per share calculations—basic and diluted		54,619			48,254			46,157			45,697

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS (Policies)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Basis of Presentation

Estimates and Uncertainties

Reclassification

Fair Value Measurements

•

Level 1—quoted prices for identical instruments in active markets;

•

Level 3—valuations derived from valuation techniques in which one or more significant value drivers are unobservable.

Cash and Cash Equivalents

Only investments that are highly liquid and readily convertible to cash and have original maturities of three months or less are considered cash equivalents.

Available-For-Sale Debt Securities

Accounts Receivable

The Company’s accounts receivable arise from product sales, government research contracts and other grants. They are generally stated at the invoiced amount and do not bear interest.

Concentration of Credit Risk

Inventory

The following table summarizes the components of the Company’s inventory for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Raw materials	$	53,875	$	9,531
Work in progress		27,442		3,175
Finished goods		2,288		107
Total inventory	$	83,605	$	12,813

Property and Equipment

The Company generally depreciates the cost of its property and equipment using the straight-line method over the estimated useful lives of the respective assets, which are summarized as follows:

Asset Category		Useful lives
Lab equipment		5 years
Office equipment		5 years
Software and computer equipment		3 - 5 years
Leasehold improvements		Lesser of the useful life or the term of the respective lease
Land		Not depreciated
Building		30 years
Construction in Progress		Not depreciated until put into service

Intangible assets

The following table summarizes the estimated future amortization for intangible assets for the next five years:

	As of December 31, 2017 (in thousands)
2018		1,681
2019		1,507
2020		1,465
2021		1,453
2022		1,446
Total	$	7,552

Impairment of Long-Lived Assets

Convertible Debt Transactions

Revenue Recognition

The Company recognizes revenue when all of the following criteria are met:

persuasive evidence of an arrangement exists;

delivery has occurred or services have been rendered;

price to the customer is fixed or determinable; and

collectability is reasonably assured.

Product revenues

Revenue from product sales is recognized when title and risk of loss have passed to the customer and is recorded net of applicable reserves for rebates, discounts and allowances.

Reserves for rebates, discounts and allowances

•

Revenue from research contracts and other grants

Deferred revenue

Research and Development

Stock-Based Compensation

Income Taxes

Rent Expense

For the years ended December 31, 2017, 2016 and 2015, the Company recognized rent expense and occupancy costs of $4.4 million, $5.6 million and $5.2 million, respectively.

Commitments and Contingencies

Subsequent Events

Recent Accounting Pronouncements

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Components of Inventory

The following table summarizes the components of the Company’s inventory for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Raw materials	$	53,875	$	9,531
Work in progress		27,442		3,175
Finished goods		2,288		107
Total inventory	$	83,605	$	12,813

Summary of Estimated Useful Lives of Property and Equipment

The Company generally depreciates the cost of its property and equipment using the straight-line method over the estimated useful lives of the respective assets, which are summarized as follows:

Asset Category		Useful lives
Lab equipment		5 years
Office equipment		5 years
Software and computer equipment		3 - 5 years
Leasehold improvements		Lesser of the useful life or the term of the respective lease
Land		Not depreciated
Building		30 years
Construction in Progress		Not depreciated until put into service

Summary of Estimated Future Amortization for Intangible Assets

The following table summarizes the estimated future amortization for intangible assets for the next five years:

	As of December 31, 2017 (in thousands)
2018		1,681
2019		1,507
2020		1,465
2021		1,453
2022		1,446
Total	$	7,552

FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Assets Measured and Carried at Fair Value

	Fair Value Measurement as of December 31, 2017
	Total		Level 1		Level 2		Level 3
	(in thousands)
Assets
Cash equivalents	$	352,370	$	352,370	$	—	$	—
Commercial paper		133,368		—		133,368		—
Government and government agency bonds		294,717		284,745		9,972		—
Corporate bonds		127,956		127,956		—		—
Certificates of deposit		648		648		—		—
Total assets	$	909,059	$	765,719	$	143,340	$	—


	Fair Value Measurement as of December 31, 2016
	Total		Level 1		Level 2		Level 3
	(in thousands)
Cash equivalents	$	1,147	$	1,147	$	—	$	—
Commercial paper		69,304		—		69,304		—
Government and government agency bonds		105,287		—		105,287		—
Corporate bonds		20,834		—		20,834		—
Certificates of deposit		11,343		11,343		—		—
Total assets	$	207,915	$	12,490	$	195,425	$	—

CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES (Tables)

12 Months Ended

Dec. 31, 2017

Cash And Cash Equivalents [Abstract]

Summary of Company Financial Assets with Maturities of Less Than 90 Days Included in Cash Equivalents

	As of December 31
	2017		2016
	(in thousands)
Money market funds		352,370		1,147
Corporate bonds		16,720		—
Government and government agency bonds		49,972		—
Total		419,062		1,147

Summary of Company Cash, Cash Equivalents and Investments

. The following tables summarize the Company’s cash, cash equivalents and investments for each of the periods indicated:

	As of December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Market Value
	(in thousands)
Cash and money market funds	$	532,999	$	—	$	—		$	532,999
Commercial paper - current		133,368		—		—			133,368
Government and government agency bonds - current		294,915		2		(200	)		294,717
Corporate bonds
Current		118,121		—		(145	)		117,976
Non-current		10,016		—		(36	)		9,980
Total assets	$	1,089,419	$	2	$	(381	)	$	1,089,040
As reported:
Cash and cash equivalents	$	599,698	$	2	$	(9	)	$	599,691
Short-term investments		479,705		—		(336	)		479,369
Long-term investments		10,016		—		(36	)		9,980
Total assets	$	1,089,419	$	2	$	(381	)	$	1,089,040
	As of December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Market Value
	(in thousands)
Cash and money market funds	$	122,420	$	—	$	—		$	122,420
Commercial paper - current		69,355		—		(51	)		69,304
Government and government agency bonds - current		105,340		—		(53	)		105,287
Corporate bonds - current		20,850		—		(16	)		20,834
Total assets	$	317,965	$	—	$	(120	)	$	317,845
As reported:
Cash and cash equivalents	$	122,420	$	—	$	—		$	122,420
Short-term investments		195,545		—		(120	)		195,425
Total assets	$	317,965	$	—	$	(120	)	$	317,845

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES (Tables)

12 Months Ended

Dec. 31, 2017

Receivables Net Current [Abstract]

Summary of Components of Accounts Receivable

The following table summarizes the components of the Company’s accounts receivable for the periods indicated:

	As of December 31,
	2017		2016
	(in thousands)
Product sales, net of discounts and allowances	$	28,539	$	4,002
Government contract receivables		929		1,226
Total accounts receivable	$	29,468	$	5,228

Summary of Change in Reserves for Discounts and Allowances

The following table summarizes an analysis of the change in reserves for discounts and allowances for the periods indicated:

	Chargebacks			Rebates			Prompt Pay			Other Accruals			Total
	(in thousands)
Balance, as of December 31, 2016	$	1		$	238		$	—		$	67		$	306
Provision		6,455			7,659			469			1,293			15,876
Payments/credits		(5,461	)		(938	)		(300	)		(896	)		(7,595	)
Balance, as of December 31, 2017	$	995		$	6,959		$	169		$	464		$	8,587

Summary of Total Reserves Included in Consolidated Balance Sheets

The following table summarizes the total reserves above included in the Company’s consolidated balance sheets for the periods indicated:

	As of December 31,
	2017		2016
	(in thousands)
Reduction to accounts receivable	$	1,285	$	1
Component of accrued expenses		7,302		305
Total reserves	$	8,587	$	306

OTHER ASSETS (Tables)

12 Months Ended

Dec. 31, 2017

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Summary of Other Current Assets

The following table summarizes the Company’s other current assets for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Manufacturing-related deposits and prepaids	$	18,650	$	23,604
Prepaid clinical and preclinical expenses		5,175		1,225
Prepaid research expenses		2,896		—
Prepaid maintenance services		1,711		109
Prepaid commercial expenses		1,589		54
Asset held for sale		1,501		—
Other prepaids		2,726		989
Other		2,263		914
Total other current assets	$	36,511	$	26,895

Summary of Investments and Other Assets

The following table summarizes the Company’s investments and other assets for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Long-term available-for-sale securities	$	9,980	$	—
Prepaid clinical expenses		7,488		3,725
Alternative minimum tax credit		3,315		—
Restricted cash and investments		784		784
Other		242		242
Total investments and other assets	$	21,809	$	4,751

PROPERTY AND EQUIPMENT (Tables)

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Summarizes Components of Property and Equipment, Net

Property and equipment are recorded at historical cost, net of accumulated depreciation. The following table summarizes components of property and equipment, net for each of the periods indicated:

	As of December 31,
	2017			2016
	(in thousands)
Land	$	4,158		$	4,158
Building and improvements		14,543			12,729
Software and computer equipment		8,965			6,775
Lab equipment		9,860			14,149
Office equipment		2,211			3,028
Leasehold improvements		13,633			22,872
Construction in progress		7,808			4,436
Property and equipment, gross		61,178			68,147
Less: accumulated depreciation		(18,022	)		(30,346	)
Property and equipment, net	$	43,156		$	37,801

