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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 diseases. Applying its proprietary, highly-differentiated and innovative platform technologies, the Company is able to target a broad range of diseases and disorders.

Its first commercial product in the U.S., EXONDYS 51^® (eteplirsen) Injection (“EXONDYS 51”), was granted accelerated approval by the U.S. Food and Drug Administration (the “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. In December 2018, the Company completed the submission of its rolling new drug application (“NDA”) to the FDA seeking accelerated approval for golodirsen, the Company’s PMO chemistry and exon-skipping technology to skip exon 53 of the DMD gene.

As of December 31, 2018, the Company had approximately $1,174.9 million of cash, cash equivalents and investments, consisting of $370.8 million of cash and cash equivalents, $803.1 million of short-term investments, and $1.0 million of a long-term restricted investment. The Company believes that its balance of cash, cash equivalents and investments as of December 31, 2018 is sufficient to fund its current operational plan for at least the next twelve months, though it may pursue additional cash resources through public or private debt and equity financings, seek additional government contracts and establish collaborations with or license its technology to other companies.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS

12 Months Ended

Dec. 31, 2018

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS

2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES AND RECENT ACCOUNTING PRONOUNCEMENTS

Basis of Presentation

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

Estimates and Uncertainties

The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, equity, revenue, expenses and the disclosure of contingent assets and liabilities. Actual results could differ from those estimates.

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:

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

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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

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

The fair value of the majority of the Company’s financial assets is categorized as Level 1 within the fair value hierarchy. These assets include money market funds, publicly traded debt, and equity securities. For additional information related to fair value measurements, please read Note 5, Fair Value Measurements to the consolidated financial statements.

Cash and Cash Equivalents

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

Investments

Available-For-Sale Debt Securities

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

Equity Investments

The Company’s equity investments include its investments in a publicly traded biotechnology company and a privately held biotechnology company and are included in investments and other assets in the Company’s consolidated balance sheets. The equity investment in the publicly traded biotechnology company has a readily determinable fair value and is carried at fair value with changes in value recorded in the Company’s consolidated statements of operations and comprehensive loss. The equity investment in the privately held biotechnology company does not have readily determinable fair value and is measured at cost less any impairment, plus or minus changes resulting from observable price changes for the identical or a similar investment of the same issuer, which is recorded as a gain or loss on the Company’s consolidated statements of operations and comprehensive loss.

Accounts Receivable

The Company’s accounts receivable arise from product sales, government research contracts and other grants. They are generally stated at the invoiced amount and do not bear interest. Revenues from product sales are recorded at the net sales price (transaction price), which includes estimates of variable consideration for which reserves are established and which result from Medicaid rebates, governmental chargebacks including Public Health Services (“PHS”) chargebacks, prompt pay 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 payments are required of the Company) for PHS chargebacks, prompt pay discounts and certain distribution fees, or a current liability (if a payment is required of us), for Medicaid rebates, co-pay assistance and certain distribution fees.

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, Brazil, Israel and the Middle East (collectively, “Customers”). The Company monitors the financial performance and creditworthiness of its Customers so that it can properly assess and respond to changes in the Customers’ credit profiles. The Company provides reserves against trade receivables for estimated losses that may result from a customer’s inability to pay. Amounts determined to be uncollectible are written-off against the established reserve. As of December 31, 2018, the credit profiles for the Company’s Customers are deemed to be in good standing and an allowance for doubtful accounts receivable is not considered necessary. Further, no accounts receivable amounts related to government research contracts and other grants have historically been written off and, thus, an allowance for doubtful accounts receivable related to government research contracts and other grants is also not considered necessary.

Concentration of Credit Risk

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

For the year ended December 31, 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 60 and 120 days. Three individual customers accounted for 42%, 38% and 18% of net product revenues and 51%, 28% and 10% of accounts receivable from product sales, respectively. As of December 31, 2018, the Company believes that such Customers are of high credit quality.

As of December 31, 2018, the Company’s cash equivalents and investments were concentrated at three financial institutions. The Company does not believe that there is significant risk of non-performance by the financial institutions.

Inventories

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

Property and Equipment

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

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

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

Intangible assets

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

The in-licensed rights relates to agreements with BioMarin (defined in Note 3) and the University of Western Australia (“UWA”). The in-licensed rights are being amortized on a straight-line basis over the remaining life of the related patent because the life of the related patent reflects the expected time period that the Company will benefit from the in-licensed rights.

Impairment of Long-Lived Assets

Long-lived assets held and used by the Company, intangible assets with definite lives and equity investments without a readily determinable fair value are reviewed for impairment whenever events or circumstances indicate that the carrying amount of assets may not be recoverable. The Company evaluates recoverability of assets to be held and used by comparing the carrying amount of an asset to future net undiscounted cash flows to be generated by the asset. If the asset is considered to be impaired, the impairment to be recognized is measured as the amount by which the carrying amount of the assets exceeds the fair value of the assets. Such reviews assess the fair value of the assets based upon estimates of future cash flows that the assets are expected to generate.

Convertible Debt

The Company separately accounts for the liability and equity components of convertible debt instruments that can be settled in cash by allocating the proceeds from issuance between the liability component and the embedded conversion option. The value of the equity component is calculated by first measuring the fair value of the liability component, using the interest rate of a similar liability that does not have a conversion feature, as of the issuance date. The difference between the proceeds from the convertible debt issuance and the amount measured as the liability component is recorded as the equity component with a corresponding discount recorded on the debt. The Company recognizes the amortization of the resulting discount as interest expense using the effective interest method. Simultaneously, the Company bought capped call options from certain counterparties to minimize the impact of potential dilution upon conversion. The premium for the capped call options was recorded as additional paid-in capital. For additional information related to the convertible debt transactions, please read Note 14, Indebtedness to the consolidated financial statements.

Revenue Recognition

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 Topic 606, “Revenue from Contracts with Customers”, 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 Company 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 or provides 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 Topic 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 its 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 years ended 2018, 2017 and 2016, the 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 PHS chargebacks, prompt payment discounts, co-pay assistance and distribution fees. 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:

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Medicaid rebates relate to the Company’s estimated obligations to states under established reimbursement arrangements. Medicaid 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.

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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.

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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.

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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.

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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 Customers, these payments are accounted for as selling, general and administrative expenses. Reserves for distribution fees result in an increase in a liability if payments are required of the Company or a reduction of accounts receivable if no payments are required of the Company.

Deferred revenue

As of December 31, 2018, the Company had deferred revenue of $3.3 million, which primarily represents up-front fees which it may recognize as revenue upon settlement of certain contingencies related to an agreement with Charley’s Fund, Inc.

Research and Development

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

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

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

Stock-Based Compensation

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

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

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

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

Income Taxes

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

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

The Tax Cuts and Jobs Act (the “TCJA”) was enacted on December 22, 2017. The TCJA reduces the U.S. federal corporate tax rate from 35% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign sourced earnings. The Company continues to evaluate the overall impact of the TCJA on its business. There is continuing uncertainty in the TCJA, and although changes or challenges cannot be predicted, the Company believes it has used reasonable assumptions and interpretations in applying the TCJA. The Company continues to monitor for legislative developments, issuance of regulations and technical memorandum to provide further clarification and/or interpretations of the TCJA and will adjust its financial statements as needed.

Rent Expense

The Company’s operating leases for its Cambridge, Andover and Burlington, Massachusetts and Dublin and Columbus, Ohio facilities provide for scheduled annual rent increases throughout each lease’s term. The Company recognizes the effects of the scheduled rent increases on a straight-line basis over the full term of the leases. Tenant improvement allowances provided by a landlord are recorded as deferred rent and amortized as reduction to rent expense over the lease term.

For the years ended December 31, 2018, 2017 and 2016, the Company recognized rent expense and occupancy costs of $7.9 million, $4.4 million and $5.6 million, respectively.

Commitments and Contingencies

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

Recent Accounting Pronouncements

In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02, “Leases (Topic 842)”, which supersedes ASC Topic 840, “Leases”, and issued additional clarifications throughout 2018. 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. The Company expects to elect the optional transition method to apply the standard as of the January 1, 2019 the effective date and therefore, it will not apply the standard to comparative periods. The Company also expects to elect the available package of practical expedients in transition which would allow it to not re-assess whether existing or expired arrangements contain a lease, the lease classification of existing or expired leases, or whether previous initial direct costs would qualify for capitalization under the new lease standard. The Company continues to finalize the impact of the ASU on its processes, disclosures and internal controls over financial reporting. Based on the procedures performed to date, the Company has estimated that the adoption of this ASU will result in an increase between $40.0 million and $45.0 million in right of use assets and between $60.0 million and $65.0 million in lease liabilities.

In March 2017, the FASB 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 December 31, 2018, the Company has elected to early adopt this new standard and the adoption did not have a material impact on its financial position and results of operations.

In June 2018, the FASB issued ASU No. 2018-07, “Compensation - Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting.” This ASU expands the scope of Topic 718 to include share based payment transactions for acquiring goods and services from nonemployees. An entity should apply the requirements of Topic 718 to nonemployee awards except for specific guidance on inputs to an option pricing model and the attribution of cost. ASU No. 2018-07 will be effective for fiscal years beginning after December 15, 2018, with early adoption permitted, although no earlier than the adoption date of Topic 606. The Company elected to early adopt this ASU in the quarter ended June 30, 2018 and determined that the adoption of this new standard did not have a material impact on its consolidated financial statements.

In August 2018, the FASB issued ASU No. 2018-13, “Fair Value Measurement (Topic 820), Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement”. This ASU removed the following disclosure requirements: (1) the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; (2) the policy for timing of transfers between levels; and (3) the valuation processes for Level 3 fair value measurements. Additionally, this update added the following disclosure requirements: (1) the changes in unrealized gains and losses for the period included in other comprehensive income and loss for recurring Level 3 fair value measurements held at the end of the reporting period; (2) the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. For certain unobservable inputs, an entity may disclose other quantitative information (such as the median or arithmetic average) in lieu of the weighted average if the entity determines that other quantitative information would be a more reasonable and rational method to reflect the distribution of unobservable inputs used to develop Level 3 fair value measurements. ASU No. 2018-13 will be effective for fiscal years beginning after December 15, 2019 with early adoption permitted. As of December 31, 2018, the Company has not elected to early adopt this guidance but does not expect that the adoption of this guidance will have a material effect on its consolidated financial statements.

In August 2018, the FASB issued ASU No. 2018-15, “Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract”. This ASU requires a customer in a cloud computing arrangement (i.e., hosting arrangement) that is a service contract to follow the internal-use software guidance contained in ASC Subtopic 350-40 to determine which implementation costs to capitalize as assets or expense as incurred. Capitalized implementation costs related to a hosting arrangement that is a service contract will be amortized over the term of the hosting arrangement, beginning when the module or component of the hosting arrange is ready for its intended use. ASU No. 2018-15 will be effective for fiscal years beginning after December 15, 2019, with early adoption permitted. As of December 31, 2018, the Company has not elected to early adopt this guidance but believes that the adoption of this guidance will not have a material effect on its consolidated financial statements.

LICENSE AND COLLABORATION AGREEMENTS

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

LICENSE AND COLLABORATION AGREEMENTS

3. LICENSE AND COLLABORATION AGREEMENTS

Lysogene S.A.

On October 15, 2018, the Company entered into a license and collaboration agreement (the “Lysogene License and Collaboration Agreement”) for developing and commercializing LYS-SAF302, a gene therapy to treat Mucopolysaccharidosis type IIIA (“MPS IIIA”) with Lysogene S.A. (“Lysogene”). Under the terms of the Lysogene License and Collaboration Agreement, Lysogene will be responsible for completion of the pivotal trial, which commenced in the fourth quarter of 2018. The Company will have exclusive rights to commercialize LYS-SAF302 in the U.S. and all territories outside of Europe. Lysogene will retain exclusive commercial rights to LYS-SAF302 in Europe. The Company will be responsible for global manufacturing of LYS-SAF302 and will supply drug products to Lysogene for Europe. Lysogene also granted the Company certain option rights to an additional central nervous system targeted gene therapy candidate.

Concurrently with the execution of the Lysogene License and Collaboration Agreement, the Company entered into an equity investment agreement with Lysogene. Under the equity investment agreement, the Company purchased 950,606 shares of common stock issued by Lysogene, representing 8% of the outstanding equity of Lysogene at the time of the transaction.

The Company considered whether it would have to consolidate the operations of Lysogene and concluded that, while Lysogene is a variable interest entity, the Company is not the primary beneficiary as it does not have the power to direct the activities that would most significantly impact the economic performance of Lysogene.

As a result of execution of the agreements, the Company made a payment of $28.0 million to Lysogene related to the up-front license fee, the option fee, the equity investment and reimbursement of certain development activities. Additionally, the Company may be liable for up to approximately $102.8 million in development, regulatory and sales milestones associated with the Lysogene License and Collaboration Agreement. Furthermore, the Company may be required to make tiered royalty payments based on net sales of the LYS-SAF302 product subsequent to its commercialization.

