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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2018	Apr. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Trading Symbol	SRPT
Entity Registrant Name	Sarepta Therapeutics, Inc.
Entity Central Index Key	0000873303
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		65,531,046

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 557,234	$ 599,691
Short-term investments	491,757	479,369
Accounts receivable	39,848	29,468
Inventory	99,375	83,605
Other current assets	31,203	36,511
Total current assets	1,219,417	1,228,644
Property and equipment, net of accumulated depreciation of $19,817 and $18,022 as of March 31, 2018 and December 31, 2017, respectively	53,927	43,156
Intangible assets, net of accumulated amortization of $4,659 and $4,145 as of March 31, 2018 and December 31, 2017, respectively	14,473	14,355
Other assets	12,466	21,809
Total assets	1,300,283	1,307,964
Current liabilities:
Accounts payable	17,379	8,467
Accrued expenses	65,648	68,982
Current portion of long-term debt	3,446	6,175
Other current liabilities	4,723	4,708
Total current liabilities	91,196	88,332
Long-term debt	427,365	424,876
Deferred rent and other	4,962	5,539
Total liabilities	523,523	518,747
Commitments and contingencies (Note 16)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 3,333,333 shares authorized; none issued and outstanding
Common stock, $0.0001 par value, 99,000,000 shares authorized; 65,493,293 and 64,791,670 issued and outstanding at March 31, 2018 and December 31, 2017, respectively	7	6
Additional paid-in capital	2,029,767	2,006,598
Accumulated other comprehensive loss	(643)	(379)
Accumulated deficit	(1,252,371)	(1,217,008)
Total stockholders’ equity	776,760	789,217
Total liabilities and stockholders’ equity	$ 1,300,283	$ 1,307,964

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Property and equipment, accumulated depreciation	$ 19,817	$ 18,022
Intangible assets, accumulated amortization	$ 4,659	$ 4,145
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	65,493,293	64,791,670
Common stock, outstanding	65,493,293	64,791,670

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues:
Product, net	$ 64,604	$ 16,342
Total revenues	64,604	16,342
Costs and expenses:
Cost of sales (excluding amortization of in-licensed rights)	5,582	223
Research and development	46,204	29,119
Selling, general and administrative	43,341	26,216
Amortization of in-licensed rights	216	29
Total costs and expenses	95,343	55,587
Operating loss	(30,739)	(39,245)
Other (loss) income:
Gain from sale of Priority Review Voucher		125,000
Interest (expense) income and other, net	(4,485)	335
Other (loss) income	(4,485)	125,335
(Loss) income before income tax expense	(35,224)	86,090
Income tax expense	139	2,000
Net (loss) income	(35,363)	84,090
Other comprehensive (loss) income:
Unrealized (loss) gain on cash equivalents and short-term investments	(264)	65
Total other comprehensive (loss) income	(264)	65
Comprehensive (loss) income	$ (35,627)	$ 84,155
Net (loss) income per share
Basic (loss) earnings per share	$ (0.55)	$ 1.53
Diluted (loss) earnings per share	$ (0.55)	$ 1.50
Weighted average number of shares of common stock used in computing:
Basic (loss) earnings per share	64,631	54,850
Diluted (loss) earnings per share	64,631	56,012

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net (loss) income	$ (35,363)	$ 84,090
Adjustments to reconcile net loss to cash flows from operating activities:
Gain from sale of Priority Review Voucher		(125,000)
Depreciation and amortization	2,252	1,637
Amortization of investment discount	(1,259)	(100)
Non-cash interest expense	4,940	82
Loss on disposal of assets	10	485
Stock-based compensation	10,526	5,712
Changes in operating assets and liabilities, net:
Net increase in accounts receivable	(10,380)	(7,050)
Net increase in inventory	(15,770)	(17,632)
Net decrease in other assets	4,672	774
Net increase (decrease) in accounts payable, accrued expenses, deferred revenue and other liabilities	4,704	(886)
Net cash used in operating activities	(35,668)	(57,888)
Cash flows from investing activities:
Purchase of property and equipment	(12,166)	(4,465)
Purchase of intangible assets	(673)	(1,245)
Purchase of available-for-sale securities	(91,514)
Proceeds from sale of Priority Review Voucher		125,000
Maturity of restricted investment		10,695
Maturity and sale of available-for-sale securities	90,093	80,000
Net cash (used in) provided by investing activities	(14,260)	209,985
Cash flows from financing activities:
Proceeds from revolving line of credit	96,235
Payments on mortgage loans	(1,265)	(43)
Payment of term loan		(2,500)
Payments on revolving line of credit	(100,142)
Proceeds from exercise of options and purchase of stock under the Employee Stock Purchase Program	12,643	2,749
Net cash provided by financing activities	7,471	206
(Decrease) increase in cash and cash equivalents	(42,457)	152,303
Cash, cash equivalents and restricted cash:
Beginning of period	599,827	122,556
End of period	557,370	274,859
Supplemental disclosure of cash flow information:
Cash paid during the period for interest	853	291
Supplemental schedule of non-cash investing activities and financing activities:
Shares withheld for taxes		165
Reclassification of long term investments to short term investments	9,980
Intangible assets included in accrued expenses	202	179
Accrual for debt issuance costs related to the term loans		400
Property and equipment included in accrued expenses	$ 2,980	$ 330