ACCRUED EXPENSES (Tables)

12 Months Ended

Dec. 31, 2017

Payables And Accruals [Abstract]

Summary of Accrued Expenses

The following table summarizes the Company’s accrued expenses for each of the periods indicated:

	As of December 31, 2017		As of December 31, 2016
	(in thousands)
Accrued clinical and preclinical costs	$	15,975	$	10,033
Accrued employee compensation costs		14,402		8,748
Accrued contract manufacturing costs		14,019		4,673
Product revenue related reserves		7,302		305
Accrued professional fees		6,794		2,799
Accrued BioMarin royalties		2,846		—
Accrued property and equipment		2,525		1,186
Accrued interest expenses		1,291		100
Accrued income taxes		943		—
Accrued research costs		401		1,186
Other		2,484		1,986
Total accrued expenses	$	68,982	$	31,016

RESTRUCTURING (Tables)

12 Months Ended

Dec. 31, 2017

Restructuring And Related Activities [Abstract]

Summary of Restructuring Costs by Function

The following table summarizes the restructuring costs by function for the period indicated:

	For the Year Ended December 31
	2017						2016
	(in thousands)
	Cash		Non-cash		Total		Cash		Non-cash		Total
Research and development	$	188	$	—	$	188	$	1,631	$	382	$	2,013
Selling, general and administrative		2,832		—		2,832		2,020		529		2,549
Total restructuring expenses	$	3,020	$	—	$	3,020	$	3,651	$	911	$	4,562

Summary of Restructuring Reserve

The following table summarizes the restructuring reserve for each of the periods indicated:

	For the Year Ended December 31
	2017			2016
	(in thousands)
Restructuring reserve beginning balance	$	1,588		$	—
Restructuring expenses incurred during the period		3,020			3,651
Amounts paid during the period		(1,675	)		(2,063	)
Restructuring reserve ending balance	$	2,933		$	1,588

INDEBTEDNESS (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Summary of 2024 Notes

The following table summarizes the 2024 Notes for the period indicated:

	As of December 31, 2017
	(in thousand)
Par value of the 2024 Notes	$	570,000
Unamortized discounts	$	(158,890	)
Debt issuance expenses		(10,449	)
Net carrying value of convertible debt	$	400,661

Fair value of convertible debt	$	619,641

Summary of Term Loans

The following table summarizes term loans for each of the periods indicated:

	As of December 31
	2017			2016
	(in thousands)
Principal amount of the 2017 Term Loan	$	30,000		$	—
Principal amount of the 2015 Term Loan		—			15,000
Unamortized debt issuance expense		(918	)		(223	)
Net carrying value of term loan	$	29,082		$	14,777

Summarizes Total Gross Payments Due under Company's Debt Arrangements

The following table summarizes the total gross payments due under the Company’s debt arrangements:

	As of December 31, 2017
	(in thousands)
2018	$	6,328
2019		10,000
2020		10,000
2021		5,000
2022		—
Thereafter		570,000
Total Payments	$	601,328

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

+ Details

STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2017

Summary of Stock Option Activity

The following tables summarize the Company’s stock option activity for each of the periods indicated:

	For the Year Ended December 31,
	2017							2016					2015
						Weighted					Weighted					Weighted
						Average					Average					Average
						Exercise					Exercise					Exercise
	Shares					Price		Shares			Price		Shares			Price
Grants outstanding at beginning of the period		5,436,951				$	22.70		6,515,976		$	23.91		5,216,203		$	24.45
Granted		4,805,722		(1	)		35.09		1,285,051			14.89		2,830,078			20.28
Exercised		(792,845	)				17.40		(1,056,821	)		18.31		(816,696	)		12.26
Cancelled		(643,624	)				25.44		(1,307,255	)		24.61		(713,609	)		26.78
Grants outstanding at end of the period		8,806,204				$	29.74		5,436,951		$	22.70		6,515,976		$	23.91

Grants exercisable at end of the period		3,288,712				$	24.76		2,942,624		$	24.42		2,617,167		$	23.85
Grants vested and expected to vest at end of the period		6,910,022				$	28.49		5,116,409		$	22.83		5,908,213		$	23.94

(1)

The weighted-average grant date fair value per share of stock options granted during the years ended December 31, 2017, 2016 and 2015 was $14.78, $12.46 and $14.98, respectively.

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
Options outstanding at December 31, 2017	$	228,081	7.6
Options exercisable at December 31, 2017	$	101,559		5.1
Options vested and expected to vest at December 31, 2017	$	187,582	7.1

Summary of Company's Stock Options Vested and Exercised

The following table summarizes the Company’s stock options vested and exercised for each of the periods indicated:

	For the Year Ended December 31,
	2017		2016		2015
	(in thousands)
Aggregate grant date fair value of stock options vested	$	18,225	$	30,651	$	27,858
Aggregate intrinsic value of stock options exercised	$	20,922	$	30,610	$	18,138

Summary of Stock Appreciation Rights Activity

The following table summarizes the Company’s SAR activity for each of the periods indicated:

	For the Year Ended December 31,
	2017				2016					2015
			Weighted					Weighted				Weighted
			Average					Average				Average
			Exercise					Exercise				Exercise
	Shares		Price		Shares			Price		Shares		Price
Grants outstanding at beginning of the period		100,000	$	23.85		170,000		$	18.18		170,000	$	18.18
Exercised		—		—		(67,812	)		10.08		—		—
Forfeited		—		—		(2,188	)		10.08		—		—
Grants outstanding at end of the period		100,000	$	23.85		100,000		$	23.85		170,000	$	18.18
Grants exercisable at end of the period		100,000	$	23.85		100,000		$	23.85		141,249	$	17.59
Grants vested and expected to vest at end of the period		100,000	$	23.85		100,000		$	23.85		167,813	$	18.29

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
SARs outstanding, exercisable and vested at December 31, 2017	$	3,179	4.9

Summary of Employee Stock Purchase Plan Activity and Expense

The following table summarizes the Company’s ESPP activity for each of the periods indicated:

	For the Year Ended December 31,
	2017		2016		2015
Number of shares purchased		102,698		137,113		75,539
Proceeds received (in millions)	$	1.4	$	1.6	$	0.9

Summary of Stock-Based Compensation Expense by Function Included within Consolidated Statement of Operations and Comprehensive Loss

The following table summarizes stock-based compensation expense by function included within the consolidated statements of operations and comprehensive loss:

	For the Year Ended December 31,
	2017		2016		2015
	(in thousands)
Research and development	$	8,542	$	9,499	$	10,403
Selling, general and administrative		21,923		20,463		21,714
Total stock-based compensation	$	30,465	$	29,962	$	32,117

Summary of Stock-Based Compensation Expense by Grant Type Included within Consolidated Statement of Operations and Comprehensive Loss

The following table summarizes stock-based compensation expense by grant type included within the consolidated statements of operations and comprehensive loss:

	For the Year Ended December 31,
	2017		2016		2015
	(in thousands)
Stock options	$	23,416	$	26,320	$	29,014
Employee stock purchase plan		1,754		2,380		2,165
Restricted stock awards/units		5,295		872		446
Other		—		390		492
Total stock-based compensation	$	30,465	$	29,962	$	32,117

Restricted Stock Awards (RSAs) [Member]

Summary of Restricted Stock Award and Restricted Stock Units Activity

The following table summarizes the Company’s RSA activity for each of the periods indicated:

	For the Year Ended December 31,
	2017					2016					2015
				Weighted					Weighted					Weighted
				Average					Average					Average
				Exercise					Exercise					Exercise
	Shares			Price		Shares			Price		Shares			Price
Grants outstanding at beginning of the period		153,170		$	34.53		161,320		$	21.50		6,000		$	34.92
Granted		341,500			34.58		117,553			41.22		181,783			20.80
Vested		(63,264	)		14.60		(59,703	)		13.62		(24,463	)		20.21
Forfeited		(19,625	)		43.02		(66,000	)		33.51		(2,000	)		13.90
Grants outstanding at end of the period		411,781		$	37.23		153,170		$	34.53		161,320		$	21.50

Restricted Stock Units (RSUs) [Member]

Summary of Restricted Stock Award and Restricted Stock Units Activity

The following table summarizes the Company’s RSU activity for the period indicated:

	For the Year Ended December 31, 2017
				Weighted
				Average
				Exercise
	Shares			Price
Granted		181,029		$	33.03
Vested		(78,017	)		32.63
Forfeited		(36,460	)		32.63
Grants outstanding at end of the period		66,552		$	33.72

Stock Options [Member]

Assumptions for Measuring Fair Values of Stocks

The fair values of stock options granted during the periods presented were measured on the date of grant using the Black-Scholes-Merton option-pricing model, with the following assumptions:

	For the Year Ended December 31,
	2017	2016	2015
Risk-free interest rate (1)	1.6 - 2.1%	1.1 – 1.8%	1.1 – 1.7%
Expected dividend yield (2)	—	—	—
Expected terms (3)	4.2 - 4.8 years	4.2 – 4.8 years	4.7 – 5.0 years
Expected volatility (4)	54.0 - 63.0%	78.2 – 137.1 %	94.3 – 111.1 %

(1)	The risk-free interest rate is estimated using an average of Treasury bill interest rates over a historical period commensurate with the expected term of the option that correlates to the prevailing interest rates at the time of grant.