Of the $28.0 million payment to Lysogene, $1.9 million was allocated to the equity investment in Lysogene, based on the closing price of Lysogene’s common stock traded on the Euronext Paris Exchange on October 12, 2018, and was recorded as an other non-current asset. The remaining $26.1 million was allocated to the up-front license fee, the option fee, and reimbursement of certain development activities, and represents rights to potential future benefits associated with ongoing research and development activities that have no alternative future use. Accordingly, this amount was recorded as research and development expense as incurred. As of December 31, 2018, one development milestone was deemed as probable of being achieved. As a result, the Company recorded an accrued expense of $18.7 million which was classified as research and development expense.

Changes in the fair value of the equity investment in Lysogene are recorded to other income and expense in the Company’s consolidated statements of operations and other comprehensive loss. As of December 31, 2018, the Company recognized a loss of $0.2 million related to the change in fair market value of the equity investment.

Lacerta Therapeutics

On August 8, 2018 (the “Effective Date”), the Company entered into a License, Development and Option Agreement (the “Lacerta License Agreement”) with Lacerta Therapeutics, Inc. (“Lacerta”). Pursuant to the Lacerta License Agreement, the Company licensed exclusive worldwide rights to develop, manufacture and commercialize a pre-clinical Pompe product candidate (the “Pompe License”). Lacerta also granted the Company exclusive options to enter into exclusive license agreements to develop, manufacture and commercialize other gene therapy product candidates for Sanfilipo syndrome and L-Amino Acid Decarboxylase Deficiency for additional consideration of $42.0 million (collectively, the “Options”) when (and if) the Options are exercised. Additionally, the Company may be liable for up to approximately $44.0 million in development, regulatory and sales milestones associated with the Pompe License and may be required to make a high-single-digit royalty payments based on net sales of the Pompe product subsequent to its commercialization.

Concurrently with the execution of the Lacerta License Agreement, the Company entered into a Series A Preferred Stock Purchase Agreement (the “Purchase Agreement”) with Lacerta. Under the Purchase Agreement, the Company purchased approximately 4.5 million shares of Series A preferred stock issued by Lacerta.

The Company considered whether it would have to consolidate the operations of Lacerta and concluded that, while Lacerta is a variable interest entity, the Company is not the primary beneficiary as it does not have the power to direct the activities that would most significantly impact the economic performance of Lacerta.

The Company made an up-front payment of $38.0 million to Lacerta in consideration of both the Lacerta License Agreement and the Purchase Agreement. This payment was allocated to the fair value of the Series A preferred stock investment, the Pompe License and the Options based on their respective relative fair values on the Effective Date. The fair value of the Options were determined using an option pricing model, whereas the Series A preferred stock investment was determined using a cost approach corroborated by the Black-Scholes option pricing model. The fair value of the Pompe License was determined by the income approach, using a discounted cash flow model based on projections of future cash flows that will arise from the Pompe product candidate. Accordingly: (i) $30.0 million was allocated to the Series A preferred stock investment, (ii) $8.0 million was allocated to the Pompe License, and (iii) no amount was allocated to the Options as they are far out of money and were determined to have a fair value of zero. The Series A preferred stock investment was initially measured at cost and classified as an other non-current asset in the accompanying consolidated balance sheets. Subsequently, changes in the carrying value of the investment will be reported as a component of earnings whenever there are observable price changes in orderly transactions for identical or similar investments of Lacerta in the future. The amount allocated to the Pompe License represents rights to potential future benefits associated with ongoing research and development activities that have no alternative future use. Accordingly, this amount has been recorded as research and development expense in the accompanying consolidated statements of operations and comprehensive loss for the year ended December 31, 2018. As of December 31, 2018, no development milestones were deemed as probable of being achieved and, accordingly, no research and development expense was recognized related to these development milestones.

Myonexus Therapeutics

On May 3, 2018, the Company entered into a Warrant to Purchase Common Stock Agreement (“Warrant Agreement”) with Myonexus Therapeutics, Inc. (“Myonexus”). Pursuant to the terms of the Warrant Agreement, the Company made an up-front payment of $60.0 million to purchase an exclusive option to acquire Myonexus for $200.0 million plus sales-related and regulatory-related contingent payments. Prior to the exercise of the option to acquire Myonexus, the Company may be required to make additional development milestone payments to Myonexus of up to $45.0 million over an approximately two-year evaluation period.

The Company considered whether it would have to consolidate the operations of Myonexus and concluded that, while Myonexus is a variable interest entity, the Company is not the primary beneficiary as it does not have the power to direct the activities that would most significantly impact the economic performance of Myonexus.

As of December 31, 2018, the Company made an up-front payment of $60.0 million and milestone payments totaling $20.0 million to Myonexus corresponding to execution of the Warrant Agreement and achievement of two development milestones, respectively. Additionally, the third development milestone for $5.0 million was deemed probable of being achieved as of December 31, 2018, and has been recorded as an accrued expense. Prior to regulatory approval, all consideration paid to Myonexus represents rights to potential future benefits associated with Myonexus’s ongoing research and development activities, which have not reached technological feasibility and have no alternative future use. Accordingly, the Company recorded $85.0 million for the year ended December 31, 2018 as research and development expense in the accompanying consolidated statements of operations and comprehensive loss.

Nationwide Children’s Hospital

On December 29, 2016, the Company entered into an exclusive option agreement with Nationwide Children’s Hospital (“Nationwide”) from which the Company obtained an exclusive right to acquire a worldwide license of the micro-dystrophin gene therapy technology for DMD and Becker muscular dystrophy. On October 8, 2018, the Company exercised the option and entered into a license agreement with Nationwide (“Nationwide License Agreement”). Pursuant to the Nationwide License Agreement, the Company licensed exclusive worldwide rights to develop, manufacture and commercialize micro-dystrophin gene therapy product candidates. Under the agreement, the Company made an up-front payment of $1.0 million to Nationwide, which was recorded as research and development expense in the accompanying consolidated statements of operations and comprehensive loss for the year ended December 31, 2018. Additionally, the Company may be required to make up to $29.0 million in development, regulatory and sale milestone payments per micro-dystrophin product and a low-single-digit royalty payments based on net sales of the micro-dystrophin products upon commercialization. As of December 31, 2018, no development milestones were deemed as probable of being achieved and, accordingly, no research and development expense was recognized related to these development milestones.

BioMarin Pharmaceutical, Inc.

In July 2017, the Company and UWA entered into a settlement agreement with BioMarin Leiden Holding BV, its subsidiaries BioMarin Nederlands BV and BioMarin Technologies BV (collectively, “BioMarin”). On the same day, the Company entered into a license agreement with BioMarin and Academisch Ziekenhuis Leiden (“AZL”) (collectively with the Company, UWA and BioMarin, the “Settlement Parties”). Under these agreements, BioMarin agreed to provide the Company with an exclusive license to certain intellectual property with an option to convert the exclusive license into a co-exclusive license and the Settlement Parties agreed to stop most existing efforts to continue with ongoing litigation and opposition and other administrative proceedings concerning BioMarin’s intellectual property. Under terms of the agreements, the Company agreed to make total up-front payments of $35.0 million upon execution of the agreements, consisting of $20.0 million under the settlement agreement and $15.0 million under the license agreement. Additionally, the Company may be liable for up to approximately $65.0 million in regulatory and sales milestones for eteplirsen as well as exon 45 and exon 53 skipping product candidates. BioMarin will also be eligible to receive royalty payments, ranging from 4% - 8%, for exon 51 skipping products, exon 45 skipping products and exon 53 skipping products. The royalty terms under the license agreement will expire in December 2023 in the U.S. and September 2024 in the EU.

In connection with the above agreements, in July 2017, the Company made a cash payment of $35.0 million to BioMarin. Of this amount, $6.6 million was recorded as an intangible asset on the Company’s consolidated balance sheets. The remaining amount of $28.4 million was expensed as incurred as EXONDYS 51 settlement and license charges in the Company’s consolidated statements of operations and comprehensive loss.

The $6.6 million intangible asset represents the fair value of the U.S. license to BioMarin’s intellectual property related to EXONDYS 51, which was determined by an income-based approach, and is being amortized on a straight-line basis over the remaining life of the patent. For the year ended December 31, 2018, the Company recognized intangible asset amortization expense and royalty expense of approximately $0.9 million and $15.1 million, respectively. For the year ended December 31, 2017, the Company recognized intangible asset amortization expense and royalty expense of approximately $1.0 million and $4.7 million, respectively. The royalties are included in cost of sales in the Company’s consolidated statements of operations and comprehensive loss. As of December 31, 2018, no regulatory or sales milestones were deemed as probable of being achieved and, accordingly, no additional in-licensed rights were recognized related to these milestones.

Summit (Oxford) Ltd.

In October 2016, the Company entered into an exclusive Collaboration and License Agreement (the “Summit Collaboration Agreement”) with Summit which grants the Company the exclusive right to commercialize products in Summit’s utrophin modulator pipeline in the EU, Switzerland, Norway, Iceland, Turkey and the Commonwealth of Independent States (the “Licensed Territory”).

Under the terms of the Summit Collaboration Agreement, the Company made an up-front payment of $40.0 million to Summit, with additional payments of up to $192.0 million based on achievement of certain development and regulatory milestones for ezutromid, a Summit product candidate in its utrophin modulator pipeline. For each of Summit’s future generation small molecule utrophin modulators, the Company may be required to make up to $290.0 million in development and regulatory milestone payments. Additionally, on a product-by-product basis, the Company may be required to make up to $330.0 million in sales milestone payments. The Summit Collaboration Agreement also grants the Company an option to expand the Licensed Territory (“Option Territory”). If the Company exercises this option, it will be liable for a one-time $10.0 million option fee as well as up to $7.0 million in regulatory milestone payments. For each licensed product in the Option Territory, the Company may be liable for up to $82.5 million in sales milestone payments. Additionally, the Company may be required to make tiered royalty payments ranging from a low to high teens percentage of net sales on a product-by-product basis in the Licensed Territory.

Under the Summit Collaboration Agreement, a joint steering committee was established to plan, monitor and coordinate future development activities for ezutromid and future generation small molecule utrophin modulators. Summit was solely responsible for all research and development costs for the licensed products until December 31, 2017. Thereafter, Summit will be responsible for 55.0% of the budgeted research and development costs related to the licensed products in the Licensed Territory, and the Company will be responsible for 45.0% of such costs. Any costs in excess of 110.0% of the budgeted amount are borne by the party that incurred such costs. Summit is also obligated to spend a specified minimum amount on the research and development of certain licensed products prior to the end of 2019. For the year ended December 31, 2018, the Company incurred approximately $8.6 million in collaboration expense for ezutromid, with no similar activity in 2017 and 2016.

In June 2018, Summit announced that it is discontinuing the Phase 2 clinical trial on ezutromid. As a result, no development, regulatory or sales milestones were deemed as probable of being achieved as of December 31, 2018. For the years ended December 31, 2017, and 2016, the Company recorded a $22.0 million milestone payment and a $40.0 million up-front payment to Summit, respectively, as research and development expense in its consolidated statement of operations and comprehensive loss.

University of Western Australia

In April 2013, the Company and UWA entered into an agreement under which an existing exclusive license agreement between the Company and UWA was amended and restated (the “Amended and Restated UWA License Agreement”). The Amended and Restated UWA License Agreement grants the Company specific rights to the treatment of DMD by inducing the skipping of certain exons. EXONDYS 51 falls under the scope of the license agreement. Under the Amended and Restated UWA License Agreement, the Company may be required to make payments of up to $6.0 million in the aggregate to UWA based on the successful achievement of certain development and regulatory milestones relating to EXONDYS 51 and up to five additional product candidates. The Company may also be obligated to make payments to UWA of up to $20.0 million upon the achievement of certain sales milestones. Additionally, the Company may be required to pay a low-single-digit percentage royalty on net sales of products covered by issued patents licensed from UWA during the term of the Amended and Restated UWA License Agreement. However, the Company has the option to purchase future royalties up-front. Under this option, prior to the First Amendment (defined below), the Company could elect to make a one-time royalty payment of $30.0 million to UWA.

In June 2016, the Company and UWA entered into the first amendment to the Amended and Restated UWA License Agreement (the “First Amendment”). Under the First Amendment, the Company was obligated to make an up-front payment of $7.0 million to UWA upon execution of the amendment. Under the terms of the First Amendment, UWA has waived certain rights and amended the timing of certain payments under the Amended and Restated UWA License Agreement, including lowering the up-front payment that is due by the Company upon exercise of the option to purchase future royalties up-front. Upon exercise of the option to purchase future royalties up-front, the Company will be obligated to make a $23.0 million payment to UWA. Additionally, the Company would still be obligated to make up to $20.0 million in payments to UWA upon achievement of certain sales milestones.