ORGANIZATION AND NATURE OF BUSINESS	3 Months Ended
ORGANIZATION AND NATURE OF BUSINESS	Mar. 31, 2018
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 United States Food and Drug Administration (“FDA”) on September 19, 2016. EXONDYS 51 is indicated for the treatment of Duchenne muscular dystrophy (“DMD”) in patients who have a confirmed mutation of the 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”). In November 2016, the Company submitted a marketing authorization application (“MAA”) for eteplirsen to the European Medicines Agency (“EMA”) and the application was validated in December 2016. The Company continues to work with the EMA during their review process and anticipate they will complete their review and make a final decision on the approvability of the Company’s MAA for eteplirsen in the first half of 2018. The Company has also initiated a market access program (“MAP”) for eteplirsen in select countries in Europe, North America, South America and Asia where it currently has not been approved. The MAP provides a mechanism through which physicians can prescribe eteplirsen, within their professional responsibility, to patients who meet pre-specified medical and other criteria and can secure funding. The Company has commenced shipments through the MAP and continue to expand the MAP to include more countries. In addition, the Company contracted with third party distributors and service providers to distribute eteplirsen in certain areas outside the U.S., such as Israel and certain countries in the Middle East, on a named patient basis. As of March 31, 2018, the Company had approximately $1,049.8 million of cash, cash equivalents and investments, consisting of $557.2 million of cash and cash equivalents, $491.8 million of short-term investments, and $0.8 million of restricted cash and investments. The Company believes that its balance of cash, cash equivalents and investments as of the date of the issuance of this report 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

3 Months Ended

Mar. 31, 2018

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 unaudited condensed 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 unaudited condensed 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.

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 equivalents and investments held at financial institutions.

As of March 31, 2018, 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 45 and 120 days. Three individual customers accounted for 44%, 34% and 18% of net product revenues and 60%, 22% and 9% 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 March 31, 2018, the Company believes that such customers are of high credit quality.

As of March 31, 2018 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.

Significant Accounting Policies

For details about the Company’s accounting policies, please read Note 2, Summary of Significant Accounting Policies and Recent Accounting Pronouncements of the Annual Report on Form 10-K for the year ended December 31, 2017.

The Company has adopted Accounting Standards Codification Topic 606, “Revenue from Contracts with Customers” (“ASC 606”) effective as of January 1, 2018. The Company has chosen to use the full retrospective transition method, under which it is required to revise its consolidated financial statements for the years ended December 31, 2016 and 2017 as well as any applicable interim periods within those years, as if ASC 606 had been effective for those periods. Under ASC 606, the Company recognizes revenue when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the Company expects to receive in exchange for the goods or services provided. To determine revenue recognition for arrangements within the scope of ASC 606, the Company performs the following five steps: (1) identify the contracts with a customer; (2) identify the performance obligations in the contract; (3) determine the transaction price; (4) allocate the transaction price to the performance obligations in the contract; and (5) recognize revenue when or as the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when or as the performance obligation is satisfied. For all contracts that fall into the scope of ASC 606, only one performance obligation has been identified by the Company: to timely deliver drug products to the customer’s designated warehouses.

Product Revenues

The Company distributes its product principally through a limited number of specialty distributor and specialty pharmacies in the U.S. and certain distributors in the European Union (“EU”), Israel and Middle East (collectively, “Customers”). The Customers subsequently resell the product to patients and health care providers. The Company provides no right of return to the Customers except in cases of shipping error or product defect. Product revenues are recognized when the Customers take control of the product, which typically occurs upon delivery to the Customers. For the three months ended March 31, 2018, majority of the revenues recognized were generated by the specialty distributor and specialty pharmacies in the U.S.

Variable Consideration

Product revenues are recorded at the net sales price (“transaction price”) which includes estimated reserves for variable consideration, such as Medicaid rebates, governmental chargebacks, including Public Health Service (“PHS”) chargebacks, prompt payment discounts, co-pay assistance and distribution fees. These reserves are based on the amounts earned or to be claimed on the related sales and are classified as reductions of accounts receivable (if no payment is required by the Company) or a current liability (if a payment is required by the Company). These reserves reflect the Company’s best estimates of the amount of consideration to which it is entitled based on the terms of the contracts. Additional details relating to variable consideration follows:

•

Medicaid rebates relate to the Company’s estimated obligations to states under established reimbursement arrangements. Rebate reserves 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 PHS chargebacks, relate to 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 recorded in the same period the related revenue is recognized, resulting in a reduction of 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 the Company’s estimated obligations for credits to be granted to a specialty pharmacy for remitting payment on its purchases within established incentive periods. Reserves for prompt payment discounts are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and accounts receivable.

•

Co-pay assistance relates to financial assistance provided to qualified patients, whereby the Company may assist them with prescription drug co-payments required by the patient’s insurance provider. Reserves for co-pay assistance 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.

•

Distribution fees relate to fees paid to Customers in the distribution channel that provide the Company with inventory management, data and distribution services and are generally accounted for as a reduction of revenue. To the extent that the services received are distinct from the Company’s sale of products to the Customer, these payments are accounted for as selling, general and administrative expenses.

The impact of adopting ASC 606 was not material. There have not been any other material changes to the Company’s accounting policies as of March 31, 2018.

Recent Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board 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 March 31, 2018, 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 March 2017, the Financial Accounting Standards Board issued ASU No. 2017-08, “Receivables - Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities”. This new standard amends the amortization period for certain purchased callable debt securities held at a premium by shortening the amortization period for the premium to the earliest call date. ASU No. 2017-02 will be effective for fiscal years beginning after December 15, 2018, with early adoption permitted. As of March 31, 2018, we are currently evaluating the potential impact that this new standard may have on our financial position and results of operations.

Reclassification

The Company has revised the presentation as well as the caption of certain items within the unaudited condensed consolidated balance sheets to conform to the current period presentation. “Restricted cash and investments” of $0.8 million and “deferred revenue” of $3.3 million as December 31, 2017 are grouped into “other assets” and “other current liabilities”, respectively. These revisions had no impact on total assets nor total liabilities.

Additionally, the Company has revised the presentation as well as caption of certain items within the unaudited condensed consolidated statements of operations and comprehensive loss to conform to the 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 unaudited condensed consolidated statements of operations and comprehensive loss. The reclassification had no impact on operating loss or net income.

Furthermore, the Company has also revised the presentation as well as caption of certain items within the unaudited condensed consolidated statements of cash flows to conform to the current period presentation. “Accretion of discount on available-for-sale securities” of $0.1 million and “Non-cash interest expense” of approximately $0.1 million are presented separately in the unaudited condensed consolidated statements of cash flows. These revisions had no impact on the net cash used in operating activities or cash, cash equivalents and restricted cash at end of period.