(2)	The expected dividend yield is zero as the Company has not paid any dividends to date and does not expect to pay dividends in the future.

(3)	The expected terms are estimated using historical exercise behavior.

(4)

OTHER INCOME AND LOSS (Tables)

12 Months Ended

Dec. 31, 2017

Other Income And Expenses [Abstract]

Summary of Other Income and Loss

The following table summarizes other income and loss for the periods indicated:

	For the year ended December 31,
	2017			2016			2015
	(in thousand)
Interest expenses	$	(5,781	)	$	(1,892	)	$	(1,365	)
Interest income		1,809			507			890
Other income		1,982			850			629
Gain from sale of Priority Review Voucher		125,000			—			—
Total other income (loss)	$	123,010		$	(535	)	$	154

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Reconciliation Between Effective Tax Rate and Income Tax Rate

The following table summarizes the reconciliation between the Company’s effective tax rate and the income tax rate for each of the periods indicated:

	For the Year Ended December 31,
	2017				2016				2015
Federal income tax rate		34.0		%		34.0		%		34.0		%
State taxes		(27.7	)			—				—
Research and development and other tax credits		8.5				1.5				0.3
Valuation allowance		(93.2	)			(17.3	)			(19.1	)
Permanent differences		6.4				(3.4	)			(1.7	)
Sarepta International C.V. return to provision		62.1				—				—
Impact of tax reform, net of valuation allowance		5.9				—				—
Foreign rate differential		(0.6	)			(14.8	)			(13.5	)
Other		0.4				—				—
Effective tax rate		(4.2	)	%		—		%		—		%

Analysis of Deferred Tax Assets and Liabilities

The following table summarizes the analysis of the deferred tax assets and liabilities for each of the periods indicated:

	As of December 31,
	2017			2016
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	144,897		$	174,569
Difference in depreciation and amortization		1,210			2,589
Research and development and other tax credits		44,306			37,812
Stock-based compensation		14,392			16,166
Deferred rent		1,210			2,306
Deferred revenue		904			1,315
Capitalized inventory		22,127			36,282
Other		4,071			3,047
Total deferred tax assets		233,117			274,086
Deferred tax liabilities:
Debt discount		(28,548	)		—
Total deferred tax liabilities		(28,548	)		—
Valuation allowance		(204,569	)		(274,086	)
Net deferred tax assets	$	—		$	—

Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits

The follow table summarizes the reconciliation of the beginning and ending amount of total unrecognized tax benefits for each of the periods indicated:

	For the Year Ended December 31,
	2017			2016		2015
	(in thousands)
Balance at beginning of the period	$	4,644		$	3,706	$	—
Increase related to current year tax positions		735			801		613
Increase related to prior year tax positions		—			137		3,093
Decrease related to prior year tax position		(245	)		—		—
Balance at end of the period	$	5,134		$	4,644	$	3,706

NET LOSS PER SHARE (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Basic and Diluted Net Loss Per Share

Given that the Company recorded a net loss for each of the periods presented, there is no difference between basic and diluted net loss per share since the effect of common stock equivalents would be anti-dilutive and are, therefore, excluded from the diluted net loss per share calculation.

	For the Year Ended December 31,
	2017			2016			2015
	(in thousands, except per share amounts)
Net loss	$	(50,688	)	$	(267,265	)	$	(220,030	)
Weighted-average number of shares of common stock and common stock equivalents outstanding:
Weighted-average number of shares of common stock outstanding for computing basic loss per share		58,818			48,697			42,290
Dilutive effect of outstanding stock awards and stock options after application of the treasury stock method*		—			—			—
Weighted-average number of shares of common stock and dilutive common stock equivalents outstanding for computing diluted loss per share		58,818			48,697			42,290
Net loss per share — basic and diluted	$	(0.86	)	$	(5.49	)	$	(5.20	)

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Summary of Aggregate Non-Cancelable Future Minimum Payments under Company's Leases

The following table summarizes the aggregate non-cancelable future minimum payments under the Company’s leases:

	As of December 31, 2017 (in thousands)
2018		4,946
2019		4,986
2020		5,089
2021		332
Total minimum lease payments	$	15,353

Non-Cancelable Contractual Obligations

The following table presents non-cancelable contractual obligations arising from these arrangements:

	As of December 31, 2017 (in thousands)
2018		81,677
2019		13,939
Total purchase commitments	$	95,616

FINANCIAL INFORMATION BY QUARTER (UNAUDITED) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Financial Information by Quarter

	2017 for Quarter Ended
	December 31			September 30			June 30			March 31
	(in thousands)
Revenues:
Product, net	$	57,277		$	45,954		$	35,011		$	16,342
Total revenues		57,277			45,954			35,011			16,342

Operating expenses:
Cost of sales (excluding amortization of in-licensed rights)		3,546			3,078			505			224
Research and development		44,441			34,239			58,908			29,119
Selling, general and administrative		32,221			28,176			36,069			26,216
Settlement and license charges		—			25,588			2,839			—
Amortization of in-licensed rights		216			780			29			28
Total cost and operating expenses		80,424			91,861			98,350			55,587
Operating loss		(23,147	)		(45,907	)		(63,339	)		(39,245	)
Other (loss) income:
Interest (expense) income and other, net		(2,693	)		184			184			335
Gain from sale of Priority Review Voucher		—			—			—			125,000
Total other (loss) income		(2,693	)		184			184			125,335
(Loss) income before income tax (benefit) expense		(25,840	)		(45,723	)		(63,155	)		86,090
Income tax (benefit) expense		(1,842	)		2,011			(109	)		2,000
Net (loss) income	$	(23,998	)	$	(47,734	)	$	(63,046	)	$	84,090

Net income (loss) per share:
Basic (loss) earnings per share	$	(0.37	)	$	(0.78	)	$	(1.15	)	$	1.53
Diluted (loss) earnings per share	$	(0.37	)	$	(0.78	)	$	(1.15	)	$	1.50
Weighted average number of shares of common stock used in calculating:
Basic (loss) earnings per share		64,277			61,528			54,976			54,850
Diluted (loss) earnings per share		64,277			61,258			54,976			56,012

	2016 for Quarter Ended
	December 31			September 30			June 30			March 31
	(in thousands)
Revenues:
Product, net	$	5,421		$	—		$	—		$	—
Total revenues		5,421			—			—			—

Cost and operating expenses:
Cost of sales (excluding amortization of in-licensed rights)		101			—			—			—
Amortization of in-licensed rights		29			—			—			—
Research and development		70,749			34,349			44,348			38,826
Selling, general and administrative		22,937			22,184			17,752			20,876
Total operating expenses		93,816			56,533			62,100			59,702
Operating loss		(88,395	)		(56,533	)		(62,100	)		(59,702	)
Other (loss) income:
Interest expense and other, net		(57	)		(209	)		(201	)		(68	)
Total other loss		(57	)		(209	)		(201	)		(68	)
Net loss	$	(88,452	)	$	(56,742	)	$	(62,301	)	$	(59,770	)

Net loss per share—basic and diluted	$	(1.62	)	$	(1.18	)	$	(1.35	)	$	(1.31	)
Shares used in per share calculations—basic and diluted		54,619			48,254			46,157			45,697

Organization and Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Cash, cash equivalents and investments	$ 1,089,900
Cash and cash equivalents	599,691	$ 122,420
Short-term investments	479,369	195,425
Long-term investments	10,000
Long-term restricted cash and investments	$ 784	$ 784