For the year ended December 31, 2016, the Company recorded $7.6 million relating to the development milestone and up-front payments to UWA as research and development expense in the consolidated statement of operations and comprehensive loss as the Amended and Restated UWA License Agreement and its First Amendment were entered into before the FDA approval of EXONDYS 51. The Company did not incur any milestone expense for the years ended December 31, 2018 and 2017. Additionally, corresponding to the FDA approval and the subsequent commercial sale of EXONDYS 51, the Company recorded a $1.0 million milestone payment as an in-license right in its consolidated balance sheet in 2016. As of December 31, 2018, the in-license right is recorded net of $0.3 million accumulated amortization on the consolidated balance sheets. The amortization of the in-licensed right is recorded as cost of sales in the Company’s consolidated statements of operations and comprehensive loss. During the years ended December 31, 2018, 2017, and 2016, the Company did not incur royalty expense nor did it make any royalty payments to UWA but may be obligated to make these payments in the future.

GAIN FROM SALE OF PRIORITY REVIEW VOUCHER

12 Months Ended

Dec. 31, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

GAIN FROM SALE OF PRIORITY REVIEW VOUCHER

4. GAIN FROM SALE OF PRIORITY REVIEW VOUCHER

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

FAIR VALUE MEASUREMENTS

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

FAIR VALUE MEASUREMENTS

5. FAIR VALUE MEASUREMENTS

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

	Fair Value Measurement as of December 31, 2018
	Total		Level 1		Level 2		Level 3
	(in thousands)
Assets
Money market funds	$	42,920	$	42,920	$	—	$	—
Commercial paper		125,907		—		125,907		—
Government and government agency bonds		760,235		760,235		—		—
Corporate bonds		43,468		43,468		—		—
Strategic equity investments		31,739		1,739		—		30,000
Certificates of deposit		1,001		1,001		—		—
Total	$	1,005,270	$	849,363	$	125,907	$	30,000

	Fair Value Measurement as of December 31, 2017
	Total		Level 1		Level 2		Level 3
	(in thousands)
Assets
Money market funds	$	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	$	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, certificate of deposit, and the Company’s strategic investment in Lysogene, a publicly traded company in France, as more fully described in Note 3, License and Collaboration Agreements. Certain of the government and government agency bonds and corporate bonds are publicly traded fixed income securities and are presented as cash equivalents on the consolidated balance sheets.

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 Company’s assets with fair value categorized as Level 3 within the fair value hierarchy consists of a strategic investment in Series A preferred stock of Lacerta as more fully described in Note 3, License and Collaboration Agreements. The fair value of the asset is based on a cost approach corroborated by the Black-Scholes option pricing model. The most significant assumptions in the option pricing model include historical volatility of similar public companies, estimated term through Lacerta’s potential exit and a risk free rate based on certain U.S. Treasury rates.

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

CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES

12 Months Ended

Dec. 31, 2018

Cash And Cash Equivalents [Abstract]

CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES

6. CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES

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

	As of December 31
	2018		2017
	(in thousands)
Money market funds	$	42,920	$	352,370
Corporate bonds		—		16,720
Government and government agency bonds		111,587		49,972
Commercial paper		14,940		—
Total	$	169,447	$	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 December 31, 2018 and 2017 was approximately two and seven months, respectively. The following tables summarize the Company’s cash, cash equivalents and investments for each of the periods indicated:

	As of December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Market Value
	(in thousands)
Cash and money market funds	$	244,302	$	—	$	—		$	244,302
Commercial paper		125,907		—		—			125,907
Government and government agency bonds		760,258		12		(35	)		760,235
Corporate bonds		43,544		—		(76	)		43,468
Total cash, cash equivalents and investments		1,174,011		12		(111	)		1,173,912
As reported:
Cash and cash equivalents	$	370,827	$	3	$	(1	)	$	370,829
Short-term investments		803,184		9		(110	)		803,083
Total cash, cash equivalents and investments	$	1,174,011	$	12	$	(111	)	$	1,173,912

	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		133,368		—		—			133,368
Government and government agency bonds		294,915		2		(200	)		294,717
Corporate bonds
Current		118,121		—		(145	)		117,976
Non-current		10,016		—		(36	)		9,980
Total cash, cash equivalents and investments		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 cash, cash equivalents and investments	$	1,089,419	$	2	$	(381	)	$	1,089,040

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES

12 Months Ended

Dec. 31, 2018

Receivables Net Current [Abstract]

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES

7. ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES

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

	As of December 31,
	2018		2017
	(in thousands)
Product sales, net of discounts and allowances	$	48,252	$	28,539
Government contract receivables		792		929
Total accounts receivable, net	$	49,044	$	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 decrease in government contract receivables is related to contract finalization and subsequent collection of the EU SKIP-NMD Agreement related to the Company’s exon 53 product candidate.

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

	Chargebacks			Rebates			Prompt Pay			Other Accruals			Total
	(in thousands)
Balance, as of December 31, 2017	$	995		$	6,959		$	169		$	464		$	8,587
Provision		12,284			28,420			2,624			5,286			48,614
Payments/credits		(11,901	)		(11,103	)		(2,255	)		(3,432	)		(28,691	)
Balance, as of December 31, 2018	$	1,378		$	24,276		$	538		$	2,318		$	28,510

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

	As of December 31,
	2018		2017
	(in thousands)
Reduction to accounts receivable	$	2,364	$	1,285
Component of accrued expenses		26,146		7,302
Total reserves	$	28,510	$	8,587

INVENTORY

12 Months Ended

Dec. 31, 2018

Inventory Disclosure [Abstract]

INVENTORY

8. INVENTORY

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

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Raw materials	$	71,313	$	53,875
Work in progress		47,279		27,442
Finished goods		6,853		2,288
Total inventory	$	125,445	$	83,605

OTHER ASSETS

12 Months Ended

Dec. 31, 2018

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

OTHER ASSETS

9. OTHER ASSETS

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

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Manufacturing-related deposits and prepaids	$	39,036	$	18,650
Leasehold improvement receivable		13,474		—
Prepaid clinical and pre-clinical expenses		9,706		5,175
Prepaid maintenance services		2,994		1,711
Prepaid income tax		2,130		—
Prepaid research expenses		1,932		2,896
Interest receivable		1,918		709
Prepaid commercial expenses		1,573		1,589
Asset held for sale		—		1,501
Other prepaids		3,671		2,726
Other		1,348		1,554
Total other current assets	$	77,782	$	36,511

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

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Manufacturing-related deposits	$	62,821	$	—
Strategic investments		31,739		—
Prepaid clinical expenses		7,541		7,488
Alternative minimum tax credit		3,367		3,315
Restricted investments		1,001		784
Long-term available-for-sale securities		—	$	9,980
Other		825		242
Total investments and other assets	$	107,294	$	21,809

PROPERTY AND EQUIPMENT, NET

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

PROPERTY AND EQUIPMENT, NET

10. PROPERTY AND EQUIPMENT, NET

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

	As of December 31,
	2018			2017
	(in thousands)
Land	$	4,158		$	4,158
Building and improvements		22,972			14,543
Software and computer equipment		15,774			8,965
Lab equipment		17,659			9,860
Office equipment		3,663			2,211
Leasehold improvements		20,937			13,633
Construction in progress		40,010			7,808
Property and equipment, gross		125,173			61,178
Less: accumulated depreciation		(28,149	)		(18,022	)
Property and equipment, net	$	97,024		$	43,156

For the years ended December 31, 2018, 2017 and 2016, depreciation expense totaled $10.2 million, $6.4 million and $5.0 million, respectively.

INTANGIBLE ASSETS

12 Months Ended

Dec. 31, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

INTANGIBLE ASSETS

11. INTANGIBLE ASSETS

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

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Patents	$	5,773	$	7,660
In-licensed rights		5,626		6,491
Software licenses		175		204
Total	$	11,574	$	14,355

The in-licensed rights relate to agreements with BioMarin and UWA. Following the execution of the settlement and license agreements with BioMarin in July 2017, the Company recorded a $6.6 million intangible asset related to EXONDYS 51 in the U.S. Additionally, as a result of the FDA approval and the subsequent commercial sale of EXONDYS 51, the Company made a $1.0 million sales milestone to UWA and, accordingly, recorded an in-licensed right. The in-licensed rights are being amortized on a straight-line basis over the remaining life of the related patent because the life of the related patent reflects the expected time period that the Company will benefit from the in-licensed right. For the years ended December 31, 2018, 2017 and 2016, the Company recorded $0.9 million, $1.1 million and less than $0.1 million, respectively, of amortization related to the in-license rights.

Patent costs consist primarily of external legal costs, filing fees incurred to file patent applications and renewal fees on proprietary technology developed or licensed by the Company. Patent costs associated with applying for a patent, being issued a patent and annual renewal fees are capitalized. Costs to defend a patent and costs to invalidate a competitor’s patent or patent application are expensed as incurred. Patent costs are amortized on a straight-line basis over the shorter of the estimated economic lives or the initial term of the patents, which is generally 20 years. Patent amortization expense was $0.7 million, $0.6 million and $0.6 million for the years ended December 31, 2018, 2017 and 2016, respectively. The Company also expensed the remaining net book value of previously capitalized patents that were later abandoned of $0.1 million, $0.6 million and $0.3 million for the years ended December 31, 2018, 2017 and 2016, respectively, which were included in research and development expenses on the consolidated statements of operations and comprehensive loss.

Additionally, the Company reviewed its patent portfolio and identified technology that the Company will no longer pursue. As a result, the Company impaired these patent assets and recorded $3.8 million in impairment loss, which was included in research and development expense on the consolidated statement of operations and comprehensive loss.

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

	As of December 31, 2018 (in thousands)
2019		1,490
2020		1,300
2021		1,299
2022		1,298
2023		1,297
Total	$	6,684

ACCRUED EXPENSES

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

ACCRUED EXPENSES

12. ACCRUED EXPENSES

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

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Product revenue related reserves	$	26,146	$	7,302
Accrued employee compensation costs		24,692		14,402
Accrued milestone expense		24,020		—
Accrued contract manufacturing costs		15,794		14,019
Accrued clinical and pre-clinical costs		11,396		15,975
Accrued professional fees		11,319		6,794
Accrued BioMarin royalties		8,254		2,846
Accrued property and equipment		5,421		2,525
Accrued collaboration cost sharing		2,167		—
Accrued research costs		1,070		401
Accrued interest expenses		1,045		1,291
Accrued income taxes		301		943
Other		2,470		2,484
Total accrued expenses	$	134,095	$	68,982

RESTRUCTURING

12 Months Ended

Dec. 31, 2018

Restructuring And Related Activities [Abstract]

RESTRUCTURING

13. RESTRUCTURING

In March 2016, the Company announced a long-term 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 Company’s facility in Corvallis, Oregon 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. In June 2018, the Corvallis facility was sold, and the Company entered into a rental termination agreement with the new landlord regarding the space made available for sub-lease. As a result, we relieved the remaining $2.2 million of cease-use liability related to this space in June 2018, which was recorded as a reduction to selling, general and administrative expenses.

For the years ended December 31, 2018, 2017 and 2016, the Company recognized $(2.2) million, $3.0 million and $4.6 million as restructuring expenses, respectively.

	For the Year Ended December 31
	2018			2017		2016
	(in thousands)
Research and development	$	—		$	188	$	2,013
Selling, general and administrative		(2,222	)		2,832		2,549
Total restructuring expenses	$	(2,222	)	$	3,020	$	4,562

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

	For the Year Ended December 31
	2018			2017
	(in thousands)
Restructuring reserve beginning balance	$	2,933		$	1,588
Restructuring expenses incurred during the period		—			3,020
Amounts paid during the period		(711	)		(1,675	)
Reversal of cease-use liability		(2,222	)		—
Restructuring reserve ending balance	$	—		$	2,933

INDEBTEDNESS

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

INDEBTEDNESS

14. INDEBTEDNESS

2024 Convertible Notes

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

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

The Company allocated the proceeds received from issuance of the 2024 Notes between the liability component and the embedded conversion option, or equity component. The liability component was determined by measuring the fair value of similar notes that do not include the embedded conversion option. The Company allocated $161.1 million to the equity component, which was determined by deducting the fair value of the liability component from the par value of the 2024 Notes. The equity component, net of allocated offering costs of $4.2 million, was recorded as an increase additional paid-in capital. The equity component, plus $10.6 million of offering costs allocated to the liability component, represent the total debt discount on the 2024 Notes at issuance. The debt discount will be amortized under an effective interest method and recorded as additional interest expense over the life of the 2024 Notes. The effective interest rate on the liability component of the 2024 Notes for the year ended December 31, 2018 and 2017 was 6.9%.

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

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

Term Loan and Revolving Line of Credit

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 bore interest at a rate per annum equal to 6.25%, plus the one-month London Interbank Offered Rate (“LIBOR”). In addition to paying interest on the outstanding principal under the Amended and Restated Credit and Security Agreement, the Company paid an origination fee equal to 0.50% of the amount of the term loan when advanced under the Amended and Restated Credit and Security Agreement and would be liable for a final payment fee equal to 2.00% of the amount borrowed under the Amended and Restated Credit and Security Agreement when the July 2017 Term Loan was fully repaid. Commencing on July 1, 2018, and continuing for the remaining thirty six months of the facility, the Company was 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 would mature in July 2021.

In July 2017, the Company also 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 bore 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 fees, which was 0.50% of the amount of the revolving loan. The Company recognized this origination fee as other asset and it was amortized to interest expense over the term of the revolving loan.