FAIR VALUE MEASUREMENTS

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

FAIR VALUE MEASUREMENTS

3. 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 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 valuation techniques it utilizes to determine such fair value:

	Fair Value Measurement as of March 31, 2018
	Total		Level 1		Level 2		Level 3
	(in thousands)
Assets
Cash equivalents	$	370,052	$	370,052	$	—	$	—
Commercial paper		178,875		—		178,875		—
Government and government agency bonds		270,119		270,119		—		—
Corporate bonds		92,195		92,195		—		—
Certificates of deposit		648		648		—		—
Total assets	$	911,889	$	733,014	$	178,875	$	—

	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	$	—

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 publically traded fixed income securities and are presented as cash equivalents on the unaudited condensed consolidated balance sheets as of March 31, 2018.

The Company’s assets with fair value categorized as Level 2 within the fair value hierarchy consist of commercial paper and government and government agency 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 unaudited condensed 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

3 Months Ended

Mar. 31, 2018

Cash And Cash Equivalents [Abstract]

CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES

4. 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 unaudited condensed consolidated balance sheets for each of the periods indicated:

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Money market funds		370,052		352,370
Corporate bonds		24,451		16,720
Government and government agency bonds		24,981		49,972
Total		419,484		419,062

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 March 31, 2018 and December 31, 2017 was approximately three and seven months, respectively.

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

	As of March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Market Value
	(in thousands)
Cash and money market funds	$	507,802	$	—	$	—		$	507,802
Commercial paper		178,875		—		—			178,875
Government and government agency bonds		270,422		1		(304	)		270,119
Corporate bonds		92,535		—		(340	)		92,195
Total assets	$	1,049,634	$	1	$	(644	)	$	1,048,991
As reported:
Cash and cash equivalents	$	557,233	$	1	$	—		$	557,234
Short-term investments	$	492,401	$	—	$	(644	)	$	491,757
Total assets	$	1,049,634	$	1	$	(644	)	$	1,048,991

	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

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES

3 Months Ended

Mar. 31, 2018

Receivables Net Current [Abstract]

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES

5. 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 the EU, Israel and the 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 March 31, 2018, 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 March 31, 2018		As of December 31, 2017
	(in thousands)
Product sales, net of discounts and allowances	$	38,919	$	28,539
Government contract receivables		929		929
Total accounts receivable	$	39,848	$	29,468

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

	Chargebacks			Rebates			Prompt Pay			Other			Total
	(in thousands)
Balance, as of December 31, 2017	$	995		$	6,959		$	169		$	464		$	8,587
Provision		2,790			5,159			495			1,534			9,978
Payments/credits		(2,916	)		(989	)		(435	)		(146	)		(4,486	)
Balance, as of March 31, 2018	$	869		$	11,129		$	229		$	1,852		$	14,079

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

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Reduction to accounts receivable	$	1,367	$	1,285
Component of accrued expenses		12,712		7,302
Total reserves	$	14,079	$	8,587

INVENTORY

3 Months Ended

Mar. 31, 2018

Inventory Disclosure [Abstract]

INVENTORY

6. 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 the period indicated:

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Raw materials	$	63,494	$	53,875
Work in progress		34,609		27,442
Finished goods		1,272		2,288
Total inventory	$	99,375	$	83,605

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.

ASSET HELD FOR SALE	3 Months Ended
ASSET HELD FOR SALE	Mar. 31, 2018
Asset Held For Sale Current [Abstract]
ASSET HELD FOR SALE	7. ASSET HELD FOR SALE The Company owned a facility located at 1749 SW Airport Avenue, Corvallis, OR (“Airport Facility”). The Airport Facility was previously leased to an unrelated third party. In July 2016, the third party lessee terminated the lease and vacated the facility. The Company set up a program to actively market the Airport Facility. Accordingly, the Airport Facility with net book value of approximately $1.5 million was reclassified as an asset held for sale which is presented as a component of other current assets. In August 2017, the Company entered into a purchase and sale agreement with an unrelated third-party buyer. The sale price of as well as fees related to the Airport Facility were approximately $1.5 million and $0.2 million, respectively. The transaction was completed in January 2018 when cash was received and the two mortgage loans associated with the facility were repaid. For more information regarding the two mortgage loans, please read Note 10, Indebtedness.

OTHER CURRENT ASSETS AND OTHER NON-CURRENT ASSETS

3 Months Ended

Mar. 31, 2018

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

OTHER CURRENT ASSETS AND OTHER NON-CURRENT ASSETS

8. OTHER CURRENT ASSETS AND OTHER NON-CURRENT ASSETS

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

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Manufacturing-related deposits and prepaids	$	15,049	$	18,650
Prepaid clinical and preclinical expenses		6,103		5,175
Prepaid maintenance and license fees		2,442		1,711
Prepaid research expenses		2,140		2,896
Prepaid commercial expenses		883		1,589
Asset held for sale		—		1,501
Other prepaids		2,186		2,726
Other		2,400		2,263
Total other current assets	$	31,203	$	36,511

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

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Prepaid clinical expenses	$	6,151	$	7,488
Alternative minimum tax credit		3,315		3,315
Manufacturing-related deposits		1,975		—
Restricted cash and investments		784		784
Long-term available-for-sale securities		—		9,980
Other		241		242
Total other non-current assets	$	12,466	$	21,809

ACCRUED EXPENSES

3 Months Ended

Mar. 31, 2018

Payables And Accruals [Abstract]

ACCRUED EXPENSES

9. ACCRUED EXPENSES

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

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Product revenue related reserves	$	12,712	$	7,302
Accrued clinical and preclinical costs		10,924		15,975
Accrued professional fees		10,351		6,794
Accrued employee compensation costs		7,906		14,402
Accrued contract manufacturing costs		7,025		14,019
Accrued interest expense		3,411		1,291
Accrued collaboration cost sharing		3,197		—
Accrued BioMarin royalties		3,117		2,846
Accrued property and equipment		2,980		2,525
Accrued income taxes		943		943
Accrued research costs		226		401
Other		2,856		2,484
Total accrued expenses	$	65,648	$	68,982

INDEBTEDNESS

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

INDEBTEDNESS

10. 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. The Company recorded a total debt discount of $171.8 million upon issuance of the 2024 Notes, consisting of an equity component of $161.2 million and debt issuance costs of $10.6 million. The debt discount is being amortized under the effective interest method and recorded as additional non-cash interest expense over the life of the 2024 Notes. The effective interest rate on the liability component of the 2024 Notes for the year ended December 31, 2017 was 6.9%. The fair value of the 2024 Notes is $713.0 million as of March 31, 2018. It is based on open market trades and is classified as level 1 in the fair value hierarchy.