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail)	3 Months Ended					12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment							1
Restricted cash and investments	$ 784,000				$ 784,000		$ 784,000	$ 784,000
Amortization of in-licensed rights	216,000	$ 780,000	$ 29,000	$ 28,000	29,000		1,053,000	29,000
Debt issuance costs							15,154,000		$ 399,000
Prepaid maintenance services	1,711,000				109,000		1,711,000	109,000
Prepaid clinical and preclinical expenses	5,175,000				1,225,000		5,175,000	1,225,000
Prepaid commercial expenses	1,589,000				54,000		1,589,000	54,000
Accrued interest expenses	1,291,000				100,000		1,291,000	100,000
Product revenue related reserves	7,302,000				305,000		7,302,000	305,000
Accrued property and equipment	2,525,000				1,186,000		2,525,000	1,186,000
Intangible asset	14,355,000				8,076,000		14,355,000	$ 8,076,000
Deferred revenue	3,300,000						$ 3,300,000
U.S. federal corporate tax rate							34.00%	34.00%	34.00%
Transition tax							$ 0
Rent expense and occupancy costs							4,400,000	$ 5,600,000	$ 5,200,000
ASU No. 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash included in cash and cash equivalents	100,000						$ 100,000
Employee Stock Purchase Plan [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of closing price of common stock							85.00%
Percentage of discount recognized on graded-vesting based on stock-based compensation expense over the purchase period							15.00%
Patents [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization of in-licensed rights							$ 600,000	600,000	500,000
Initial term of patents							20 years
Patents [Member] \| Research and Development Expenses [Member]
Summary Of Significant Accounting Policies [Line Items]
Book value of patent abandoned							$ 600,000	300,000	200,000
EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization of in-licensed rights							1,100,000
Sales milestone recorded as an in-license right	6,600,000				1,000,000		6,600,000	1,000,000
EXONDYS 51 [Member] \| University of Western Australia [Member]
Summary Of Significant Accounting Policies [Line Items]
Sales milestone recorded as an in-license right	1,000,000						$ 1,000,000
Product Revenues [Member] \| Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							47.00%
Product Revenues [Member] \| Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							34.00%
Product Revenues [Member] \| Customer Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							19.00%
Accounts Receivable [Member] \| Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							56.00%
Accounts Receivable [Member] \| Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							27.00%
Accounts Receivable [Member] \| Customer Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage							16.00%
U.S. [Member] \| EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangible asset	$ 6,600,000						$ 6,600,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term							60 days
U.S. federal corporate tax rate							35.00%
Maximum [Member] \| EXONDYS 51 [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization of in-licensed rights								100,000
Maximum [Member] \| Outside of U.S. [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term							120 days
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term							30 days
Minimum [Member] \| Outside of U.S. [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term							60 days
Scenario, Previously Reported [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash and investments					800,000			800,000
Restatement Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Debt issuance costs									$ 400,000
Prepaid maintenance services					100,000			100,000
Prepaid clinical and preclinical expenses					1,200,000			1,200,000
Accrued interest expenses					100,000			100,000
Product revenue related reserves					300,000			300,000
Accrued property and equipment					1,200,000			1,200,000
Restatement Adjustment [Member] \| Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization of in-licensed rights								100,000
Prepaid commercial expenses					$ 100,000			$ 100,000
Scenario, Forecast [Member]
Summary Of Significant Accounting Policies [Line Items]
U.S. federal corporate tax rate						21.00%

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Summary of Components of Inventory (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Raw materials	$ 53,875	$ 9,531
Work in progress	27,442	3,175
Finished goods	2,288	107
Total inventory	$ 83,605	$ 12,813

Summary of Estimated Useful Lives of Plant and Equipment (Detail)	12 Months Ended
	Dec. 31, 2017
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	5 years
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	5 years
Software and Computer Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	3 years
Software and Computer Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	Lesser of the useful life or the term of the respective lease
Land [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	Not depreciated
Building [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	30 years
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	Not depreciated until put into service

Summary of Estimated Future Amortization for Intangible Assets (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Goodwill And Intangible Assets Disclosure [Abstract]
2018	$ 1,681
2019	1,507
2020	1,465
2021	1,453
2022	1,446
Total	$ 7,552

License and Collaboration Agreements - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended					12 Months Ended
	Jul. 31, 2017	Oct. 31, 2016	Jun. 30, 2016	Apr. 30, 2013	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements	$ 35,000,000
Intangible asset, net					$ 14,355,000				$ 8,076,000	$ 14,355,000	$ 8,076,000
Settlement and license charges						$ 25,588,000	$ 2,839,000			28,427,000
Intangible asset amortization expense					216,000	$ 780,000	$ 29,000	$ 28,000	29,000	1,053,000	29,000
Intangible asset, accumulated amortization					4,145,000				3,134,000	4,145,000	3,134,000
EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Intangible asset amortization expense										1,100,000
Sales milestone payment recorded as an in-license right					6,600,000				$ 1,000,000	6,600,000	1,000,000
U.S. [Member] \| EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Intangible asset, net					6,600,000					6,600,000
Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of budgeted research and development cost allocation		45.00%
Maximum percentage of budgeted research and development cost		110.00%
Development milestone and up-front payments										22,000,000	47,600,000	$ 200,000
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Regulatory and sales milestone payments	$ 65,000,000
Percentage of royalty payments	8.00%
Maximum [Member] \| EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Intangible asset amortization expense											100,000
Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of royalty payments	4.00%
BioMarin Pharmaceutical, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements	$ 35,000,000
Intangible asset, net					6,600,000					6,600,000
Intangible asset amortization expense										1,000,000
Royalty payments										4,700,000
BioMarin Pharmaceutical, Inc. [Member] \| U.S. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment expiry period	2023-12
BioMarin Pharmaceutical, Inc. [Member] \| EU [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment expiry period	2024-09
BioMarin Pharmaceutical, Inc. [Member] \| Settlement Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements	$ 20,000,000
BioMarin and Academisch Ziekenhuis Leiden [Member] \| License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements	$ 15,000,000
Summit [Member] \| Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements		$ 40,000,000
One-time option fee		$ 10,000,000
Percentage of budgeted research and development cost allocation		55.00%
Development milestone and up-front payments										22,000,000	40,000,000
Summit [Member] \| Maximum [Member] \| Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional payments upon successful achievement		$ 192,000,000
Development and regulatory milestone payments		290,000,000
Sales milestone payments		330,000,000
Regulatory milestone payments upon exercise of option		7,000,000
Sales milestone payments upon exercise of option		$ 82,500,000
University of Western Australia [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payments										0
Development milestone and up-front payments										0	$ 7,600,000	$ 200,000
Up-front payment to purchase future royalties				$ 30,000,000
Up-front payment upon execution of amendment			$ 7,000,000
Purchase option upfront payment for future royalties			23,000,000
University of Western Australia [Member] \| EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sales milestone payment recorded as an in-license right					1,000,000					1,000,000
Intangible asset, accumulated amortization					$ 100,000					$ 100,000
University of Western Australia [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional payments upon successful achievement			$ 20,000,000	20,000,000
University of Western Australia [Member] \| Maximum [Member] \| EXONDYS 51 and Up to Five Additional Products [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments to UWA on successful achievement of certain development and regulatory milestones				$ 6,000,000

Gain From Sale of Priority Review Voucher - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017
Gain From Sale Of Intangible Asset [Line Items]
Gain from sale of PRV asset	$ 125,000	$ 125,000
Gilead [Member]
Gain From Sale Of Intangible Asset [Line Items]
Gain from sale of PRV asset	$ 125,000

Assets Measured and Carried at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 909,059	$ 207,915
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	352,370	1,147
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	133,368	69,304
Government and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	294,717	105,287
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	127,956	20,834
Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	648	11,343
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	765,719	12,490
Level 1 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	352,370	1,147
Level 1 [Member] \| Government and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	284,745
Level 1 [Member] \| Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	127,956
Level 1 [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	648	11,343
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	143,340	195,425
Level 2 [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	133,368	69,304
Level 2 [Member] \| Government and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 9,972	105,287
Level 2 [Member] \| Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets		$ 20,834

Summary of Company Financial Assets with Maturities of Less Than 90 Days Included in Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 419,062	$ 1,147
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	352,370	$ 1,147
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	16,720
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 49,972

Cash, Cash Equivalents and Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Abstract]
Weighted average maturity period of available-for-sale securities	7 months	4 months

Summary of Company Cash, Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	$ 1,089,419	$ 317,965
Cash, cash equivalents and investments, Gross unrealized gains	2
Cash, cash equivalents and investments, Gross unrealized losses	(381)	(120)
Cash, cash equivalents and investments, Fair market value	1,089,040	317,845
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	532,999	122,420
Cash, cash equivalents and investments, Fair market value	532,999	122,420
Commercial Paper - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	133,368	69,355
Cash, cash equivalents and investments, Gross unrealized losses		(51)
Cash, cash equivalents and investments, Fair market value	133,368	69,304
Government and Government Agency Bonds - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	294,915	105,340
Cash, cash equivalents and investments, Gross unrealized gains	2
Cash, cash equivalents and investments, Gross unrealized losses	(200)	(53)
Cash, cash equivalents and investments, Fair market value	294,717	105,287
Corporate Bonds - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	118,121	20,850
Cash, cash equivalents and investments, Gross unrealized losses	(145)	(16)
Cash, cash equivalents and investments, Fair market value	117,976	20,834
Corporate Bonds - Non-current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	10,016
Cash, cash equivalents and investments, Gross unrealized losses	(36)
Cash, cash equivalents and investments, Fair market value	9,980
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	599,698	122,420
Cash, cash equivalents and investments, Gross unrealized gains	2
Cash, cash equivalents and investments, Gross unrealized losses	(9)
Cash, cash equivalents and investments, Fair market value	599,691	122,420
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	479,705	195,545
Cash, cash equivalents and investments, Gross unrealized losses	(336)	(120)
Cash, cash equivalents and investments, Fair market value	479,369	$ 195,425
Long-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and investments, Amortized cost	10,016
Cash, cash equivalents and investments, Gross unrealized losses	(36)
Cash, cash equivalents and investments, Fair market value	$ 9,980

Accounts Receivable and Reserves for Product Sales - Summary of Components of Accounts Receivable (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Receivables Net Current [Abstract]
Product sales, net of discounts and allowances	$ 28,539	$ 4,002
Government contract receivables	929	1,226
Total accounts receivable	$ 29,468	$ 5,228