In September 2018, the Company terminated both the Amended and Restated Credit and Security Agreement and the Revolving Credit Agreement with MidCap and paid off the remaining outstanding balance of principal and accrued and unpaid interest on the July 2017 Term Loan. As a result, the Company recorded a debt extinguishment loss of $2.3 million primarily related to the write-off of unamortized debt issuance costs and prepayment fees.

Mortgage Loans

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

As of December 31, 2018, the Company recorded approximately $420.6 million as long-term debt on the consolidated balance sheets. For the years ended December 31, 2018, 2017 and 2016, the Company recorded $33.7 million, $5.8 million and $1.9 million of interest expense, respectively.

The following table summarizes the Company’s debt facilities for the periods indicated:

	As of December 31, 2018			As of December 31, 2017
	(in thousand)			(in thousand)
Par value of the 2024 Notes	$	570,000		$	570,000
Unamortized discount - equity component		(140,206	)		(158,890	)
Unamortized discount - debt issuance costs		(9,240	)		(10,449	)
Net carrying value of convertible debt		420,554			400,661
Net carrying value of other debt facilities		—			30,390
Net carrying value of total debt facilities		420,554			431,051

Fair value of convertible debt	$	952,681		$	619,641

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

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

	As of December 31, 2018
	(in thousands)
2019	$	—
2020		—
2021		—
2022		—
2023		—
Thereafter		570,000
Total Payments	$	570,000

EQUITY FINANCING

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

EQUITY FINANCING

15. EQUITY FINANCING

In November 2018, the Company sold approximately 4.1 million shares of common stock through an underwritten public offering, including 0.3 million shares sold to the underwriters. The offering price was $131.00 per share. The Company received net proceeds of approximately $513.4 million from the offering, net of commission and offering expenses of approximately $24.6 million.

In July 2017, the Company sold approximately 8.8 million shares of common stock through an underwritten public offering, including 1.2 million shares sold to the underwriters. The offering price was $42.50 per share. The Company received net proceeds of approximately $354.0 million from the offering, net of commission and offering expenses of approximately $20.0 million.

In September 2016, the Company sold approximately 5.8 million shares of common stock through an underwritten public offering at a price of $59.75 per share. The Company received net proceeds of approximately $327.4 million from the offering net of commission and offering expenses of approximately $17.6 million.

In June 2016, the Company sold approximately 2.1 million shares of common stock through an underwritten public offering at a price of $17.84 per share. The Company received net proceeds of approximately $37.3 million from the offering net of offering expense of approximately $0.2 million.

STOCK-BASED COMPENSATION

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

STOCK-BASED COMPENSATION

16. STOCK-BASED COMPENSATION

In June 2011, the Company’s stockholders approved the 2011 Equity Incentive Plan (“2011 Plan”). The 2011 Plan, which authorized 13.0 million shares of common stock to be issued, allows for the grant of stock options, SARs, RSAs, RSUs, performance shares and performance units. In June 2016 and 2015, shareholders authorized the issuance of an additional 1.3 million and 1.7 million shares, respectively, of common stock under the 2011 Plan. As of December 31, 2018, the 2011 Plan was merged into the 2018 Plan (defined below). As a result, there were no shares of common stock remaining available for future grant under the 2011 Plan.

In June 2013, the Company’s stockholders approved the 2013 ESPP with approximately 0.3 million shares of common stock available to be issued. In June 2016, the Company’s stockholders approved an additional approximately 0.3 million shares of common stock available to be issued to be added to the 2013 ESPP. As of December 31, 2018, 0.2 million shares of common stock remain available for future grant under the 2013 ESPP.

In September 2014, the Company initiated the 2014 Employment Commencement Incentive Plan (“2014 Plan”) with approximately 0.6 million shares of common stock available to be issued. In October 2015, June 2017 and July 2018, the 2014 Plan was increased by 1.0 million, 3.8 million and 1.2 million shares of common stock available to be issued, respectively. As of December 31, 2018, 0.9 million shares of common stock remain available for future grant under the 2014 Plan.

In June 2018, the Company’s stockholders approved the 2018 Equity Incentive Plan (“2018 Plan”). The 2018 Plan, which authorized 2.9 million shares of common stock to be issued, allows for the grant of stock options, SARs, RSAs, RSUs, performance shares and performance units. The 2011 Plan was merged into the 2018 Plan and, as a result, all remaining shares in the 2011 Plan were transferred into the 2018 Plan. As of December 31, 2018, 4.4 million shares of common stock remain available for future grant under the 2018 Plan.

Stock Options

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

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

	For the Year Ended December 31,
	2018	2017	2016
Risk-free interest rate (1)	2.5 - 3.0%	1.6 - 2.1%	1.1 – 1.8%
Expected dividend yield (2)	—	—	—
Expected term (3)	5.06 years	4.2 - 4.8 years	4.2 – 4.8 years
Expected volatility (4)	52.4 - 60.8%	54.0 - 63.0%	78.2 – 137.1 %

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

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

(3)	The expected term is estimated using historical exercise behavior.

(4)

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

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

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

	For the Year Ended December 31,
	2018					2017							2016
				Weighted							Weighted					Weighted
				Average							Average					Average
				Exercise							Exercise					Exercise
	Shares			Price		Shares					Price		Shares			Price
Grants outstanding at beginning of the period		8,806,204		$	29.74		5,436,951				$	22.70		6,515,976		$	23.91
Granted		2,152,439			90.15		4,805,722		(1	)		35.09		1,285,051			14.89
Exercised		(2,119,306	)		22.89		(792,845	)				17.40		(1,056,821	)		18.31
Cancelled		(448,166	)		46.11		(643,624	)				25.44		(1,307,255	)		24.61
Grants outstanding at end of the period		8,391,171		$	46.09		8,806,204				$	29.74		5,436,951		$	22.70

Grants exercisable at end of the period		2,304,791		$	27.69		3,288,712				$	24.76		2,942,624		$	24.42
Grants vested and expected to vest at end of the period		6,643,835		$	45.43		6,910,022				$	28.49		5,116,409		$	22.83

(1)

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

The weighted-average grant date fair value per share of stock options granted during the years ended December 31, 2018, 2017 and 2016 was $44.66, $14.78 and $12.46, respectively.

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
Options outstanding at December 31, 2018	$	551,131	7.7
Options exercisable at December 31, 2018	$	187,693		5.5
Options vested and expected to vest at December 31, 2018	$	433,681	7.4

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

	For the Year Ended December 31,
	2018		2017		2016
	(in thousands)
Aggregate grant date fair value of stock options vested	$	16,316	$	18,225	$	30,651
Aggregate intrinsic value of stock options exercised	$	158,936	$	20,922	$	30,610

In February 2016, the Company granted to executives approximately 0.3 million stock options with service- and performance-based conditions (“February 2016 Performance Grants”). Vesting is achieved based upon the FDA approval of EXONDYS 51 and submission of MAA for eteplirsen to the EMA and continuing service over a four-year period. As a result of both milestones achieved in 2016, 50% of the February 2016 performance grants were triggered to be vested immediately and the remaining 50% are subject to the remaining service conditions of the awards. Additionally, in June 2013, the Company granted to executives approximately 0.4 million stock options with service- and performance-based conditions (“June 2013 Performance Grants”). Vesting was achieved based upon various regulatory approval for EXONDYS 51 and filings of investigational new drug submissions for other drug candidates and continuing service over a four-year period. 80% of the June 2013 Performance Grants was vested as of December 31, 2016 and the remaining 20% were forfeited.

For the years ended December 31, 2018, 2017 and 2016, the Company has recognized approximately $0.2 million, $0.9 million and $5.0 million in stock-based compensation expense related to the options with performance-based criteria, respectively.

As of December 31, 2018, the total stock-based compensation expense related to non-vested awards with only service-vesting conditions not yet recognized is approximately $73.0 million, those with service- and performance-based conditions approximates $0.2 million and those with market-based conditions approximates $31.0 million.

Restricted Stock Awards

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

	For the Year Ended December 31,
	2018					2017					2016
				Weighted					Weighted					Weighted
				Average					Average					Average
				Exercise					Exercise					Exercise
	Shares			Price		Shares			Price		Shares			Price
Grants outstanding at beginning of the period		411,781		$	37.23		153,170		$	34.53		161,320		$	21.50
Granted		27,590			98.57		341,500			34.58		117,553			41.22
Vested		(187,050	)		39.34		(63,264	)		14.60		(59,703	)		13.62
Forfeited		—			—		(19,625	)		43.02		(66,000	)		33.51
Grants outstanding at end of the period		252,321		$	42.37		411,781		$	37.23		153,170		$	34.53

In September 2016, the Company granted certain executives RSAs with a performance condition relating to certain sales targets. If the sales targets are achieved within the required time frame, the number of RSAs may be increased from 71,925 to 89,906 shares. In December 2017, the Company modified the expiration date of these RSAs from June 30, 2018 to January 1, 2019. As a result of this modification, the fair value was changed from $48.94 to $54.29. Through December 31, 2017, the Company had not recorded any stock-based compensation expense associated with these awards as the achievement of the performance conditions was not deemed probable. However, as of December 31, 2018, the first sales target related to these RSAs was achieved and the second target was not deemed as probable of being achieved. Accordingly, the Company recognized approximately $3.3 million of stock-based compensation expense during the year ended December 31, 2018, related to the achievement of the first performance condition.

Restricted Stock Units

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

	For the Year Ended December 31
	2018					2017
				Weighted					Weighted
				Average					Average
				Exercise					Exercise
	Shares			Price		Shares			Price
Grants outstanding at beginning of the period		66,552		$	33.72		—		$	—
Granted		230,736			87.95		181,029			33.03
Vested		(30,276	)		33.23		(78,017	)		32.63
Forfeited		(15,714	)		71.45		(36,460	)		32.63
Grants outstanding at end of the period		251,298		$	81.21		66,552		$	33.72

In March 2017, the Company granted executives 156,029 RSUs with performance conditions relating to certain sales target and regulatory milestones. The first performance condition of these RSUs was achieved in June 2017. The second performance condition was deemed as probable of being achieved in September 2017 and was achieved in May 2018. For the years ended December 31, 2018 and 2017, the Company recognized approximately $0.2 million and $2.9 million of stock-based compensation expense, respectively, related to the performance conditions of these RSUs. The third performance milestone was not deemed as probable of being achieved as of December 31, 2018. If and when deemed probable that the last performance milestone may be achieved within the required time frame, the Company may recognize up to $0.7 million of stock-based compensation related to the third performance milestone of these grants.

Stock Appreciation Rights

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

As of December 31, 2018, there were 100,000 SARs vested and outstanding. These SARs were granted to the Company’s Executive Vice President and CFO in November 2012 and have an exercise price of $23.85 per share. The SARs are classified as equity as the agreements require settlement in shares of stock.

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

	For the Year Ended December 31,
	2018				2017				2016
			Weighted				Weighted				Weighted
			Average				Average				Average
			Exercise				Exercise				Exercise
	Shares		Price		Shares		Price		Shares		Price
Grants outstanding at beginning of the period		100,000	$	23.85		100,000	$	23.85		100,000	$	23.85
Grants outstanding at end of the period		100,000	$	23.85		100,000	$	23.85		100,000	$	23.85
Grants exercisable at end of the period		100,000	$	23.85		100,000	$	23.85		100,000	$	23.85
Grants vested and expected to vest at end of the period		100,000	$	23.85		100,000	$	23.85		100,000	$	23.85

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
SARs outstanding, exercisable and vested at December 31, 2018	$	8,528	3.9

2013 Employee Stock Purchase Plan

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

	For the Year Ended December 31,
	2018		2017		2016
Number of shares purchased		75,094		102,698		137,113
Proceeds received (in millions)	$	2.3	$	1.4	$	1.6

Stock-based Compensation Expense

For the years ended December 31, 2018, 2017 and 2016, total stock-based compensation expense was $50.1 million, $30.5 million and $30 million, respectively. Included in the amounts for the year ended December 31, 2017 is $2.1 million of stock-based compensation expense incurred in connection with the resignation of the Company’s former CEO. The following table summarizes stock-based compensation expense by function included within the consolidated statements of operations and comprehensive loss:

	For the Year Ended December 31,
	2018		2017		2016
	(in thousands)
Research and development	$	14,214	$	8,542	$	9,499
Selling, general and administrative		35,913		21,923		20,463
Total stock-based compensation	$	50,127	$	30,465	$	29,962

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

	For the Year Ended December 31,
	2018		2017		2016
	(in thousands)
Stock options	$	37,671	$	23,416	$	26,320
Restricted stock awards/units		10,632		5,295		872
Employee stock purchase plan		1,824		1,754		2,380
Other		—		—		390
Total stock-based compensation	$	50,127	$	30,465	$	29,962

401 (K) PLAN

12 Months Ended

Dec. 31, 2018

Compensation And Retirement Disclosure [Abstract]

401 (K) PLAN

17. 401 (K) PLAN

The Company sponsors a 401 (k) Plan (“the Plan”) which is a defined contribution plan. It is available to all employees who are age 21 or older. Participants may make voluntary contributions and the Company makes matching contributions according to the Plan’s matching formula. Matching contributions fully vest after one year of service for all employees. The expense related to the Plan primarily consists of the Company’s matching contributions.