Term Loan

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”). 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 was in compliance with all affirmative and negative covenants associated with the Amended and Restated Credit and Security Agreement at March 31, 2018.

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. 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.

Mortgage Loans

The Company had two loans outstanding which bear interest at 4.75%, mature in February 2027 and are collateralized by the Airport Facility in Corvallis, Oregon. In connection with the sale of the Airport Facility in January 2018, the two long-term mortgage loans were repaid.

As of March 31, 2018, the Company recorded approximately $3.4 million as current portion of long-term debt and approximately $427.4 million as long-term debt on the unaudited condensed consolidated balance sheets related to the 2024 convertible notes, the term loan, the revolving line of credit and the mortgage loans. The following table summarizes the Company’s debt facilities for the periods indicated:

	As of March 31, 2018			As of December 31, 2017
	(in thousand)
Par value of the 2024 Notes		570,000			570,000
Unamortized discounts		(154,340	)		(158,890	)
Debt issuance expenses		(10,151	)		(10,450	)
Net carrying value of convertible debt		405,509			400,660
Other debt facilities		25,302			30,391
Net carrying value of total debt facilities		430,811			431,051

For the three months ended March 31, 2018 and 2017, the Company recorded $7.6 million and $0.3 million in interest expense, respectively.

RESTRUCTURING

3 Months Ended

Mar. 31, 2018

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 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 December 31, 2017, the relocations and terminations 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 $1.5 million and $2.3 million, in restructuring expenses for the second and the first floor, respectively. As of March 31, 2018, the Company continues to be obligated to make $4.5 million of minimum lease payments and certain other contractual maintenance costs for the whole facility.

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

	For the Three Months Ended March 31, 2018			For the Year Ended December 31, 2017
	(in thousands)
Restructuring reserve beginning balance	$	2,933		$	1,588
Restructuring expenses incurred during the period		—			3,020
Amounts paid during the period		(355	)		(1,675	)
Restructuring reserve ending balance	$	2,578		$	2,933

STOCK-BASED COMPENSATION

3 Months Ended

Mar. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

STOCK-BASED COMPENSATION

12. STOCK-BASED COMPENSATION

The following table summarizes the Company’s stock awards granted for each of the periods indicated:

	For the Three Months Ended March 31,
	2018				2017
	Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value
Stock options		1,124,585	$	34.08		877,492	$	16.30
Restricted stock units		150,725	$	71.45		161,029	$	32.63
Restricted stock awards		17,090	$	71.45		6,500	$	31.06

Stock-based Compensation Expense

For the three months ended March 31, 2018 and 2017, total stock-based compensation expense was $10.5 million and $5.7 million, respectively. The following table summarizes stock-based compensation expense by function included within the unaudited condensed consolidated statements of operations and comprehensive loss:

	For the Three Months Ended March 31,
	2018		2017
	(in thousands)
Research and development	$	2,060	$	1,874
Selling, general and administrative		8,466		3,838
Total stock-based compensation expense	$	10,526	$	5,712

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

	For the Three Months Ended March 31,
	2018		2017
	(in thousands)
Stock options	$	8,973	$	5,038
Restricted stock awards/units		1,260		204
Employee stock purchase plan ("ESPP")		293		470
Total stock-based compensation expense	$	10,526	$	5,712

OTHER INCOME AND LOSS

3 Months Ended

Mar. 31, 2018

Other Income And Expenses [Abstract]

OTHER INCOME AND LOSS

13. OTHER INCOME AND LOSS

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

	For the Three Months Ended March 31,
	2018			2017
	(in thousand)
Interest expenses	$	(7,634	)	$	(257	)
Interest income		1,872			186
Amortization of investment discount		1,259			100
Other income		18			306
Gain from sale of Priority Review Voucher		—			125,000
Total other (loss) income	$	(4,485	)	$	125,335

INCOME TAXES	3 Months Ended
INCOME TAXES	Mar. 31, 2018
Income Tax Disclosure [Abstract]
INCOME TAXES	14. INCOME TAXES The Company’s tax provision for interim periods is determined using an estimate of its annual effective tax rate, adjusted for discrete items arising in that quarter. In each quarter, the Company updates its estimate of the annual effective tax rate, and if the estimated annual tax rate changes, the Company makes a cumulative adjustment in that quarter. For the three months ended March 31, 2018, the Company recorded a provision for income taxes of $0.1 million, representing an effective tax rate of 0.4%. The Company’s estimated annual effective tax rate is lower than the federal statutory rate due to the jurisdictional mix of earnings and the release of valuation allowance against its federal and state tax attributes which can be used to offset current year earnings. For the three months ended March 31, 2017, the Company recorded a provision for income taxes of $2.0 million, representing an effective tax rate of 2.3%. The Company computed its tax provision for the three months ended March 31, 2017 based upon the year-to-date effective tax rate as opposed to an estimated annual effective tax rate. The Company concluded that the year-to-date effective tax rate was the most appropriate method to use for the three months ended March 31, 2017, given a reliable estimate of the annual effective tax rate could not be made.

NET (LOSS) EARNINGS PER SHARE

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

NET (LOSS) EARNINGS PER SHARE

15. NET (LOSS) EARNINGS PER SHARE

Basic net (loss) earnings per share is computed by dividing net (loss) income 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 generated a net loss for the three month period ended March 31, 2018, there is no difference between basic and diluted net loss per share since the effect of common stock equivalents would be anti-dilutive and, therefore, would be excluded from the diluted net loss per share calculation.