Accounts Receivable and Reserves for Product Sales - Summary of Change in Reserves for Discounts and Allowances (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2016	$ 306
Provision	15,876
Payments/credits	(7,595)
Balance, as of December 31, 2017	8,587
Chargebacks [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2016	1
Provision	6,455
Payments/credits	(5,461)
Balance, as of December 31, 2017	995
Rebates [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2016	238
Provision	7,659
Payments/credits	(938)
Balance, as of December 31, 2017	6,959
Prompt Pay [Member]
Accounts Notes And Loans Receivable [Line Items]
Provision	469
Payments/credits	(300)
Balance, as of December 31, 2017	169
Other Accruals [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2016	67
Provision	1,293
Payments/credits	(896)
Balance, as of December 31, 2017	$ 464

Accounts Receivable and Reserves for Product Sales - Summary of Total Reserves Included in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Receivables Net Current [Abstract]
Reduction to accounts receivable	$ 1,285	$ 1
Component of accrued expenses	7,302	305
Total reserves	$ 8,587	$ 306

Summary of Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepaid Expense And Other Assets Current [Abstract]
Manufacturing-related deposits and prepaids	$ 18,650	$ 23,604
Prepaid clinical and preclinical expenses	5,175	1,225
Prepaid research expenses	2,896
Prepaid maintenance services	1,711	109
Prepaid commercial expenses	1,589	54
Asset held for sale	1,501
Other prepaids	2,726	989
Other	2,263	914
Total other current assets	$ 36,511	$ 26,895

Summary of Investments and Other Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepaid Expense And Other Assets Noncurrent [Abstract]
Long-term available-for-sale securities	$ 9,980
Prepaid clinical expenses	7,488	$ 3,725
Alternative minimum tax credit	3,315
Restricted cash and investments	784	784
Other	242	242
Total investments and other assets	$ 21,809	$ 4,751

- Definition

Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

+ References + Details

Summarizes Components of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 61,178	$ 68,147
Less: accumulated depreciation	(18,022)	(30,346)
Property and equipment, net	43,156	37,801
Land [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,158	4,158
Building and Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,543	12,729
Software and Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,965	6,775
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	9,860	14,149
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,211	3,028
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	13,633	22,872
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 7,808	$ 4,436

Property and Equipment - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 6,400,000	$ 5,000,000	$ 4,700,000
Loss on write-offs of fully deprecated property and equipment	0
Deprecated property and equipment, value	$ 17,100,000
Deprecated property and equipment, basis of valuation	Cost Basis
Property and equipment, accumulated depreciation	$ 18,022,000	$ 30,346,000
Corvallis Plan [Member]
Property, Plant and Equipment [Line Items]
Accelerated depreciation	$ 800,000

Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued clinical and preclinical costs	$ 15,975	$ 10,033
Accrued employee compensation costs	14,402	8,748
Accrued contract manufacturing costs	14,019	4,673
Product revenue related reserves	7,302	305
Accrued professional fees	6,794	2,799
Accrued BioMarin royalties	2,846
Accrued property and equipment	2,525	1,186
Accrued interest expenses	1,291	100
Accrued income taxes	943
Accrued research costs	401	1,186
Other	2,484	1,986
Total accrued expenses	$ 68,982	$ 31,016

Restructuring - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Apr. 30, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost And Reserve [Line Items]
Restructuring expenses			$ 3,020,000	$ 4,562,000
Workforce Reduction [Member]
Restructuring Cost And Reserve [Line Items]
Cost incurred related to workforce reduction				$ 2,300,000
Workforce Reduction [Member] \| Maximum [Member]
Restructuring Cost And Reserve [Line Items]
Cost incurred related to workforce reduction			$ 100,000
Corvallis Plan [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring, initiation date			2016-03
Restructuring, completion date			Sep. 30, 2017
Minimum lease payment obligation			$ 4,900,000
Corvallis Plan [Member] \| Workforce Reduction [Member]
Restructuring Cost And Reserve [Line Items]
Percentage of workforce reduced			19.00%
Corvallis Plan [Member] \| Facility Closing [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses	$ 2,400,000	$ 1,600,000
Cambridge Plan [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring, initiation date			2016-08
Restructuring, completion date			Oct. 31, 2016
Cambridge Plan [Member] \| Workforce Reduction [Member]
Restructuring Cost And Reserve [Line Items]
Percentage of workforce reduced			6.00%
Restructuring expenses			$ 600,000

Summary of Restructuring Costs by Function (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses, Cash	$ 3,020	$ 3,651
Total restructuring expenses, Non-cash		911
Total restructuring expenses	3,020	4,562
Research and Development [Member]
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses, Cash	188	1,631
Total restructuring expenses, Non-cash		382
Total restructuring expenses	188	2,013
Selling, General and Administrative [Member]
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses, Cash	2,832	2,020
Total restructuring expenses, Non-cash		529
Total restructuring expenses	$ 2,832	$ 2,549

Summary of Restructuring Reserve (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Restructuring And Related Activities [Abstract]
Restructuring reserve beginning balance	$ 1,588
Restructuring expenses incurred during the period	3,020	$ 3,651
Amounts paid during the period	(1,675)	(2,063)
Restructuring reserve ending balance	$ 2,933	$ 1,588

Indebtedness - Additional Information (Detail)		1 Months Ended	12 Months Ended
Indebtedness - Additional Information (Detail)	Nov. 14, 2017 USD ($) shares $ / shares	Jul. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) Loan $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Proceeds for issuance of notes allocated to equity			$ 570,000,000
Amount paid for capped calls transactions			50,901,000
Repayment of outstanding balance			15,109,000	$ 7,603,000	$ 2,598,000
Revolving line-of-credit outstanding balance			601,328,000
Total indebtedness of unpaid principal balance			$ 1,300,000
Sale of Airport Facility date			2018-01
Long-term debt			$ 424,876,000	6,042,000
July 2017 Term Loan [Member]
Debt Instrument [Line Items]
Debt issuance costs			1,100,000
Net proceeds from issuance of debt			29,100,000
Long-term debt			30,000,000
June 2015 Term Loan [Member]
Debt Instrument [Line Items]
Repayment of outstanding balance			9,200,000
Long-term debt				15,000,000
Mortgage Loan One [Member]
Debt Instrument [Line Items]
Revolving line-of-credit outstanding balance			800,000
Mortgage Loan Two [Member]
Debt Instrument [Line Items]
Revolving line-of-credit outstanding balance			500,000
Convertible Debt, The Term Loan, The Revolver and The Mortgage Loans [Member]
Debt Instrument [Line Items]
Current portion of long-term debt			6,200,000
Long-term debt			424,900,000
Interest expense related to debt facilities			5,800,000	$ 1,900,000	$ 1,300,000
Maximum [Member] \| Revolving Loan [Member]
Debt Instrument [Line Items]
Revolving line-of-credit outstanding balance			100,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 570,000,000		$ 570,000,000
Debt instrument, maturity date	Nov. 15, 2024
Debt instrument, interest rate per annum	1.50%
Debt instrument, payment frequency			The 2024 Notes were issued at face value and bear interest at the rate of 1.50% per annum, payable semi-annually in cash on each May 15 and November 15, commencing on May 15, 2018.
Debt instrument, conversion rate	0.013621
Debt instrument, conversion price \| $ / shares	$ 73.42		$ 73.42
Debt issuance costs	$ 14,800,000
Proceeds for issuance of notes allocated to equity	161,100,000
Offering costs	4,200,000
Debt discount and issuance costs on notes	$ 10,600,000
Effective interest rate percentage	6.90%
Capped calls strike price \| $ / shares	73.42
Capped calls cap price \| $ / shares	104.88
Amount paid for capped calls transactions	$ 50,900,000
2024 Convertible Notes [Member] \| Fundamental Change [Member]
Debt Instrument [Line Items]
Debt instrument, repurchase price percentage	100.00%
2024 Convertible Notes [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Percentage of additional interest expenses from fail to register notes after 180 days and 380 days after issuance date	0.25%
2024 Convertible Notes [Member] \| Minimum [Member] \| Beneficial Owner [Member] \| Fundamental Change [Member]
Debt Instrument [Line Items]
Voting percentage	50.00%
2024 Convertible Notes [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Percentage of additional interest expenses from fail to register notes after 180 days and 380 days after issuance date	0.50%
2024 Convertible Notes [Member] \| Common Stock [Member]
Debt Instrument [Line Items]
Debt instrument, convertible into shares \| shares	7,763,552
MidCap Financial Amended and Restated Credit and Security Agreement [Member] \| July 2017 Term Loan [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate per annum		6.25%
Current borrowing capacity		$ 60,000,000
Variable interest rate description		one-month London Interbank Offered Rate
Origination fee, percentage		0.50%
Final payment fee, percentage		2.00%
Monthly principal payments commencement date		Jul. 01, 2018
Periodic principal payment frequency		monthly
Monthly principal payments		$ 800,000
Credit facility maturity date		2021-07
Debt instrument prepayment fee percent within twelve months			3.00%
Debt instrument prepayment fee percent on or after twelve months and within twenty four months			2.00%
Debt instrument prepayment fee percent on or after twenty four months			1.00%
Percentage of minimum amount of cash and/or cash equivalents on outstanding principal amounts			75.00%
Midcap Financial Revolving Credit and Security Agreement [Member] \| Revolving Loan [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate per annum		3.95%
Current borrowing capacity		$ 40,000,000
Variable interest rate description			one-month LIBOR
Origination fee, percentage		0.50%
Credit facility maturity date		2021-07
Percentage of minimum amount of cash and/or cash equivalents on outstanding principal amounts			75.00%
Line of credit facility additional tranche that may be increased		$ 20,000,000
Origination fee		$ 200,000
Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate per annum			4.75%
Credit facility maturity date			2027-02
Number of mortgage loans outstanding \| Loan			2