Expense related to the Plan totaled $2.1 million, $1.4 million and $1.2 million for the years ended December 31, 2018, 2017 and 2016, respectively.

OTHER INCOME AND LOSS

12 Months Ended

Dec. 31, 2018

Other Income And Expenses [Abstract]

OTHER INCOME AND LOSS

18. OTHER INCOME AND LOSS

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

	For the year ended December 31,
	2018			2017			2016
	(in thousand)
Interest expense	$	(33,709	)	$	(5,801	)	$	(1,892	)
Interest income		6,810			1,809			507
Amortization of investment discount (premium)		8,573			1,401			(80	)
Other (loss) income		(656	)		601			930
Gain from sale of Priority Review Voucher		—			125,000			—
Total other (loss) income	$	(18,982	)	$	123,010		$	(535	)

INCOME TAXES

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

INCOME TAXES

19. INCOME TAXES

The following table summarizes the loss before the provision for income taxes by jurisdiction for the periods indicated:

	For the Year Ended December 31,
	2018			2017			2016
	(in thousands)
Domestic	$	(309,294	)	$	(45,686	)	$	(267,265	)
Foreign		(53,316	)		(2,942	)		—
Total	$	(362,610	)	$	(48,628	)	$	(267,265	)

The following table summarizes provision for income taxes in the accompanying consolidated financial statements for the periods indicated:

	For the Year Ended December 31,
	2018			2017		2016
	(in thousands)
Current provision
Federal	$	(110	)	$	204	$	—
State		(653	)		1,856		—
Foreign		311			—		—
Total current provision		(452	)		2,060		—
Deferred benefit:
Federal		—			—		—
State		—			—		—
Foreign		(240	)		—		—
Total deferred benefit		(240	)		—		—
Total current provision	$	(692	)	$	2,060	$	—

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

	For the Year Ended December 31,
	2018				2017				2016
Federal income tax rate		21.0		%		34.0		%		34.0		%
State taxes		12.3				(27.7	)			—
Research and development and other tax credits		3.1				8.5				1.5
Valuation allowance		(45.5	)			(93.2	)			(17.3	)
Permanent differences		6.9				6.4				(3.4	)
Sarepta International C.V. return to provision		(0.1	)			62.1				—
Impact of tax reform, net of valuation allowance		—				5.90				—
Foreign rate differential		(0.9	)			(0.6	)			(14.8	)
Other		3.4				0.40				—
Effective tax rate		0.2		%		(4.2	)	%		—		%

Permanent differences affecting the Company’s effective tax rate primarily include excess stock-based compensation tax deductions, net of non-deductible stock-based compensation and 162(m) limitation.

In December 2012, the Company licensed certain intellectual property of Sarepta Therapeutics, Inc. to its wholly owned subsidiary, Sarepta International C.V. The parties also entered into a contract research agreement under which Sarepta Therapeutics, Inc. performs research services for Sarepta International C.V. In January 2016, Sarepta Therapeutics, Inc. entered into a manufacturing and distribution agreement as well as service agreement with Sarepta International C.V. In conjunction with its recent filings, it was determined that beginning in 2016, Sarepta International C.V. is effectively connected with the conduct of a trade or business by the entity in the U.S. and, accordingly, the 2016, 2017 and 2018 losses are subject to U.S. income taxes. For the years ended December 31, 2018, 2017, and 2016, Sarepta International C.V. incurred losses of $67.6 million, $143.8 million, and $174.9 million, respectively.

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

	As of December 31,
	2018			2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	212,342		$	144,897
Difference in depreciation and amortization		55,162			1,210
Research and development tax credits		67,309			44,306
Stock-based compensation		15,538			14,392
Deferred rent		4,831			1,210
Deferred revenue		888			904
Capitalized inventory		22,943			22,127
Other		15,727			4,071
Total deferred tax assets		394,740			233,117
Deferred tax liabilities:
Debt discount		(25,162	)		(28,548	)
Total deferred tax liabilities		(25,162	)		(28,548	)
Valuation allowance		(369,345	)		(204,569	)
Net deferred tax assets	$	233		$	—

The Company has evaluated the positive and negative evidence bearing upon the realizability of its U.S. net deferred tax assets, which are comprised principally of federal and state net operating loss carryforwards, research and development tax credit carryforwards, stock-based compensation expense, capitalized inventory and intangibles. Under the applicable accounting standards, management has considered the Company’s history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of net federal and state deferred tax assets. Accordingly, a full valuation allowance of the U.S. net deferred tax asset had been established at December 31, 2018 and 2017. The net change in the valuation allowance for deferred tax assets was an increase of $164.8 million and a decrease of $69.5 million for the years ended December 31, 2018 and 2017, respectively. This increase for the year ended December 31, 2018 was primarily due to the generation of federal and state net operating losses.

The Company generated foreign deferred tax assets mainly consisting of net operating loss carryforwards, stock-based compensation and unrealized gain/losses. Based upon the income projections in the majority of the foreign jurisdictions, the Company believes it will realize the benefit of its future deductible differences. As such, the Company has not recorded a valuation allowance against all foreign jurisdictions except for Brazil and Switzerland. Brazil and Switzerland generated deferred tax assets, which consist of net operating loss carryforwards and stock-based compensation expense. A full valuation allowance has been recorded against these deferred tax assets.

As of December 31, 2018, the Company had federal and state net operating loss carryforwards of $438.5 million and $503.3 million, respectively, available to reduce future taxable income. The federal and state net operating loss carry forwards of $240.0 million and $503.3 million will expire at various dates between 2019 and 2038. Federal net operating loss of $198.5 million can be carried forward indefinitely. In addition, the Company had $386.3 million of federal net operating loss carryforwards generated by its wholly-owned subsidiary, Sarepta International C.V., of which $318.7 million will expire between 2036 and 2037. $67.6 million of Sarepta International C.V.’s net operating losses can be carried forward indefinitely. Utilization of these net operating losses could be limited under Section 382 of the Internal Revenue Code and similar state laws based on ownership changes and the value of the Company’s stock. Additionally, the Company has $50.3 million and $20.9 million of federal and state research and development credits, respectively, available to offset future taxable income. These federal and state research and development credits begin to expire between 2019 and 2038 and between 2019 and 2033, respectively. The Company also has foreign net operating loss carryforwards of $55.2 million, mainly derived from the net operating loss generated by Sarepta International Holdings GmbH, which will begin to expire in 2024.

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

	For the Year Ended December 31,
	2018		2017			2016
	(in thousands)
Balance at beginning of the period	$	5,134	$	4,644		$	3,706
Increase related to current year tax positions		2,164		735			801
Increase related to prior year tax positions		30,246		—			137
Decrease related to prior year tax positions		—		(245	)		—
Balance at end of the period	$	37,544	$	5,134		$	4,644

The balance of total unrecognized tax benefits at December 31, 2018, if recognized, would not affect the effective tax rate on income from continuing operations, due to a full valuation allowance against the Company’s U.S. deferred tax assets. The Company does not expect that the amount of unrecognized tax benefits to change significantly in the next twelve months. The Company, including its domestic subsidiaries, files consolidated U.S. federal and state income tax returns. The Company’s policy is to recognize interest and/or penalties related to income tax matters in income tax expense. It had no accrual for interest or penalties on its balance sheet at December 31, 2018 or 2017. No interest and/or penalties were recognized in 2017 or 2016.

NET LOSS PER SHARE

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

NET LOSS PER SHARE

20. NET LOSS PER SHARE

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

	For the Year Ended December 31,
	2018			2017			2016
	(in thousands, except per share amounts)
Net loss	$	(361,918	)	$	(50,688	)	$	(267,265	)
Weighted-average number of shares of common stock and common stock equivalents outstanding:
Weighted-average number of shares of common stock outstanding for computing basic loss per share		66,250			58,818			48,697
Dilutive effect of outstanding stock awards and stock options after application of the treasury stock method*		—			—			—
Weighted-average number of shares of common stock and dilutive common stock equivalents outstanding for computing diluted loss per share		66,250			58,818			48,697
Net loss per share — basic and diluted	$	(5.46	)	$	(0.86	)	$	(5.49	)

For the years ended December 31, 2018 and 2017, stock options, RSAs, RSUs, SARs and ESPP to purchase approximately 9.1 million and 9.4 million shares of common stock, respectively, were excluded from the net loss per share calculation as their effect would have been anti-dilutive. For the year ended December 31, 2016, stock options, RSAs and SARs to purchase approximately 5.7 million shares of common stock, were excluded from the net loss per share calculation as their effect would have been anti-dilutive. The Company accounts for the effect of the 2024 Notes on diluted net earnings per share using the if-converted method as they may be settled in cash or shares at the Company’s option. While the closing price on December 31, 2018 exceeded the conversion price of $73.42, the potential shares issuable under the 2024 Notes were excluded from the calculation of diluted loss per share as they were anti-dilutive using the if-converted method. In the period of conversion, the 2024 Notes will have no impact on diluted net earnings (loss) if they are settled in cash and will have an impact on diluted earnings per share if the Notes are settled in shares upon conversion and when the Company in an income position.

COMMITMENTS AND CONTINGENCIES

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

COMMITMENTS AND CONTINGENCIES

21. COMMITMENTS AND CONTINGENCIES

Lease Obligations

In June 2013, the Company entered into a lease agreement (“Cambridge lease”) for its headquarters located in Cambridge, Massachusetts. In April and September 2018, the Company entered into the seventh and eighth amendment, respectively, to its Cambridge lease which extended the original term of the lease to September 30, 2025 and increased the total rental space to approximately 163,842 square feet. The agreement calls for a security deposit in the form of a letter of credit totaling $1.0 million. The Company purchased a certificate of deposit to meet the requirement and it is recorded as a long-term restricted investment in the consolidated balance sheets as of December 31, 2018 and 2017.

In March and July of 2018, the Company entered into sublease agreements in Andover, Massachusetts, to lease a total of 23,102 square feet of additional office space. These subleases expires in December 2020.

In November 2018, the Company entered into a sublease agreement in Burlington, Massachusetts, to lease 44,740 square feet of additional office and lab space. The sublease expires in January 2022.

In December 2018, the Company entered into lease agreements in Dublin and Columbus, Ohio, to lease 22,600 square feet and 77,679 square feet, respectively, of additional office and lab space. The leases expire in November 2019 and June 2026, respectively.

In August 2015 and December 2016, the Company entered into agreements to sublease 15,077 square feet and 7,461 square feet, respectively, of office space to two unrelated third parties. The subleases expired in March and December 2017, respectively.

The Company also leased laboratory and office space in Corvallis, Oregon which would expire in December 2020. The second floor and the first floor of the facility were vacated and closed and made available for sub-leasing in December 2016 and April 2017, respectively. The lease was terminated in June 2018.

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

	As of December 31, 2018 (in thousands)
2019	$	11,588
2020		11,395
2021		12,558
2022		10,757
2023		10,898
Thereafter		20,524
Total minimum lease payments	$	77,720

Royalty Obligations

The Company is obligated to pay royalties on net sales of certain of its products. The royalty rates range from the low-single-digit to high teens percentages for both inside and outside the U.S. Under the license agreement with BioMarin signed in July 2017, the Company is required to make a mid-single-digit percentage of royalty on the net product sales inside the U.S. and maybe required to make a high-single-digit percentage of royalty on net product sales outside the U.S. Under the Amended and Restated License Agreement with UWA signed in April 2013, the Company may be obligated to pay a low-single-digit percentage of royalty on the net sales of products exceeding certain amounts, which includes EXONDYS 51. For the years ended December 31, 2018 and 2017, the Company recorded $15.1 and $4.7 million, respectively, of royalty payments to BioMarin on its consolidated statements of operations and comprehensive loss.

Milestone Obligations

The Company has collaboration and license agreements for which it could be obligated to pay up-front, development, regulatory and sales milestones as a product candidate proceeds from the submission of an investigational new drug application through approval for commercial sale and beyond. As of December 31, 2018, the Company may be obligated to make up to $378.8 million of future development, regulatory, up-front royalty and sales milestone payments associated with its collaboration and license agreements. For the years ended December 31, 2018, 2017 and 2016, the Company recognized approximately $142.4million, $22.0 million and $47.6 million relating to certain up-front and development milestone payments as research and development expense, respectively, under these agreements. As of December 31, 2017 and 2016, the Company also recorded $6.6 million and $1.0 million as in-licensed right assets, respectively, corresponding to the execution of the settlement and license agreements with BioMarin and the first sale of EXONDYS 51, with no similar activity as of December 31, 2018.

Other Funding Commitments

The Company has several on-going clinical trials in various clinical trial stages. Its most significant clinical trial expenditures are to contract research organizations (“CROs”). The CRO contracts are generally cancellable at the Company’s option. As of December 31, 2018, the Company has approximately $71.6 million in cancellable future commitments based on existing CRO contracts. For the years ended December 31, 2018, 2017 and 2016, the Company recognized approximately $19.6 million, $13.9 million and $8.5 million, respectively, for expenditures incurred by CROs.