	For the Three Months Ended March 31,
	2018			2017
	(in thousands, except per share amounts)
Net (loss) income	$	(35,363	)	$	84,090
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) earnings per share		64,631			54,850
Dilutive effect of outstanding stock awards and stock options after application of the treasury stock method*		—			1,162
Weighted-average number of shares of common stock and dilutive common stock equivalents outstanding for computing diluted loss per share		64,631			56,012
Net (loss) earnings per share - basic and diluted
Basic (loss) earnings per share	$	(0.55	)	$	1.53
Diluted (loss) earnings per share	$	(0.55	)	$	1.50

For the three months ended March 31, 2018, stock options, RSAs, RSUs, stock appreciation rights and potentially issuable stock for ESPP to purchase 9.9 million shares of the Company’s common stock were excluded from the net loss per share calculation as their effect would have been anti-dilutive. For the three months ended March 31, 2017, out of money stock options, unvested performance-based RSUs and RSAs whose performance milestones were not achieved and potentially issuable stock for ESPP to purchase approximately 3.4 million were excluded from the net earnings per share calculation as their effect would have been anti-dilutive.

COMMITMENTS AND CONTINGENCIES	3 Months Ended
COMMITMENTS AND CONTINGENCIES	Mar. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	16. COMMITMENTS AND CONTINGENCIES 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. The period for filing a petition with the U.S. Supreme Court for a writ of certiorari has elapsed without a filing from the plaintiffs. As such, there is no risk of loss in connection with this litigation. 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. Oral argument took place on December 4, 2017 and the First Circuit affirmed the District Court’s dismissal of this case on April 4, 2018. Plaintiffs did not file a petition for rehearing before the operative deadline. As such, the risk of loss is negligible, barring a successful and highly unlikely petition for a writ of certiorari from the U.S. Supreme Court. 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.

SUBSEQUENT EVENTS	3 Months Ended
SUBSEQUENT EVENTS	Mar. 31, 2018
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	17. SUBSEQUENT EVENTS On April 27, 2018, the Company entered into a seventh amendment (the “Amendment”) to its Cambridge, Massachusetts headquarters lease which extended the original term of the lease to September 30, 2025 and increased the total rental space by approximately 63,698 square feet. The Company expects to incur an incremental $54.4 million in rent expense as a result of the Amendment through the remainder of the extended lease term. On May 3, 2018, the Company and Myonexus Therapeutics, Inc. (“Myonexus”) entered into a warrant to purchase common stock of Myonexus, which, in combination with amendments to the Myonexus certificate of incorporation, provides the Company with an exclusive option to acquire Myonexus. The Company may exercise the option at any time prior to the 60th day after Myonexus: (i) completes a successful biopsy analysis on each of the patients within cohort 2 capable of providing a biopsy at the end of the 60-day period of Myonexus’ Phase 1/2A clinical trial of MYO-101, and (ii) provides the Company with the study’s final biopsy results, subject to certain conditions. The Company is liable for a $60 million non-refundable up-front payment upon execution of the agreement and may be obligated to make up to $45 million in development milestone payments. An additional payment would be required if and when the Company exercises the exclusive option to acquire Myonexus.

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

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Estimates and Uncertainties

Concentration of Credit Risk

Revenue Recognition

Product Revenues

Variable Consideration

•

The impact of adopting ASC 606 was not material. There have not been any other material changes to the Company’s accounting policies as of March 31, 2018.

Recent Accounting Pronouncements

Reclassification

FAIR VALUE MEASUREMENTS (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Assets Measured and Carried at Fair Value

	Fair Value Measurement as of March 31, 2018
	Total		Level 1		Level 2		Level 3
	(in thousands)
Assets
Cash equivalents	$	370,052	$	370,052	$	—	$	—
Commercial paper		178,875		—		178,875		—
Government and government agency bonds		270,119		270,119		—		—
Corporate bonds		92,195		92,195		—		—
Certificates of deposit		648		648		—		—
Total assets	$	911,889	$	733,014	$	178,875	$	—

	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	$	—

CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES (Tables)

3 Months Ended

Mar. 31, 2018

Cash And Cash Equivalents [Abstract]

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

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Money market funds		370,052		352,370
Corporate bonds		24,451		16,720
Government and government agency bonds		24,981		49,972
Total		419,484		419,062

Summary of Company Cash, Cash Equivalents and Short-Term Investments

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

	As of March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Market Value
	(in thousands)
Cash and money market funds	$	507,802	$	—	$	—		$	507,802
Commercial paper		178,875		—		—			178,875
Government and government agency bonds		270,422		1		(304	)		270,119
Corporate bonds		92,535		—		(340	)		92,195
Total assets	$	1,049,634	$	1	$	(644	)	$	1,048,991
As reported:
Cash and cash equivalents	$	557,233	$	1	$	—		$	557,234
Short-term investments	$	492,401	$	—	$	(644	)	$	491,757
Total assets	$	1,049,634	$	1	$	(644	)	$	1,048,991

	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

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES (Tables)

3 Months Ended

Mar. 31, 2018

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 March 31, 2018		As of December 31, 2017
	(in thousands)
Product sales, net of discounts and allowances	$	38,919	$	28,539
Government contract receivables		929		929
Total accounts receivable	$	39,848	$	29,468

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			Total
	(in thousands)
Balance, as of December 31, 2017	$	995		$	6,959		$	169		$	464		$	8,587
Provision		2,790			5,159			495			1,534			9,978
Payments/credits		(2,916	)		(989	)		(435	)		(146	)		(4,486	)
Balance, as of March 31, 2018	$	869		$	11,129		$	229		$	1,852		$	14,079

Summary of Total Reserves Included in Consolidated Balance Sheets

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

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Reduction to accounts receivable	$	1,367	$	1,285
Component of accrued expenses		12,712		7,302
Total reserves	$	14,079	$	8,587

INVENTORY (Tables)

3 Months Ended

Mar. 31, 2018

Inventory Disclosure [Abstract]