Indebtedness - Summary of 2024 Notes (Detail) - 2024 Convertible Notes [Member] - USD ($)	Dec. 31, 2017	Nov. 14, 2017
Debt Instrument [Line Items]
Par value of the 2024 Notes	$ 570,000,000	$ 570,000,000
Unamortized discounts	(158,890,000)
Debt issuance expenses	(10,449,000)
Net carrying value of convertible debt	400,661,000
Fair value of convertible debt	$ 619,641,000

Indebtedness - Summary of Term Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Unamortized debt issuance expense	$ (918)	$ (223)
Net carrying value of term loan	29,082	14,777
2017 Term Loan [Member]
Debt Instrument [Line Items]
Principal amount of term loan	$ 30,000
2015 Term Loan [Member]
Debt Instrument [Line Items]
Principal amount of term loan		$ 15,000

Indebtedness - Summarizes Total Gross Payments Due under Company's Debt Arrangements (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Debt Disclosure [Abstract]
2018	$ 6,328
2019	10,000
2020	10,000
2021	5,000
Thereafter	570,000
Total Payments	$ 601,328

Equity Financing - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended
	Jul. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Oct. 31, 2015
Class Of Stock [Line Items]
Common stock issued		5.8	2.1	3.3
Common stock average price per share	$ 42.50	$ 59.75	$ 17.84	$ 39.00
Net proceeds from issue of common stock	$ 354.0	$ 327.4	$ 37.3	$ 119.9
Underwriting discounts, commission and offering expenses	$ 20.0	$ 17.6	$ 0.2	$ 6.8
Underwriting discount and commission per share			$ 1.60
Underwritten Public Offering [Member]
Class Of Stock [Line Items]
Common stock issued	8.8
Underwritten Offering [Member]
Class Of Stock [Line Items]
Common stock issued	1.2

Stock Based Compensation - Additional Information (Detail) - USD ($)

1 Months Ended

12 Months Ended

Dec. 31, 2017

Jun. 30, 2017

Mar. 31, 2017

Sep. 30, 2016

Jun. 30, 2016

Feb. 29, 2016

Oct. 31, 2015

Sep. 30, 2015

Jun. 30, 2015

Jun. 30, 2013

Nov. 30, 2012

Aug. 31, 2012

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Dec. 31, 2014

Sep. 30, 2014

Jun. 30, 2011

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Term of award

10 years

Share based compensation granted under plan vest period

4 years

Weighted-average grant date fair value per share of stock-based awards, granted to employees

$ 14.78

$ 12.46

$ 14.98

Stock option grants with performance conditions

4,805,722

^[1]

1,285,051

2,830,078

Stock-based compensation expense

$ 30,465,000

$ 29,962,000

$ 32,117,000

Former Chief Executive Officer [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation expense

$ 2,100,000

8,600,000

Stock Options [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Share based compensation granted under plan vest period

4 years

Stock option grants with performance conditions

300,000

400,000

Forfeited awards percentage

20.00%

Stock-based compensation expense

$ 23,416,000

26,320,000

29,014,000

Stock Options [Member] | EXONDYS 51 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting percentage

80.00%

Stock Options [Member] | Performance Grants Vested Immediately [Member] | EXONDYS 51 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting percentage

50.00%

Stock Options [Member] | Vesting Subject to the Remaining Service Conditions [Member] | EXONDYS 51 [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Vesting percentage

50.00%

Stock Options with Performance-based Conditions [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation expense

900,000

$ 5,000,000

$ 500,000

Service-Vesting Conditions [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock based compensation expense not yet recognized

$ 47,500,000

47,500,000

Stock Options with Service-and Performance-Based Conditions [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock based compensation expense not yet recognized

$ 400,000

Restricted Stock Awards (RSAs) [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Grants, Restricted stock units and awards

341,500

117,553

181,783

Grant date fair value, Restricted stock units and awards

$ 34.58

$ 41.22

$ 20.80

Restricted Stock Awards (RSAs) [Member] | Executives [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Grants, Restricted stock units and awards

65,000

Grant date fair value, Restricted stock units and awards

$ 54.29

Restricted Stock Awards (RSAs) [Member] | Minimum [Member] | Executives [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Grants, Restricted stock units and awards

71,925

Share-based compensation award, expiration date

Jun. 30, 2018

Restricted Stock Awards (RSAs) [Member] | Maximum [Member] | Executives [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Grants, Restricted stock units and awards

89,906

Share-based compensation award, expiration date

Dec. 31, 2018

Stock-based compensation related to grants

$ 4,900,000

Restricted Stock Units (RSUs) [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Grants, Restricted stock units and awards

181,029

Grant date fair value, Restricted stock units and awards

$ 33.03

Restricted Stock Units (RSUs) [Member] | Executives [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Grants, Restricted stock units and awards

156,029

Restricted Stock Units (RSUs) [Member] | Maximum [Member] | Executives [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation related to grants

$ 700,000

Restricted Stock Units (RSUs) [Member] | Performance Grants Vested Immediately [Member] | Executives [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation expense

2,500,000

Percentage of RSUs vested upon achievement of first condition

50.00%

Restricted Stock Units (RSUs) [Member] | Vesting upon Achievement of Second Milestone [Member] | Executives [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Stock-based compensation expense

400,000

Stock-based compensation related to grants

$ 300,000

Stock Appreciation Rights [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Grants, Restricted stock units and awards

100,000

70,000

Exercise price of SARs

$ 23.85

$ 10.08

2011 Equity Incentive Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock remain available for future grant

2,300,000

Number of shares approved under the plan

13,000,000

Number of additional shares approved under the plan

1,300,000

1,700,000

2013 Employee Stock Purchase Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock remain available for future grant

200,000

Number of shares approved under the plan

300,000

Term of award

24 months

Percentage of closing price of common stock

85.00%

2013 Employee Stock Purchase Plan [Member] | Minimum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Share-based compensation award, expiration date

Feb. 28, 2018

2013 Employee Stock Purchase Plan [Member] | Maximum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Share-based compensation award, expiration date

Aug. 31, 2019

2014 Employment Commencement Incentive Plan (2014 Plan) [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock remain available for future grant

600,000

Number of shares approved under the plan

600,000

Number of additional shares approved under the plan

3,800,000

1,000,000

[1]

Assumptions for Measuring Fair Values of Stocks (Detail)

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Expected dividend yield

0.00%

Stock Options [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Risk-free interest rate, minimum

^[1]

1.60%

1.10%

Risk-free interest rate, maximum

^[1]

2.10%

1.80%

1.70%

Expected volatility, minimum

^[2]

54.00%

78.20%

94.30%

Expected volatility, maximum

^[2]

63.00%

137.10%

111.10%

Stock Options [Member] | Minimum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Expected terms

^[3]

4 years 2 months 12 days

4 years 8 months 12 days

Stock Options [Member] | Maximum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Expected terms

^[3]

4 years 9 months 18 days

5 years

[1]	The risk-free interest rate is estimated using an average of Treasury bill interest rates over a historical period commensurate with the expected term of the option that correlates to the prevailing interest rates at the time of grant.
[2]	Upon commercialization of EXONDYS 51 in the U.S., the Company’s future risk profile changed. As a result, starting January 1, 2017, the Company estimates expected volatility by using only implied volatility in exchange-traded options of the Company’s common stock. Prior to January 1, 2017, the expected volatility was estimated using a blend of calculated volatility of the Company’s common stock over a historical period and implied volatility in exchange-traded options of the Company’s common stock.
[3]	The expected terms are estimated using historical exercise behavior.