Manufacturing Obligations

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

Brammer Bio MA, LLC

In June 2018, the Company entered into a Development, Commercial Manufacturing and Supply Agreement (“Brammer Manufacturing Agreement”), with Brammer Bio MA, LLC (“Brammer”). Pursuant to the terms of the Brammer Manufacturing Agreement, Brammer agreed to provide the Company with access to clinical and commercial manufacturing capacity for its gene therapy programs.

As part of the Brammer Manufacturing Agreement, the Company will purchase product in batches from Brammer, subject to minimum and maximum annual purchase requirements. Further, the Company: (i) was required to make a $20.0 million advance payment to Brammer upon execution of the agreement, (ii) was required to make two non-refundable payments of $5.0 million each to Brammer in the third and fourth quarter of 2018 to be used in the specification, selection, and procurement of the related process equipment to be utilized under the agreement, and (iii) is required to make a $10.0 million quarterly capacity access fee payment to Brammer throughout the term of the agreement. However, through June 30, 2019, a reduced quarterly capacity access fee will be in effect as Brammer works towards achieving full capacity at its facility. In addition, one-tenth of the $20.0 million advance payment will be applied as a credit to the quarterly capacity access fees due and payable from July 1, 2019 through December 31, 2021, resulting in a net capacity access fee of $8.0 million.

The term of the Brammer Manufacturing Agreement will continue for a period of six years following the first regulatory approval of a product manufactured under the agreement. The term will automatically renew for successive two years unless the Company notifies Brammer of its intention not to renew (no less than twenty-four months prior to the expiration of the term). The Company also has the ability to terminate the agreement prior to expiration but would be required to continue remitting capacity access fees to Brammer for up to eight additional quarters.

The Company has determined that the Brammer Manufacturing Agreement does not contain an embedded lease because it does not convey the right to control the use of the facility or related equipment. This conclusion was based on the Company’s inability or right to control physical access to Brammer’s facility and the related equipment, and the ability of one or more parties, other than the Company, to take more than a minor amount of the output that will be produced during the term of the agreement.

As of December 31, 2018, the Company has made payments totaling $40.0 million to Brammer under the Brammer Manufacturing Agreement consisting of: (i) the $20.0 million advance payment made in June 2018, (ii) the two $5.0 million process equipment fees paid in July 2018 and October 2018, and (iii) the first two reduced quarterly capacity access fee payments of $5.0 million made in July 2018 and September 2018. Of the cumulative amount paid of $40.0 million, $5.0 million was recorded as an other current asset and $35.0 million as an other non-current asset, in the accompanying consolidated balance sheets.

The advance payment and process equipment fees were capitalized and will be amortized over their expected economic benefit to research and development expense, prior to regulatory approval of the related product, commencing upon the first batch delivery to the Company, which is currently estimated to occur in July 2019. Upon regulatory approval, amortization expense will be classified to cost of inventory with the recognition in cost of sales as the sales of product occur. Capacity access fee payments made prior to the first batch delivery to the Company will be capitalized and amortized to expense in a manner similar to the advance payment and process equipment fees. Capacity access fee payments made subsequent to the first batch delivery to the Company will be expensed as incurred to research and development expense, prior to regulatory approval of the related product. Upon regulatory approval, the expense associated with capacity access fee payments will be classified to cost of inventory with the recognition in cost of sales as the sales of product occur. In the event the Company does not expect services under the Brammer Manufacturing Agreement to be rendered, the capitalized payments will be charged to expense.

Paragon Bioservices, Inc.

In October 2018, the Company entered into a manufacturing collaboration agreement (“Paragon Collaboration Agreement”) with Paragon Bioservices, Inc. (“Paragon”). Pursuant to the terms of the Paragon Collaboration Agreement, Paragon agreed to provide the Company with two dedicated clean room suites and an option to reserve two additional clean room suites (“Optional Clean Room Suites”) for its gene therapy programs.

As part of the Paragon Collaboration Agreement, the Company will purchase product in batches from Paragon subject to minimum annual purchase requirements. Further, the Company is required to pay Paragon: (i) use fees of $1.0 million per year per clean room suite after the clean rooms are fully qualified and validated to manufacture the Company’s materials, and (ii) clean room reservation fees, which could total up to $48.0 million if the Company exercises the option to reserve the Optional Clean Room Suites. Additional use fees and reservation fees are required if the Company has equipment needs beyond the basic equipment package included in the initial clean room suites. In addition, Paragon will provide the Company with a credit of up to 100% of the clean room use fee if certain clean room capacity utilization thresholds are met.

The term of the Paragon Collaboration Agreement will continue until the earlier of: (i) the latest expiration date of any commercial supply agreement in effect, or (ii) December 31, 2025. The Company has the ability to terminate the Paragon Collaboration Agreement prior to expiration, subject to potential additional financial consideration.

The Company has concluded that the Paragon Collaboration Agreement contains an embedded lease as the Company has the right to operate the clean room suites in a manner it determines while also controlling more than a minor amount of the output during the term of the agreement. The Company also concluded that it is not the deemed owner during construction of the clean room suites nor does the agreement represent a capital lease under ASC 840, “Leases”. As a result, the Paragon Collaboration Agreement will be accounted for as an operating lease. As of December 31, 2018, the Company has made reservation fee payments totaling $26.0 million to Paragon, which was recorded as an other non-current asset in the accompanying consolidated balance sheets. This amount, along with any additional reservation fee payments made, will be amortized on a straight-line basis as rent expense over the term of the embedded lease, commencing upon the date that the Company controls the physical use of the clean room suites, currently anticipated to occur in the fourth quarter of 2019. Use fees associated with the clean room suites are considered contingent rental payments, and will be charged to rent expense when (and if) incurred. Rent expense recognized prior to regulatory approval of the related product will be classified to research and development expense. Upon regulatory approval, rent expense will be classified to cost of inventory with the recognition in cost of sales as the sales of product occur.

Aldevron, LLC

In December 2018, the Company entered into a Clinical and Commercial Supply Agreement (the “CCSA”) with Aldevron LLC (“Aldevron”) for the supply of plasmid DNA to fulfill its needs for gene therapy clinical trials and commercial supply. Pursuant to the terms of the CCSA, Aldevron agreed to reserve a certain number of manufacturing slots (“Reserved Slots”) on a quarterly basis. The initial term of the CCSA expires on December 31, 2019. The Company has an option to extend the CCSA to December 31, 2020 (the “2020 Option”) and other option to extend the term of the CCSA for an additional year to December 31, 2021 (the “2021 Renewal Right”).

The Company may be required to make up to $60.0 million in prepayments associated with the CCSA should the Company exercise both the 2020 Option and the 2021 Renewal Right. The prepayments will be credited back to Sarepta, until exhausted, for each batch of product delivered by Aldevron, in an amount equal to 50% of the batch invoice amount. The Company has determined that the CCSA does not contain an embedded lease because it does not convey the right to control the use of Aldevron’s facility or related equipment therein. As of December 31, 2018, the Company has made a prepayment of $10.0 million to Aldevron under the CCSA, which was recorded as an other current asset in the accompanying consolidated balance sheets. In the event the Company does not expect services under the CCSA to be rendered to fully exhaust any prepayments made to Aldevron, the applicable balance will be charged to expense at the time this determination is made.

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

	As of December 31, 2018 (in thousands)
2019	$	182,352
2020		34,000
2021		34,000
2022		42,000
2023		42,000
Thereafter		124,000
Total manufacturing commitments	$	458,352

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, a 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. 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.

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 alleged 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 sought 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. On July 10, 2018, Plaintiff filed a Notice of Nonsuit as to all causes of action asserted in the complaint. On July 11, 2018, the court accepted the Notice of Nonsuit and all causes of action asserted in the complaint were dismissed with prejudice. As such, there is no risk of loss in connection with this litigation.

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 alleged 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 sought unspecified damages, actions to reform and improve corporate governance and internal procedures, and attorneys’ fees. On July 23, 2018, Plaintiffs filed a Notice of Voluntary Dismissal and dismissed their claims without prejudice. As such, there is no risk of loss in connection with this litigation.

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 agreed to a settlement, which was approved by the Delaware Court of Chancery on September 4, 2018. The Company does not believe that disposition of the McDonald suit will have a material financial impact on the Company.

SUBSEQUENT EVENT

12 Months Ended

Dec. 31, 2018

Subsequent Events [Abstract]

SUBSEQUENT EVENT

22. SUBSEQUENT EVENT

On February 27, 2019, the Company announced that it exercised the exclusive option to acquire Myonexus as more fully described in Note 3, License and Collaboration Agreements. The final exercise price as negotiated between the Company and Myonexus was $165.0 million. The Company may also be required to make up to $200.0 million in additional payments to the selling shareholders of Myonexus based on the achievement of certain milestones.

The Company has evaluated subsequent events from the date of the consolidated balance sheet through the date the consolidated financial statements were issued.

FINANCIAL INFORMATION BY QUARTER (UNAUDITED)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

FINANCIAL INFORMATION BY QUARTER (UNAUDITED)

23. FINANCIAL INFORMATION BY QUARTER (UNAUDITED)

	2018 for Quarter Ended
	December 31			September 30			June 30			March 31
	(in thousands)
Revenues:
Product, net	$	84,415		$	78,486		$	73,529		$	64,604
Total revenues		84,415			78,486			73,529			64,604

Cost and expenses:
Cost of sales (excluding amortization of in-licensed rights)		13,135			8,741			6,735			5,582
Research and development		146,207			86,584			122,848			46,204
Selling, general and administrative		64,220			53,044			47,156			43,341
Amortization of in-licensed rights		216			216			217			216
Total cost and operating expenses		223,778			148,585			176,956			95,343
Operating loss		(139,363	)		(70,099	)		(103,427	)		(30,739	)
Other loss:
Interest expense and other, net		(2,311	)		(6,968	)		(5,218	)		(4,485	)
Total other loss		(2,311	)		(6,968	)		(5,218	)		(4,485	)
Loss before income (benefit) tax expense		(141,674	)		(77,067	)		(108,645	)		(35,224	)
Income tax (benefit) expense		(779	)		(674	)		622			139
Net loss	$	(140,895	)	$	(76,393	)	$	(109,267	)	$	(35,363	)

Net loss per share - basic and diluted	$	(2.05	)	$	(1.15	)	$	(1.67	)	$	(0.55	)

Weighted average number of shares of common stock used in computing basic and diluted net loss per share		68,653			66,209			65,484			64,631

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

Cost and expenses:
Cost of sales (excluding amortization of in-licensed rights)		3,546			3,078			505			224
Research and development		44,441			34,239			58,908			29,119
Selling, general and administrative		32,221			28,176			36,069			26,216
EXONDYS 51 settlement and license charges		—			25,588			2,839			—
Amortization of in-licensed rights		216			780			29			28
Total cost and operating expenses		80,424			91,861			98,350			55,587
Operating loss		(23,147	)		(45,907	)		(63,339	)		(39,245	)
Other (loss) income:
Interest (expense) income and other, net		(2,693	)		184			184			335
Gain from sale of Priority Review Voucher		—			—			—			125,000
Total other (loss) income		(2,693	)		184			184			125,335
(Loss) income before income tax (benefit) expense		(25,840	)		(45,723	)		(63,155	)		86,090
Income tax (benefit) expense		(1,842	)		2,011			(109	)		2,000
Net (loss) income	$	(23,998	)	$	(47,734	)	$	(63,046	)	$	84,090

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

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

12 Months Ended

Dec. 31, 2018

Estimates and Uncertainties

Basis of Presentation

Fair Value Measurements

•

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

•

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

Cash and Cash Equivalents

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

Investments

Available-For-Sale Debt Securities

Equity Investments

Accounts Receivable

Concentration of Credit Risk

Inventories

Property and Equipment

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

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

Intangible assets

Impairment of Long-Lived Assets

Convertible Debt

Revenue Recognition

Product revenues

Variable Consideration

•

Deferred revenue

Research and Development

Stock-Based Compensation

Income Taxes

Rent Expense

For the years ended December 31, 2018, 2017 and 2016, the Company recognized rent expense and occupancy costs of $7.9 million, $4.4 million and $5.6 million, respectively.