Summary of Components of Inventory

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

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Raw materials	$	63,494	$	53,875
Work in progress		34,609		27,442
Finished goods		1,272		2,288
Total inventory	$	99,375	$	83,605

OTHER CURRENT ASSETS AND OTHER NON-CURRENT ASSETS (Tables)

3 Months Ended

Mar. 31, 2018

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 March 31, 2018		As of December 31, 2017
	(in thousands)
Manufacturing-related deposits and prepaids	$	15,049	$	18,650
Prepaid clinical and preclinical expenses		6,103		5,175
Prepaid maintenance and license fees		2,442		1,711
Prepaid research expenses		2,140		2,896
Prepaid commercial expenses		883		1,589
Asset held for sale		—		1,501
Other prepaids		2,186		2,726
Other		2,400		2,263
Total other current assets	$	31,203	$	36,511

Summary of Other Non-current Assets

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

	As of March 31, 2018		As of December 31, 2017
	(in thousands)
Prepaid clinical expenses	$	6,151	$	7,488
Alternative minimum tax credit		3,315		3,315
Manufacturing-related deposits		1,975		—
Restricted cash and investments		784		784
Long-term available-for-sale securities		—		9,980
Other		241		242
Total other non-current assets	$	12,466	$	21,809

ACCRUED EXPENSES (Tables)

3 Months Ended

Mar. 31, 2018

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 March 31, 2018		As of December 31, 2017
	(in thousands)
Product revenue related reserves	$	12,712	$	7,302
Accrued clinical and preclinical costs		10,924		15,975
Accrued professional fees		10,351		6,794
Accrued employee compensation costs		7,906		14,402
Accrued contract manufacturing costs		7,025		14,019
Accrued interest expense		3,411		1,291
Accrued collaboration cost sharing		3,197		—
Accrued BioMarin royalties		3,117		2,846
Accrued property and equipment		2,980		2,525
Accrued income taxes		943		943
Accrued research costs		226		401
Other		2,856		2,484
Total accrued expenses	$	65,648	$	68,982

INDEBTEDNESS (Tables)

3 Months Ended

Mar. 31, 2018

Debt Disclosure [Abstract]

Summary of Debt Facilities

	As of March 31, 2018			As of December 31, 2017
	(in thousand)
Par value of the 2024 Notes		570,000			570,000
Unamortized discounts		(154,340	)		(158,890	)
Debt issuance expenses		(10,151	)		(10,450	)
Net carrying value of convertible debt		405,509			400,660
Other debt facilities		25,302			30,391
Net carrying value of total debt facilities		430,811			431,051

- 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

RESTRUCTURING (Tables)

3 Months Ended

Mar. 31, 2018

Restructuring And Related Activities [Abstract]

Summary of Restructuring Reserve

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

	For the Three Months Ended March 31, 2018			For the Year Ended December 31, 2017
	(in thousands)
Restructuring reserve beginning balance	$	2,933		$	1,588
Restructuring expenses incurred during the period		—			3,020
Amounts paid during the period		(355	)		(1,675	)
Restructuring reserve ending balance	$	2,578		$	2,933

STOCK-BASED COMPENSATION (Tables)

3 Months Ended

Mar. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Awards Granted

The following table summarizes the Company’s stock awards granted for each of the periods indicated:

	For the Three Months Ended March 31,
	2018				2017
	Grants		Weighted Average Grant Date Fair Value		Grants		Weighted Average Grant Date Fair Value
Stock options		1,124,585	$	34.08		877,492	$	16.30
Restricted stock units		150,725	$	71.45		161,029	$	32.63
Restricted stock awards		17,090	$	71.45		6,500	$	31.06

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

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

	For the Three Months Ended March 31,
	2018		2017
	(in thousands)
Research and development	$	2,060	$	1,874
Selling, general and administrative		8,466		3,838
Total stock-based compensation expense	$	10,526	$	5,712

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

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

	For the Three Months Ended March 31,
	2018		2017
	(in thousands)
Stock options	$	8,973	$	5,038
Restricted stock awards/units		1,260		204
Employee stock purchase plan ("ESPP")		293		470
Total stock-based compensation expense	$	10,526	$	5,712

OTHER INCOME AND LOSS (Tables)

3 Months Ended

Mar. 31, 2018

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 Three Months Ended March 31,
	2018			2017
	(in thousand)
Interest expenses	$	(7,634	)	$	(257	)
Interest income		1,872			186
Amortization of investment discount		1,259			100
Other income		18			306
Gain from sale of Priority Review Voucher		—			125,000
Total other (loss) income	$	(4,485	)	$	125,335

NET (LOSS) EARNINGS PER SHARE (Tables)

3 Months Ended

Mar. 31, 2018

Earnings Per Share [Abstract]

Basic and Diluted Net (Loss) Earnings Per Share

Given that the Company generated a net loss for the three month period ended March 31, 2018, there is no difference between basic and diluted net loss per share since the effect of common stock equivalents would be anti-dilutive and, therefore, would be excluded from the diluted net loss per share calculation.

	For the Three Months Ended March 31,
	2018			2017
	(in thousands, except per share amounts)
Net (loss) income	$	(35,363	)	$	84,090
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) earnings per share		64,631			54,850
Dilutive effect of outstanding stock awards and stock options after application of the treasury stock method*		—			1,162
Weighted-average number of shares of common stock and dilutive common stock equivalents outstanding for computing diluted loss per share		64,631			56,012
Net (loss) earnings per share - basic and diluted
Basic (loss) earnings per share	$	(0.55	)	$	1.53
Diluted (loss) earnings per share	$	(0.55	)	$	1.50

Organization and Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Cash, cash equivalents and investments	$ 1,049,800
Cash and cash equivalents	557,234	$ 599,691
Short-term investments	491,757	479,369
Restricted cash and investments	$ 784	$ 784