Assumptions for Measuring Fair Values of Stocks (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected dividend yield	0.00%

Summary of Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Stock Option Shares

Grants outstanding at beginning of the period

5,436,951

6,515,976

5,216,203

Granted

4,805,722

^[1]

1,285,051

2,830,078

Exercised

(792,845)

(1,056,821)

(816,696)

Cancelled

(643,624)

(1,307,255)

(713,609)

Grants outstanding at end of the period

8,806,204

5,436,951

6,515,976

Grants exercisable at end of the period

3,288,712

2,942,624

2,617,167

Grants vested and expected to vest at end of the period

6,910,022

5,116,409

5,908,213

Weighted Average Exercise Price

Grants outstanding at beginning of the period

$ 22.70

$ 23.91

$ 24.45

Granted

35.09

14.89

20.28

Exercised

17.40

18.31

12.26

Cancelled

25.44

24.61

26.78

Grants outstanding at end of the period

29.74

22.70

23.91

Grants exercisable at end of the period

24.76

24.42

23.85

Grants vested and expected to vest at end of the period

$ 28.49

$ 22.83

$ 23.94

Aggregate Intrinsic Value

Outstanding at end of year

$ 228,081

Exercisable at end of year

101,559

Vested and expected to vest

$ 187,582

Weighted Average Remaining Contractual Life (Years)

Outstanding at end of year

7 years 7 months 6 days

Exercisable at end of year

5 years 1 month 6 days

Vested and expected to vest

7 years 1 month 6 days

[1]

Summary of Stock Option Activity (Parenthetical) (Detail) - $ / shares	1 Months Ended			12 Months Ended
	Jun. 30, 2017	Feb. 29, 2016	Jun. 30, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of options granted				$ 14.78	$ 12.46	$ 14.98
Vesting period				4 years
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period		4 years	4 years
Vest Monthly Thereafter [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	0.02%
CEO [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted	3,300,000
Cliff vesting schedule	5 years
Fair value of options granted	$ 13.48
Vesting period	4 years
CEO [Member] \| Vest First Anniversary [Member] \| Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage	25.00%

Summary of Company's Stock Options Vested and Exercised (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Aggregate grant date fair value of stock options vested	$ 18,225	$ 30,651	$ 27,858
Aggregate intrinsic value of stock options exercised	$ 20,922	$ 30,610	$ 18,138

Summary of Restricted Stock Award Activity (Detail) - Restricted Stock Awards (RSAs) [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Grants outstanding at beginning of the period	153,170	161,320	6,000
Shares, Granted	341,500	117,553	181,783
Shares, Vested	(63,264)	(59,703)	(24,463)
Shares, Forfeited	(19,625)	(66,000)	(2,000)
Shares, Grants outstanding at end of the period	411,781	153,170	161,320
Weighted Average Grant Date Fair Value per Share, Grants outstanding at beginning of the period	$ 34.53	$ 21.50	$ 34.92
Weighted Average Grant Date Fair Value per Share, Granted	34.58	41.22	20.80
Weighted Average Grant Date Fair Value per Share, Vested	14.60	13.62	20.21
Weighted Average Grant Date Fair Value per Share, Forfeited	43.02	33.51	13.90
Weighted Average Grant Date Fair Value per Share, Grants outstanding at end of the period	$ 37.23	$ 34.53	$ 21.50

Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Granted \| shares	181,029
Shares, Vested \| shares	(78,017)
Shares, Forfeited \| shares	(36,460)
Shares, Grants outstanding at end of the period \| shares	66,552
Weighted Average Grant Date Fair Value per Share, Granted \| $ / shares	$ 33.03
Weighted Average Grant Date Fair Value per Share, Vested \| $ / shares	32.63
Weighted Average Grant Date Fair Value per Share, Forfeited \| $ / shares	32.63
Weighted Average Grant Date Fair Value per Share, Grants outstanding at end of the period \| $ / shares	$ 33.72

Summary of Stock Appreciation Rights Activity (Detail) - Stock Appreciation Rights [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Grants outstanding at beginning of the period	100,000	170,000	170,000
Shares, Exercised		(67,812)
Shares, Forfeited		(2,188)
Shares, Grants outstanding at end of the period	100,000	100,000	170,000
Shares, Grants exercisable at end of the period	100,000	100,000	141,249
Shares, Grants vested and expected to vest at end of the period	100,000	100,000	167,813
Weighted Average Exercise Price per Share, Grants outstanding at beginning of the period	$ 23.85	$ 18.18	$ 18.18
Weighted Average Exercise Price per Share, Exercised		10.08
Weighted Average Grant Date Fair Value per Share, Forfeited		10.08
Weighted Average Exercise Price per Share, Grants outstanding at end of the period	23.85	23.85	18.18
Weighted Average Exercise Price per Share, Grants exercisable at end of the period	23.85	23.85	17.59
Weighted Average Exercise Price per Share, Grants vested and expected to vest at end of the period	$ 23.85	$ 23.85	$ 18.29
Stock Appreciation Rights, Aggregate Intrinsic Value
SARs outstanding, exercisable and vested at December 31, 2017	$ 3,179
Stock Appreciation Rights, Weighted Average Remaining Contractual Life
SARs outstanding, exercisable and vested at December 31, 2017	4 years 10 months 24 days

Summary of Employee Stock Purchase Plan Activity and Expense (Detail) - 2013 Employee Stock Purchase Plan [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares purchased	102,698	137,113	75,539
Proceeds received (in millions)	$ 1.4	$ 1.6	$ 0.9

Summary of Stock-Based Compensation Expense by Function Included within Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 30,465	$ 29,962	$ 32,117
Research and Development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	8,542	9,499	10,403
Selling, General and Administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 21,923	$ 20,463	$ 21,714

Summary of Stock-Based Compensation Expense by Grant Type Included within Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 30,465	$ 29,962	$ 32,117
Stock Options [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	23,416	26,320	29,014
Employee Stock Purchase Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	1,754	2,380	2,165
Restricted Stock Awards/Units (RSAs/RSUs) [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation	$ 5,295	872	446
Other [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation		$ 390	$ 492

401 (K) PLAN - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Expense related to 401 (K) Plan	$ 1.4	$ 1.2	$ 0.9
Defined contribution plan, employers matching contribution, service period	3 years		1 year
Defined contribution plan, employers matching contribution, vesting percentage after one years of service			100.00%
Description of 401(k) plan	The Company sponsors a 401 (k) Plan (“the Plan”) which is a defined contribution plan. It is available to all employees who are age 21 or older. Participants may make voluntary contributions and the Company makes matching contributions according to the Plan’s matching formula. For employees who started before January 1, 2016, matching contributions fully vest after one year of service. For employees who started on and after January 1, 2016, matching contributions vest on a straight line basis over three years. The expense related to the Plan primarily consists of the Company’s matching contributions.
Age limit for eligibility to participate in the defined contribution plan	21 years

Summary of Other Income and Loss (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Nonoperating Income Expense [Abstract]
Interest expenses									$ (5,781)	$ (1,892)	$ (1,365)
Interest income									1,809	507	890
Other income									1,982	850	629
Gain from sale of Priority Review Voucher				$ 125,000					125,000
Total other income (loss)	$ (2,693)	$ 184	$ 184	$ 125,335	$ (57)	$ (209)	$ (201)	$ (68)	$ 123,010	$ (535)	$ 154

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
U.S. federal corporate tax rate						34.00%	34.00%	34.00%
State income tax liability	$ 1,800,000					$ 1,800,000
Current federal income tax						$ 200,000
Alternative minimum tax partially refundable year one						2018
Alternative minimum tax partially refundable year two						2019
Alternative minimum tax partially refundable year three						2020
Alternative minimum tax fully refundable year one						2021
Current federal income tax							$ 0	$ 0
State income taxes							0	0
Foreign income taxes							0	0
Domestic pre-tax losses						$ 45,700,000
Foreign pre-tax losses						2,900,000
Reduction to deferred tax assets						87,000,000
Impact on net tax						0
Losses incurred	(25,840,000)	$ (45,723,000)	$ (63,155,000)	$ 86,090,000		(48,628,000)	(267,265,000)	(220,030,000)
Net change in valuation allowance for deferred tax assets						69,500,000	54,200,000
Accrual for interest or penalties	0					0	0
Recognized interest and/or penalties						0	0	$ 0
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	249,300,000					249,300,000
State [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	174,800,000					$ 174,800,000
Foreign
Income Taxes [Line Items]
Operating loss carryforwards, ending of expiration year						2024
Net operating loss carryforwards	2,900,000					$ 2,900,000
Research and Development Credits [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Description						These federal and state research and development credits begin to expire between 2018 and 2037 and between 2018 and 2032, respectively.
Research and Development Credits [Member] \| Federal [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Amount	34,300,000					$ 34,300,000
Research and Development Credits [Member] \| State [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Amount	11,500,000					11,500,000
Sarepta International C.V. [Member]
Income Taxes [Line Items]
Losses incurred						172,800,000	$ 214,300,000
Sarepta International C.V. [Member] \| Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 387,100,000					$ 387,100,000
Scenario, Forecast [Member]
Income Taxes [Line Items]
U.S. federal corporate tax rate					21.00%
Maximum [Member]
Income Taxes [Line Items]
U.S. federal corporate tax rate						35.00%
Operating loss carryforwards, ending of expiration year						2037
Maximum [Member] \| Research and Development Credits [Member] \| Federal [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period						2037
Maximum [Member] \| Research and Development Credits [Member] \| State [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period						2032
Maximum [Member] \| Sarepta International C.V. [Member]
Income Taxes [Line Items]
Operating loss carryforwards, ending of expiration year						2037
Minimum [Member]
Income Taxes [Line Items]
Operating loss carryforwards, ending of expiration year						2018
Minimum [Member] \| Research and Development Credits [Member] \| Federal [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period						2018
Minimum [Member] \| Research and Development Credits [Member] \| State [Member]
Income Taxes [Line Items]
Research and development credits carryforward, Expiration period						2018
Minimum [Member] \| Sarepta International C.V. [Member]
Income Taxes [Line Items]
Operating loss carryforwards, ending of expiration year						2036