Commitments and Contingencies

Recent Accounting Pronouncements

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

12 Months Ended

Dec. 31, 2018

Summary of Estimated Useful Lives of Property and Equipment

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

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

FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Assets Measured and Carried at Fair Value

	Fair Value Measurement as of December 31, 2018
	Total		Level 1		Level 2		Level 3
	(in thousands)
Assets
Money market funds	$	42,920	$	42,920	$	—	$	—
Commercial paper		125,907		—		125,907		—
Government and government agency bonds		760,235		760,235		—		—
Corporate bonds		43,468		43,468		—		—
Strategic equity investments		31,739		1,739		—		30,000
Certificates of deposit		1,001		1,001		—		—
Total	$	1,005,270	$	849,363	$	125,907	$	30,000

	Fair Value Measurement as of December 31, 2017
	Total		Level 1		Level 2		Level 3
	(in thousands)
Assets
Money market funds	$	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	$	909,059	$	765,719	$	143,340	$	—

CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES (Tables)

12 Months Ended

Dec. 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 December 31
	2018		2017
	(in thousands)
Money market funds	$	42,920	$	352,370
Corporate bonds		—		16,720
Government and government agency bonds		111,587		49,972
Commercial paper		14,940		—
Total	$	169,447	$	419,062

Summary of Company Cash, Cash Equivalents and Investments

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

	As of December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Market Value
	(in thousands)
Cash and money market funds	$	244,302	$	—	$	—		$	244,302
Commercial paper		125,907		—		—			125,907
Government and government agency bonds		760,258		12		(35	)		760,235
Corporate bonds		43,544		—		(76	)		43,468
Total cash, cash equivalents and investments		1,174,011		12		(111	)		1,173,912
As reported:
Cash and cash equivalents	$	370,827	$	3	$	(1	)	$	370,829
Short-term investments		803,184		9		(110	)		803,083
Total cash, cash equivalents and investments	$	1,174,011	$	12	$	(111	)	$	1,173,912

	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		133,368		—		—			133,368
Government and government agency bonds		294,915		2		(200	)		294,717
Corporate bonds
Current		118,121		—		(145	)		117,976
Non-current		10,016		—		(36	)		9,980
Total cash, cash equivalents and investments		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 cash, cash equivalents and investments	$	1,089,419	$	2	$	(381	)	$	1,089,040

ACCOUNTS RECEIVABLE AND RESERVES FOR PRODUCT SALES (Tables)

12 Months Ended

Dec. 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 December 31,
	2018		2017
	(in thousands)
Product sales, net of discounts and allowances	$	48,252	$	28,539
Government contract receivables		792		929
Total accounts receivable, net	$	49,044	$	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 Accruals			Total
	(in thousands)
Balance, as of December 31, 2017	$	995		$	6,959		$	169		$	464		$	8,587
Provision		12,284			28,420			2,624			5,286			48,614
Payments/credits		(11,901	)		(11,103	)		(2,255	)		(3,432	)		(28,691	)
Balance, as of December 31, 2018	$	1,378		$	24,276		$	538		$	2,318		$	28,510

Summary of Total Reserves Included in Consolidated Balance Sheets

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

	As of December 31,
	2018		2017
	(in thousands)
Reduction to accounts receivable	$	2,364	$	1,285
Component of accrued expenses		26,146		7,302
Total reserves	$	28,510	$	8,587

INVENTORY (Tables)

12 Months Ended

Dec. 31, 2018

Inventory Disclosure [Abstract]

Summary of Components of Inventory

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

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Raw materials	$	71,313	$	53,875
Work in progress		47,279		27,442
Finished goods		6,853		2,288
Total inventory	$	125,445	$	83,605

OTHER ASSETS (Tables)

12 Months Ended

Dec. 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 December 31, 2018		As of December 31, 2017
	(in thousands)
Manufacturing-related deposits and prepaids	$	39,036	$	18,650
Leasehold improvement receivable		13,474		—
Prepaid clinical and pre-clinical expenses		9,706		5,175
Prepaid maintenance services		2,994		1,711
Prepaid income tax		2,130		—
Prepaid research expenses		1,932		2,896
Interest receivable		1,918		709
Prepaid commercial expenses		1,573		1,589
Asset held for sale		—		1,501
Other prepaids		3,671		2,726
Other		1,348		1,554
Total other current assets	$	77,782	$	36,511

Summary of Investments and Other Assets

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

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Manufacturing-related deposits	$	62,821	$	—
Strategic investments		31,739		—
Prepaid clinical expenses		7,541		7,488
Alternative minimum tax credit		3,367		3,315
Restricted investments		1,001		784
Long-term available-for-sale securities		—	$	9,980
Other		825		242
Total investments and other assets	$	107,294	$	21,809

PROPERTY AND EQUIPMENT, NET (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Summarizes Components of Property and Equipment, Net

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

	As of December 31,
	2018			2017
	(in thousands)
Land	$	4,158		$	4,158
Building and improvements		22,972			14,543
Software and computer equipment		15,774			8,965
Lab equipment		17,659			9,860
Office equipment		3,663			2,211
Leasehold improvements		20,937			13,633
Construction in progress		40,010			7,808
Property and equipment, gross		125,173			61,178
Less: accumulated depreciation		(28,149	)		(18,022	)
Property and equipment, net	$	97,024		$	43,156

INTANGIBLE ASSETS (Table)

12 Months Ended

Dec. 31, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

Summary of Components of Intangible Assets

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

	As of December 31, 2018		As of December 31, 2017
	(in thousands)
Patents	$	5,773	$	7,660
In-licensed rights		5,626		6,491
Software licenses		175		204
Total	$	11,574	$	14,355

Summary of Estimated Future Amortization for Intangible Assets

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

	As of December 31, 2018 (in thousands)
2019		1,490
2020		1,300
2021		1,299
2022		1,298
2023		1,297
Total	$	6,684

ACCRUED EXPENSES (Tables)

12 Months Ended

Dec. 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 December 31, 2018		As of December 31, 2017
	(in thousands)
Product revenue related reserves	$	26,146	$	7,302
Accrued employee compensation costs		24,692		14,402
Accrued milestone expense		24,020		—
Accrued contract manufacturing costs		15,794		14,019
Accrued clinical and pre-clinical costs		11,396		15,975
Accrued professional fees		11,319		6,794
Accrued BioMarin royalties		8,254		2,846
Accrued property and equipment		5,421		2,525
Accrued collaboration cost sharing		2,167		—
Accrued research costs		1,070		401
Accrued interest expenses		1,045		1,291
Accrued income taxes		301		943
Other		2,470		2,484
Total accrued expenses	$	134,095	$	68,982

RESTRUCTURING (Tables)

12 Months Ended

Dec. 31, 2018

Restructuring And Related Activities [Abstract]

Summary of Restructuring Costs

	For the Year Ended December 31
	2018			2017		2016
	(in thousands)
Research and development	$	—		$	188	$	2,013
Selling, general and administrative		(2,222	)		2,832		2,549
Total restructuring expenses	$	(2,222	)	$	3,020	$	4,562

Summary of Restructuring Reserve

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

	For the Year Ended December 31
	2018			2017
	(in thousands)
Restructuring reserve beginning balance	$	2,933		$	1,588
Restructuring expenses incurred during the period		—			3,020
Amounts paid during the period		(711	)		(1,675	)
Reversal of cease-use liability		(2,222	)		—
Restructuring reserve ending balance	$	—		$	2,933

INDEBTEDNESS (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Summary of Debt Facilities

The following table summarizes the Company’s debt facilities for the periods indicated:

	As of December 31, 2018			As of December 31, 2017
	(in thousand)			(in thousand)
Par value of the 2024 Notes	$	570,000		$	570,000
Unamortized discount - equity component		(140,206	)		(158,890	)
Unamortized discount - debt issuance costs		(9,240	)		(10,449	)
Net carrying value of convertible debt		420,554			400,661
Net carrying value of other debt facilities		—			30,390
Net carrying value of total debt facilities		420,554			431,051

Fair value of convertible debt	$	952,681		$	619,641

Summarizes Total Gross Payments Due under Company's Debt Arrangements

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

	As of December 31, 2018
	(in thousands)
2019	$	—
2020		—
2021		—
2022		—
2023		—
Thereafter		570,000
Total Payments	$	570,000

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21506-112644

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
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-Section 45
-Paragraph 2
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Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21521-112644

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21538-112644

Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-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 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

STOCK-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Stock Option Activity

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

	For the Year Ended December 31,
	2018					2017							2016
				Weighted							Weighted					Weighted
				Average							Average					Average
				Exercise							Exercise					Exercise
	Shares			Price		Shares					Price		Shares			Price
Grants outstanding at beginning of the period		8,806,204		$	29.74		5,436,951				$	22.70		6,515,976		$	23.91
Granted		2,152,439			90.15		4,805,722		(1	)		35.09		1,285,051			14.89
Exercised		(2,119,306	)		22.89		(792,845	)				17.40		(1,056,821	)		18.31
Cancelled		(448,166	)		46.11		(643,624	)				25.44		(1,307,255	)		24.61
Grants outstanding at end of the period		8,391,171		$	46.09		8,806,204				$	29.74		5,436,951		$	22.70

Grants exercisable at end of the period		2,304,791		$	27.69		3,288,712				$	24.76		2,942,624		$	24.42
Grants vested and expected to vest at end of the period		6,643,835		$	45.43		6,910,022				$	28.49		5,116,409		$	22.83

(1)

The weighted-average grant date fair value per share of stock options granted during the years ended December 31, 2018, 2017 and 2016 was $44.66, $14.78 and $12.46, respectively.

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
Options outstanding at December 31, 2018	$	551,131	7.7
Options exercisable at December 31, 2018	$	187,693		5.5
Options vested and expected to vest at December 31, 2018	$	433,681	7.4

Summary of Company's Stock Options Vested and Exercised

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

	For the Year Ended December 31,
	2018		2017		2016
	(in thousands)
Aggregate grant date fair value of stock options vested	$	16,316	$	18,225	$	30,651
Aggregate intrinsic value of stock options exercised	$	158,936	$	20,922	$	30,610

Summary of Stock Appreciation Rights Activity

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

	For the Year Ended December 31,
	2018				2017				2016
			Weighted				Weighted				Weighted
			Average				Average				Average
			Exercise				Exercise				Exercise
	Shares		Price		Shares		Price		Shares		Price
Grants outstanding at beginning of the period		100,000	$	23.85		100,000	$	23.85		100,000	$	23.85
Grants outstanding at end of the period		100,000	$	23.85		100,000	$	23.85		100,000	$	23.85
Grants exercisable at end of the period		100,000	$	23.85		100,000	$	23.85		100,000	$	23.85
Grants vested and expected to vest at end of the period		100,000	$	23.85		100,000	$	23.85		100,000	$	23.85

			Weighted
	Aggregate		Average
	Intrinsic		Remaining
	Value		Contractual
	(in thousands)		Life (Years)
SARs outstanding, exercisable and vested at December 31, 2018	$	8,528	3.9

Summary of Employee Stock Purchase Plan Activity and Expense

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

	For the Year Ended December 31,
	2018		2017		2016
Number of shares purchased		75,094		102,698		137,113
Proceeds received (in millions)	$	2.3	$	1.4	$	1.6

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

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

	For the Year Ended December 31,
	2018		2017		2016
	(in thousands)
Research and development	$	14,214	$	8,542	$	9,499
Selling, general and administrative		35,913		21,923		20,463
Total stock-based compensation	$	50,127	$	30,465	$	29,962

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

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

	For the Year Ended December 31,
	2018		2017		2016
	(in thousands)
Stock options	$	37,671	$	23,416	$	26,320
Restricted stock awards/units		10,632		5,295		872
Employee stock purchase plan		1,824		1,754		2,380
Other		—		—		390
Total stock-based compensation	$	50,127	$	30,465	$	29,962

Restricted Stock Awards (RSAs) [Member]

Summary of Restricted Stock Award and Restricted Stock Units Activity

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

	For the Year Ended December 31,
	2018					2017					2016
				Weighted					Weighted					Weighted
				Average					Average					Average
				Exercise					Exercise					Exercise
	Shares			Price		Shares			Price		Shares			Price
Grants outstanding at beginning of the period		411,781		$	37.23		153,170		$	34.53		161,320		$	21.50
Granted		27,590			98.57		341,500			34.58		117,553			41.22
Vested		(187,050	)		39.34		(63,264	)		14.60		(59,703	)		13.62
Forfeited		—			—		(19,625	)		43.02		(66,000	)		33.51
Grants outstanding at end of the period		252,321		$	42.37		411,781		$	37.23		153,170		$	34.53

Restricted Stock Units (RSUs) [Member]

Summary of Restricted Stock Award and Restricted Stock Units Activity

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

	For the Year Ended December 31
	2018					2017
				Weighted					Weighted
				Average					Average
				Exercise					Exercise
	Shares			Price		Shares			Price
Grants outstanding at beginning of the period		66,552		$	33.72		—		$	—
Granted		230,736			87.95		181,029			33.03
Vested		(30,276	)		33.23		(78,017	)		32.63
Forfeited		(15,714	)		71.45		(36,460	)		32.63
Grants outstanding at end of the period		251,298		$	81.21		66,552		$	33.72

Stock Options [Member]

Assumptions for Measuring Fair Values of Stocks

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

	For the Year Ended December 31,
	2018	2017	2016
Risk-free interest rate (1)	2.5 - 3.0%	1.6 - 2.1%	1.1 – 1.8%
Expected dividend yield (2)	—	—	—
Expected term (3)	5.06 years	4.2 - 4.8 years	4.2 – 4.8 years
Expected volatility (4)	52.4 - 60.8%	54.0 - 63.0%	78.2 – 137.1 %

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

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

(3)	The expected term is estimated using historical exercise behavior.