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Segment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	1
Restricted cash and investments	$ 784,000		$ 784,000
Amortization of in-licensed rights	216,000	$ 29,000
Accretion of discount on available-for-sale securities	1,259,000	100,000
Non-cash interest expense	4,940,000	$ 82,000
Scenario, Previously Reported [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash and investments			800,000
Deferred revenue			3,300,000
Restatement Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Accretion of discount on available-for-sale securities	100,000
Non-cash interest expense	$ 100,000
Product Revenues [Member] \| Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	44.00%
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	18.00%
Accounts Receivable [Member] \| Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	60.00%
Accounts Receivable [Member] \| Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	22.00%
Accounts Receivable [Member] \| Customer Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	9.00%
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	45 days
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term	60 days
Maximum [Member] \| Restatement Adjustment [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

Assets Measured and Carried at Fair Value (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 911,889	$ 909,059
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	370,052	352,370
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	178,875	133,368
Government and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	270,119	294,717
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	92,195	127,956
Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	648	648
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	733,014	765,719
Level 1 [Member] \| Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	370,052	352,370
Level 1 [Member] \| Government and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	270,119	284,745
Level 1 [Member] \| Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	92,195	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	648
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	178,875	143,340
Level 2 [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 178,875	133,368
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

Summary of Company Financial Assets with Maturities of Less Than 90 Days Included in Cash Equivalents (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 419,484	$ 419,062
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	370,052	352,370
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	24,451	16,720
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 24,981	$ 49,972

Cash, Cash Equivalents and Marketable Securities - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Weighted average maturity period of available-for-sale securities	3 months	7 months

Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	$ 1,049,634	$ 1,089,419
Cash, cash equivalents and short-term investments, Gross unrealized gains	1	2
Cash, cash equivalents and short-term investments, Gross unrealized losses	(644)	(381)
Cash, cash equivalents and short-term investments, Fair market value	1,048,991	1,089,040
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	507,802	532,999
Cash, cash equivalents and short-term investments, Fair market value	507,802	532,999
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	178,875	133,368
Cash, cash equivalents and short-term investments, Fair market value	178,875	133,368
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	270,422	294,915
Cash, cash equivalents and short-term investments, Gross unrealized gains	1	2
Cash, cash equivalents and short-term investments, Gross unrealized losses	(304)	(200)
Cash, cash equivalents and short-term investments, Fair market value	270,119	294,717
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	92,535
Cash, cash equivalents and short-term investments, Gross unrealized losses	(340)
Cash, cash equivalents and short-term investments, Fair market value	92,195
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	557,233	599,698
Cash, cash equivalents and short-term investments, Gross unrealized gains	1	2
Cash, cash equivalents and short-term investments, Gross unrealized losses		(9)
Cash, cash equivalents and short-term investments, Fair market value	557,234	599,691
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	492,401	479,705
Cash, cash equivalents and short-term investments, Gross unrealized losses	(644)	(336)
Cash, cash equivalents and short-term investments, Fair market value	$ 491,757	479,369
Corporate Bonds - Current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		118,121
Cash, cash equivalents and short-term investments, Gross unrealized losses		(145)
Cash, cash equivalents and short-term investments, Fair market value		117,976
Corporate Bonds - Non-current [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		10,016
Cash, cash equivalents and short-term investments, Gross unrealized losses		(36)
Cash, cash equivalents and short-term investments, Fair market value		9,980
Long-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		10,016
Cash, cash equivalents and short-term investments, Gross unrealized losses		(36)
Cash, cash equivalents and short-term investments, Fair market value		$ 9,980

Accounts Receivable and Reserves for Product Sales - Summary of Components of Accounts Receivable (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Receivables Net Current [Abstract]
Product sales, net of discounts and allowances	$ 38,919	$ 28,539
Government contract receivables	929	929
Total accounts receivable	$ 39,848	$ 29,468

Accounts Receivable and Reserves for Product Sales - Summary of Change in Reserves for Discounts and Allowances (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	$ 8,587
Provision	9,978
Payments/credits	(4,486)
Balance, as of March 31, 2018	14,079
Chargebacks [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	995
Provision	2,790
Payments/credits	(2,916)
Balance, as of March 31, 2018	869
Rebates [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	6,959
Provision	5,159
Payments/credits	(989)
Balance, as of March 31, 2018	11,129
Prompt Pay [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	169
Provision	495
Payments/credits	(435)
Balance, as of March 31, 2018	229
Other [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	464
Provision	1,534
Payments/credits	(146)
Balance, as of March 31, 2018	$ 1,852

Accounts Receivable and Reserves for Product Sales - Summary of Total Reserves Included in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Receivables Net Current [Abstract]
Reduction to accounts receivable	$ 1,367	$ 1,285
Component of accrued expenses	12,712	7,302
Total reserves	$ 14,079	$ 8,587

Inventory - Summary of Components of Inventory (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 63,494	$ 53,875
Work in progress	34,609	27,442
Finished goods	1,272	2,288
Total inventory	$ 99,375	$ 83,605

Asset Held for Sale - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Aug. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Jul. 31, 2016
Asset Held For Sale Current [Abstract]
Net book value of airport facility reclassified as an assets held for sale, current			$ 1,501	$ 1,500
Sale price of airport facility	$ 1,500
Fees related to sale of airport facility	$ 200
Sale of airport facility date		2018-01

Summary of Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Jul. 31, 2016
Prepaid Expense And Other Assets Current [Abstract]
Manufacturing-related deposits and prepaids	$ 15,049	$ 18,650
Prepaid clinical and preclinical expenses	6,103	5,175
Prepaid maintenance and license fees	2,442	1,711
Prepaid research expenses	2,140	2,896
Prepaid commercial expenses	883	1,589
Asset held for sale		1,501	$ 1,500
Other prepaids	2,186	2,726
Other	2,400	2,263
Total other current assets	$ 31,203	$ 36,511

Summary of Other Non-current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Noncurrent [Abstract]
Prepaid clinical expenses	$ 6,151	$ 7,488
Alternative minimum tax credit	3,315	3,315
Manufacturing-related deposits	1,975
Restricted cash and investments	784	784
Long-term available-for-sale securities		9,980
Other	241	242
Total other non-current assets	$ 12,466	$ 21,809