Reconciliation Between Effective Tax Rate and Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax rate	34.00%	34.00%	34.00%
State taxes	(27.70%)
Research and development and other tax credits	8.50%	1.50%	0.30%
Valuation allowance	(93.20%)	(17.30%)	(19.10%)
Permanent differences	6.40%	(3.40%)	(1.70%)
Sarepta International C.V. return to provision	62.10%
Impact of tax reform, net of valuation allowance	5.90%
Foreign rate differential	(0.60%)	(14.80%)	(13.50%)
Other	0.40%
Effective tax rate	(4.20%)

Analysis of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 144,897	$ 174,569
Difference in depreciation and amortization	1,210	2,589
Research and development and other tax credits	44,306	37,812
Stock-based compensation	14,392	16,166
Deferred rent	1,210	2,306
Deferred revenue	904	1,315
Capitalized inventory	22,127	36,282
Other	4,071	3,047
Total deferred tax assets	233,117	274,086
Deferred tax liabilities:
Debt discount	(28,548)
Total deferred tax liabilities	(28,548)
Valuation allowance	(204,569)	(274,086)
Net deferred tax assets	$ 0	$ 0

Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of the period	$ 4,644	$ 3,706
Increase related to current year tax positions	735	801	$ 613
Increase related to prior year tax positions		137	3,093
Decrease related to prior year tax position	(245)
Balance at end of the period	$ 5,134	$ 4,644	$ 3,706

Basic and Diluted Net Loss Per Share (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands

3 Months Ended

12 Months Ended

Dec. 31, 2017

Sep. 30, 2017

Jun. 30, 2017

Mar. 31, 2017

Dec. 31, 2016

Sep. 30, 2016

Jun. 30, 2016

Mar. 31, 2016

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Earnings Per Share [Abstract]

Net loss

$ (23,998)

$ (47,734)

$ (63,046)

$ 84,090

$ (88,452)

$ (56,742)

$ (62,301)

$ (59,770)

$ (50,688)

$ (267,265)

$ (220,030)

Weighted-average number of shares of common stock and common stock equivalents outstanding:

Weighted-average number of shares of common stock outstanding for computing basic loss per share

64,277

61,528

54,976

54,850

58,818

48,697

42,290

Dilutive effect of outstanding stock awards and stock options after application of the treasury stock method

^[1]

Weighted-average number of shares of common stock and dilutive common stock equivalents outstanding for computing diluted loss per share

54,619

48,254

46,157

45,697

58,818

48,697

42,290

Net loss per share — basic and diluted

$ (1.62)

$ (1.18)

$ (1.35)

$ (1.31)

$ (0.86)

$ (5.49)

$ (5.20)

[1]

For the year ended December 31, 2017, stock options, RSAs, RSUs and SARs to purchase approximately 9.4 million shares of common stock, respectively, were excluded from the net loss per share calculation as their effect would have been anti-dilutive. The Company accounts for the effect of the 2024 Notes on diluted net loss per share using the if-converted method as they may be settled in cash or shares at the Company’s option. The 2024 Notes have no effect on diluted net loss per share until the Company’s stock price exceeds the conversion price of $73.42 per share. In the period of conversion, the 2024 Notes will have no impact on diluted net loss if they are settled in cash and will have an impact on diluted loss per share if the Notes are settled in shares upon conversion. For the years ended 2016 and 2015, stock options, RSAs and SARs to purchase approximately 5.7 million and 6.8 million shares of common stock, respectively, were excluded from the net loss per share calculation as their effect would have been anti-dilutive.

Basic and Diluted Net Loss Per Share (Parenthetical) (Detail) - $ / shares shares in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 14, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of loss per share					9.4	5.7	6.8
Effect on diluted net loss per share	$ (0.37)	$ (0.78)	$ (1.15)	$ 1.50
2024 Convertible Notes [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Debt instrument, conversion price	$ 73.42				$ 73.42			$ 73.42
Effect on diluted net loss per share					$ 0

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended						12 Months Ended
	Dec. 31, 2016 USD ($) ft²	Sep. 30, 2015 USD ($)	Aug. 31, 2015 ft²	Feb. 28, 2015 ft²	Jun. 30, 2014 ft²	Jan. 31, 2014 ft²	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)
Commitments and Contingencies [Line Items]
Letters of credit amount issued		$ 10,700,000
Certificate of deposit purchased		$ 10,700,000
Certificate of deposit maturity date							2017-09
Restricted investments	$ 10,700,000							$ 10,700,000
Restricted investment maturity date							2017-09
EXONDYS 51 [Member]
Commitments and Contingencies [Line Items]
Sales milestone recorded as an in-licensed rights	$ 1,000,000						$ 6,600,000	1,000,000
Collaboration and License Agreement [Member]
Commitments and Contingencies [Line Items]
Development milestone and up-front payments							22,000,000	47,600,000	$ 200,000
Collaboration and License Agreement [Member] \| Maximum [Member]
Commitments and Contingencies [Line Items]
Development, up-front royalty and sales milestone payments							808,500,000
BioMarin Pharmaceutical, Inc. [Member]
Commitments and Contingencies [Line Items]
Royalty payments							4,700,000
CRO [Member]
Commitments and Contingencies [Line Items]
Cancellable future commitments							52,900,000
CRO [Member] \| Other Funding Commitments [Member]
Commitments and Contingencies [Line Items]
Expenditures incurred by CROs							$ 13,900,000	$ 8,100,000	$ 8,500,000
Corvallis, Oregon [Member]
Commitments and Contingencies [Line Items]
Lease expiration date								2020-12
Sublease [Member]
Commitments and Contingencies [Line Items]
Additional rental space under amendment \| ft²	7,461		15,077	7,461	10,939	15,077		7,461
Lease termination date					Dec. 31, 2016
Lease expired date	2017-12		2017-03	2016-02		2015-07
Lease Agreements [Member]
Commitments and Contingencies [Line Items]
Total rental space \| ft²							88,459
Lease expiration date							2021-01
Letters of credit amount issued	$ 600,000						$ 600,000	$ 600,000

Summary of Aggregate Non-Cancelable Future Minimum Payments under Company's Leases (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018	$ 4,946
2019	4,986
2020	5,089
2021	332
Total minimum lease payments	$ 15,353

Non-Cancelable Contractual Obligations (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2018	$ 81,677
2019	13,939
Total purchase commitments	$ 95,616

Financial Information by Quarter (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Product, net	$ 57,277	$ 45,954	$ 35,011	$ 16,342	$ 5,421				$ 154,584	$ 5,421
Total revenues	57,277	45,954	35,011	16,342	5,421				154,584	5,421	$ 1,253
Cost and operating expenses:
Cost of sales (excluding amortization of in-licensed rights)	3,546	3,078	505	224	101				7,353	101
Research and development	44,441	34,239	58,908	29,119	70,749	$ 34,349	$ 44,348	$ 38,826	166,707	188,272	146,394
Selling, general and administrative	32,221	28,176	36,069	26,216	22,937	22,184	17,752	20,876	122,682	83,749	75,043
Settlement and license charges		25,588	2,839						28,427
Amortization of in-licensed rights	216	780	29	28	29				1,053	29
Total cost and expenses	80,424	91,861	98,350	55,587	93,816	56,533	62,100	59,702	326,222	272,151	221,437
Operating loss	(23,147)	(45,907)	(63,339)	(39,245)	(88,395)	(56,533)	(62,100)	(59,702)	(171,638)	(266,730)	(220,184)
Other (loss) income:
Interest (expense) income and other, net	(2,693)	184	184	335	(57)	(209)	(201)	(68)	(1,990)	(535)	154
Gain from sale of Priority Review Voucher				125,000					125,000
Total other income (loss)	(2,693)	184	184	125,335	(57)	(209)	(201)	(68)	123,010	(535)	154
Loss before income tax expense	(25,840)	(45,723)	(63,155)	86,090					(48,628)	(267,265)	(220,030)
Income tax (benefit) expense	(1,842)	2,011	(109)	2,000					2,060
Net loss	$ (23,998)	$ (47,734)	$ (63,046)	$ 84,090	$ (88,452)	$ (56,742)	$ (62,301)	$ (59,770)	$ (50,688)	$ (267,265)	$ (220,030)
Net income (loss) per share:
Basic (loss) earnings per share	$ (0.37)	$ (0.78)	$ (1.15)	$ 1.53
Diluted (loss) earnings per share	$ (0.37)	$ (0.78)	$ (1.15)	$ 1.50
Weighted average number of shares of common stock used in calculating:
Weighted-average number of shares of common stock outstanding for computing basic loss per share	64,277	61,528	54,976	54,850					58,818	48,697	42,290
Diluted (loss) earnings per share	64,277	61,258	54,976	56,012
Net loss per share — basic and diluted					$ (1.62)	$ (1.18)	$ (1.35)	$ (1.31)	$ (0.86)	$ (5.49)	$ (5.20)
Shares used in per share calculations—basic and diluted					54,619	48,254	46,157	45,697	58,818	48,697	42,290