- 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

Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Product revenue related reserves	$ 12,712	$ 7,302
Accrued clinical and preclinical costs	10,924	15,975
Accrued professional fees	10,351	6,794
Accrued employee compensation costs	7,906	14,402
Accrued contract manufacturing costs	7,025	14,019
Accrued interest expense	3,411	1,291
Accrued collaboration cost sharing	3,197
Accrued BioMarin royalties	3,117	2,846
Accrued property and equipment	2,980	2,525
Accrued income taxes	943	943
Accrued research costs	226	401
Other	2,856	2,484
Total accrued expenses	$ 65,648	$ 68,982

Indebtedness - Additional Information (Detail)		1 Months Ended	3 Months Ended
Indebtedness - Additional Information (Detail)	Nov. 14, 2017 USD ($) shares $ / shares	Jul. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) Loan	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Sale of Airport Facility date			2018-01
Long-term debt			$ 427,365,000		$ 424,876,000
Convertible Debt, The Term Loan, The Revolver and The Mortgage Loans [Member]
Debt Instrument [Line Items]
Current portion of long-term debt			3,400,000
Long-term debt			427,400,000
Interest expense related to debt facilities			7,600,000	$ 300,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 570,000,000		$ 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
Debt discount	$ 171,800,000		$ 154,340,000		$ 158,890,000
Proceeds for issuance of notes recorded to equity component	161,200,000
Debt issuance costs	$ 10,600,000
Effective interest rate percentage					6.90%
2024 Convertible Notes [Member] \| Level 1 [Member]
Debt Instrument [Line Items]
Debt fair value			$ 713,000,000
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
Monthly principal payments commencement date		Jul. 01, 2018
Periodic principal payment frequency		monthly
Monthly principal payments		$ 800,000
Credit facility maturity date		2021-07
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
Credit facility maturity date		2021-07
Line of credit facility additional tranche that may be increased		$ 20,000,000
Origination fee		$ 200,000
Origination fee, percentage		0.50%
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 Debt Facilities (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Nov. 14, 2017
Debt Instrument [Line Items]
Other debt facilities	$ 25,302,000	$ 30,391,000
Net carrying value of total debt facilities	430,811,000	431,051,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Par value of the 2024 Notes	570,000,000	570,000,000	$ 570,000,000
Unamortized discounts	(154,340,000)	(158,890,000)	$ (171,800,000)
Debt issuance expenses	(10,151,000)	(10,450,000)
Net carrying value of convertible debt	$ 405,509,000	$ 400,660,000

Restructuring - Additional Information (Detail) - Corvallis Plan [Member] - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	Apr. 30, 2017	Dec. 31, 2016	Mar. 31, 2018
Restructuring Cost And Reserve [Line Items]
Restructuring, initiation date			2016-03
Restructuring, completion date			Dec. 31, 2017
Minimum lease payment obligation			$ 4.5
Facility Closing [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses	$ 2.3	$ 1.5

Summary of Restructuring Reserve (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Restructuring And Related Activities [Abstract]
Restructuring reserve beginning balance	$ 2,933	$ 1,588
Restructuring expenses incurred during the period		3,020
Amounts paid during the period	(355)	(1,675)
Restructuring reserve ending balance	$ 2,578	$ 2,933

Summary of Stock Awards Granted (Detail) - $ / shares	3 Months Ended
Summary of Stock Awards Granted (Detail) - $ / shares	Mar. 31, 2018	Mar. 31, 2017
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Stock options	1,124,585	877,492
Weighted Average Grant Date Fair Value, Stock options	$ 34.08	$ 16.30
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Restricted stock units and awards	150,725	161,029
Weighted Average Grant Date Fair Value, Restricted stock units and awards	$ 71.45	$ 32.63
Restricted Stock Awards (RSAs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Restricted stock units and awards	17,090	6,500
Weighted Average Grant Date Fair Value, Restricted stock units and awards	$ 71.45	$ 31.06

Stock Based Compensation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based compensation expense	$ 10,526	$ 5,712

Summary of Stock-Based Compensation Expense by Function Included within Condensed Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 10,526	$ 5,712
Research and Development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	2,060	1,874
Selling, General and Administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 8,466	$ 3,838

Summary of Stock-Based Compensation Expense by Grant Type Included within Condensed Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 10,526	$ 5,712
Stock Options [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	8,973	5,038
Restricted Stock Awards/Units (RSAs/RSUs) [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	1,260	204
Employee Stock Purchase Plan ("ESPP") [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 293	$ 470

Summary of Other Income and Loss (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Nonoperating Income Expense [Abstract]
Interest expenses	$ (7,634)	$ (257)
Interest income	1,872	186
Amortization of investment discount	1,259	100
Other income	18	306
Gain from sale of Priority Review Voucher		125,000
Other (loss) income	$ (4,485)	$ 125,335

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income tax expense	$ 139	$ 2,000
Effective tax rate	0.40%	2.30%

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

3 Months Ended

Mar. 31, 2018

Mar. 31, 2017

Earnings Per Share [Abstract]

Net (loss) income

$ (35,363)

$ 84,090

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) earnings per share

64,631

54,850

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

^[1]

1,162

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

64,631

56,012

Net (loss) earnings per share - basic and diluted

Basic (loss) earnings per share

$ (0.55)

$ 1.53

Diluted (loss) earnings per share

$ (0.55)

$ 1.50

[1]

Basic and Diluted Net (Loss) Earnings Per Share (Parenthetical) (Detail) - shares shares in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Anti-dilutive securities excluded from computation of net (loss) earnings per share	9.9	3.4

Subsequent Events - Additional Information (Detail) - Subsequent Events [Member] - Lease Agreements [Member] $ in Millions	May 03, 2018 USD ($)	Apr. 27, 2018 USD ($) ft²
Myonexus Therapeutics, Inc. [Member]
Subsequent Event [Line Items]
Up-front payment	$ 60.0
Development milestone payments	$ 45.0
Seventh Amendment [Member]
Subsequent Event [Line Items]
Extended original term of lease		Sep. 30, 2025
Total rental space \| ft²		63,698
Incremental rent expense		$ 54.4