(4)

OTHER INCOME AND LOSS (Tables)

12 Months Ended

Dec. 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 year ended December 31,
	2018			2017			2016
	(in thousand)
Interest expense	$	(33,709	)	$	(5,801	)	$	(1,892	)
Interest income		6,810			1,809			507
Amortization of investment discount (premium)		8,573			1,401			(80	)
Other (loss) income		(656	)		601			930
Gain from sale of Priority Review Voucher		—			125,000			—
Total other (loss) income	$	(18,982	)	$	123,010		$	(535	)

INCOME TAXES (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Summary of Loss before Provision for Income Taxes by Jurisdiction

The following table summarizes the loss before the provision for income taxes by jurisdiction for the periods indicated:

	For the Year Ended December 31,
	2018			2017			2016
	(in thousands)
Domestic	$	(309,294	)	$	(45,686	)	$	(267,265	)
Foreign		(53,316	)		(2,942	)		—
Total	$	(362,610	)	$	(48,628	)	$	(267,265	)

Summary of Provision for Income Taxes

The following table summarizes provision for income taxes in the accompanying consolidated financial statements for the periods indicated:

	For the Year Ended December 31,
	2018			2017		2016
	(in thousands)
Current provision
Federal	$	(110	)	$	204	$	—
State		(653	)		1,856		—
Foreign		311			—		—
Total current provision		(452	)		2,060		—
Deferred benefit:
Federal		—			—		—
State		—			—		—
Foreign		(240	)		—		—
Total deferred benefit		(240	)		—		—
Total current provision	$	(692	)	$	2,060	$	—

Reconciliation Between Effective Tax Rate and Federal Income Tax Rate

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

	For the Year Ended December 31,
	2018				2017				2016
Federal income tax rate		21.0		%		34.0		%		34.0		%
State taxes		12.3				(27.7	)			—
Research and development and other tax credits		3.1				8.5				1.5
Valuation allowance		(45.5	)			(93.2	)			(17.3	)
Permanent differences		6.9				6.4				(3.4	)
Sarepta International C.V. return to provision		(0.1	)			62.1				—
Impact of tax reform, net of valuation allowance		—				5.90				—
Foreign rate differential		(0.9	)			(0.6	)			(14.8	)
Other		3.4				0.40				—
Effective tax rate		0.2		%		(4.2	)	%		—		%

Analysis of Deferred Tax Assets and Liabilities

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

	As of December 31,
	2018			2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	212,342		$	144,897
Difference in depreciation and amortization		55,162			1,210
Research and development tax credits		67,309			44,306
Stock-based compensation		15,538			14,392
Deferred rent		4,831			1,210
Deferred revenue		888			904
Capitalized inventory		22,943			22,127
Other		15,727			4,071
Total deferred tax assets		394,740			233,117
Deferred tax liabilities:
Debt discount		(25,162	)		(28,548	)
Total deferred tax liabilities		(25,162	)		(28,548	)
Valuation allowance		(369,345	)		(204,569	)
Net deferred tax assets	$	233		$	—

Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits

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

	For the Year Ended December 31,
	2018		2017			2016
	(in thousands)
Balance at beginning of the period	$	5,134	$	4,644		$	3,706
Increase related to current year tax positions		2,164		735			801
Increase related to prior year tax positions		30,246		—			137
Decrease related to prior year tax positions		—		(245	)		—
Balance at end of the period	$	37,544	$	5,134		$	4,644

NET LOSS PER SHARE (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Basic and Diluted Net Loss Per Share

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

	For the Year Ended December 31,
	2018			2017			2016
	(in thousands, except per share amounts)
Net loss	$	(361,918	)	$	(50,688	)	$	(267,265	)
Weighted-average number of shares of common stock and common stock equivalents outstanding:
Weighted-average number of shares of common stock outstanding for computing basic loss per share		66,250			58,818			48,697
Dilutive effect of outstanding stock awards and stock options after application of the treasury stock method*		—			—			—
Weighted-average number of shares of common stock and dilutive common stock equivalents outstanding for computing diluted loss per share		66,250			58,818			48,697
Net loss per share — basic and diluted	$	(5.46	)	$	(0.86	)	$	(5.49	)

COMMITMENTS AND CONTINGENCIES (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Summary of Aggregate Future Minimum Payments under Company's Leases

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

	As of December 31, 2018 (in thousands)
2019	$	11,588
2020		11,395
2021		12,558
2022		10,757
2023		10,898
Thereafter		20,524
Total minimum lease payments	$	77,720

Summary of Non-Cancelable Contractual Obligations Arising From Long-term Contractual Arrangements

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

	As of December 31, 2018 (in thousands)
2019	$	182,352
2020		34,000
2021		34,000
2022		42,000
2023		42,000
Thereafter		124,000
Total manufacturing commitments	$	458,352

FINANCIAL INFORMATION BY QUARTER (UNAUDITED) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Financial Information by Quarter

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

Cost and expenses:
Cost of sales (excluding amortization of in-licensed rights)		3,546			3,078			505			224
Research and development		44,441			34,239			58,908			29,119
Selling, general and administrative		32,221			28,176			36,069			26,216
EXONDYS 51 settlement and license charges		—			25,588			2,839			—
Amortization of in-licensed rights		216			780			29			28
Total cost and operating expenses		80,424			91,861			98,350			55,587
Operating loss		(23,147	)		(45,907	)		(63,339	)		(39,245	)
Other (loss) income:
Interest (expense) income and other, net		(2,693	)		184			184			335
Gain from sale of Priority Review Voucher		—			—			—			125,000
Total other (loss) income		(2,693	)		184			184			125,335
(Loss) income before income tax (benefit) expense		(25,840	)		(45,723	)		(63,155	)		86,090
Income tax (benefit) expense		(1,842	)		2,011			(109	)		2,000
Net (loss) income	$	(23,998	)	$	(47,734	)	$	(63,046	)	$	84,090

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

Organization and Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Cash, cash equivalents and investments	$ 1,174,900
Cash and cash equivalents	370,829	$ 599,691	$ 122,420
Short-term investments	803,083	479,369
Long-term restricted investment	$ 1,001	$ 784

Summary of Significant Accounting Policies and Recent Accounting Pronouncements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	1
Deferred revenue	$ 3,300,000
U.S. federal corporate tax rate	21.00%	34.00%	34.00%
Rent expense and occupancy costs	$ 7,900,000	$ 4,400,000	$ 5,600,000
Employee Stock Purchase Plan [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage of closing price of common stock	85.00%
Percentage of discount recognized on graded-vesting based on stock-based compensation expense over the purchase period	15.00%
Patents [Member]
Summary Of Significant Accounting Policies [Line Items]
Initial term of patents	20 years
Product Revenues [Member] \| Customer One [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	42.00%
Product Revenues [Member] \| Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	38.00%
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	51.00%
Accounts Receivable [Member] \| Customer Two [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	28.00%
Accounts Receivable [Member] \| Customer Three [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk percentage	10.00%
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term	30 days
Minimum [Member] \| Subsequent Event [Member] \| ASU No. 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Right of use assets				$ 40,000,000
Lease liabilities				60,000,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term	60 days
U.S. federal corporate tax rate		35.00%
Maximum [Member] \| Subsequent Event [Member] \| ASU No. 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Right of use assets				45,000,000
Lease liabilities				$ 65,000,000
Outside of U.S. [Member] \| Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term	60 days
Outside of U.S. [Member] \| Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable payment term	120 days

Summary of Estimated Useful Lives of Plant and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	5 years
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	5 years
Software and Computer Equipment [Member] \| Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	3 years
Software and Computer Equipment [Member] \| Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	Lesser of the useful life or the term of the respective lease
Land [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	Not depreciated
Building [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	30 years
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated useful life	Not depreciated until put into service

License and Collaboration Agreements - Additional Information (Detail)				1 Months Ended				3 Months Ended								12 Months Ended
	Oct. 15, 2018 USD ($) Milestone shares	Oct. 08, 2018 USD ($)	May 03, 2018 USD ($)	Jul. 31, 2017 USD ($)	Oct. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Apr. 30, 2013 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Milestone	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 08, 2018 USD ($) shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment								$ 3,300,000								$ 3,300,000
Research and development expense								146,207,000	$ 86,584,000	$ 122,848,000	$ 46,204,000	$ 44,441,000	$ 34,239,000	$ 58,908,000	$ 29,119,000	401,843,000	$ 166,707,000	$ 188,272,000
Upfront fee to be paid on execution of agreements				$ 35,000,000
Intangible asset, net								11,574,000				14,355,000				11,574,000	14,355,000
Settlement and license charges													25,588,000	2,839,000			28,400,000
Intangible asset amortization expense								216,000	$ 216,000	$ 217,000	$ 216,000	216,000	$ 780,000	$ 29,000	$ 28,000	865,000	1,053,000	29,000
Intangible asset, accumulated amortization								3,852,000				4,145,000				3,852,000	4,145,000
EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sales milestone payment recorded as an in-license right								0				6,600,000				0	6,600,000	1,000,000
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Regulatory and sales milestone payments				$ 65,000,000
Percentage of royalty payments				8.00%
Intangible asset amortization expense																		100,000
Minimum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of royalty payments				4.00%
Lysogne [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Recognized loss related to fair market value of equity investment																$ 200,000
Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of development milestone \| Milestone																0
Research and development expense																$ 0
BioMarin Pharmaceutical, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements				$ 35,000,000
Regulatory and sales milestone payments																0
Intangible asset, net								6,600,000				$ 6,600,000				6,600,000	6,600,000
Intangible asset amortization expense																900,000	1,000,000
Royalty expense																15,100,000	4,700,000
Sales milestone payment recorded as an in-license right								0								0
BioMarin Pharmaceutical, Inc. [Member] \| U.S. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment expiry period				2023-12
BioMarin Pharmaceutical, Inc. [Member] \| EU [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment expiry period				2024-09
University of Western Australia [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty expense																0	0	0
Development milestone and up-front payments																0	0	7,600,000
Up-front payment to purchase future royalties							$ 30,000,000
Up-front payment upon execution of amendment						$ 7,000,000
Purchase option upfront payment for future royalties						23,000,000
University of Western Australia [Member] \| EXONDYS 51 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sales milestone payment recorded as an in-license right				$ 1,000,000				1,000,000								1,000,000
Intangible asset, accumulated amortization								300,000								300,000
University of Western Australia [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional payments upon successful achievement						$ 20,000,000	20,000,000
University of Western Australia [Member] \| Maximum [Member] \| EXONDYS 51 and Up to Five Additional Products [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments to UWA on successful achievement of certain development and regulatory milestones							$ 6,000,000
Equity Investment Agreement [Member] \| Lysogne [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock purchased,shares \| shares	950,606
Percentage of common stock outstanding	8.00%
Upfront license fee, option fee, equity investment and certain development activities reimbursement payable on execution of agreement	$ 28,000,000
Upfront license fee, option fee and certain development activities reimbursement payable on execution of agreement	$ 26,100,000
Number of development milestone \| Milestone	1
Accrued research and development expense								18,700,000								$ 18,700,000
Equity Investment Agreement [Member] \| Lysogne [Member] \| Other Non-current Asset [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Amount of investment allocated based on closing price of common stock	$ 1,900,000
Equity Investment Agreement [Member] \| Lysogne [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments to be paid	$ 102,800,000
License Agreement [Member] \| Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			$ 42,000,000
Pompe License [Member] \| Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			8,000,000
Pompe License [Member] \| Lacerta Therapeutics, Inc. [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			44,000,000
Purchase Agreement [Member] \| Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			$ 30,000,000
Purchase Agreement [Member] \| Lacerta Therapeutics, Inc. [Member] \| Series A Preferred Stock [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of shares purchased under agreement \| shares																			4,500,000
License and Purchase Agreement [Member] \| Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			$ 38,000,000
Options Agreement [Member] \| Lacerta Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front payment																			0
Fair value of options																			$ 0
Warrant Agreement [Member] \| Myonexus Therapeutics, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of development milestone \| Milestone																2
Research and development expense																$ 85,000,000
Up-front cash payment under agreements			$ 60,000,000													60,000,000
Cash consideration under agreement excluding contingent payments			$ 200,000,000
Development milestone payments evaluation period			2 years
Milestone payment																20,000,000
Milestone accured payment								$ 5,000,000								$ 5,000,000
Warrant Agreement [Member] \| Myonexus Therapeutics, Inc. [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional development milestone payments to be paid			$ 45,000,000
Nationwide License Agreement [Member] \| Nationwide [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of development milestone \| Milestone																0
Research and development expense																$ 0
Up-front cash payment under agreements		$ 1,000,000
Nationwide License Agreement [Member] \| Nationwide [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments to be paid		$ 29,000,000
Settlement Agreement [Member] \| BioMarin Pharmaceutical, Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements				20,000,000
License Agreement [Member] \| BioMarin and Academisch Ziekenhuis Leiden [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee to be paid on execution of agreements				$ 15,000,000
Collaboration and License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of budgeted research and development cost allocation					45.00%
Maximum percentage of budgeted research and development cost					110.00%
Development milestone and up-front payments																142,400,000	22,000,000	47,600,000
Collaboration and License Agreement [Member] \| Summit [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Up-front cash payment under agreements					$ 40,000,000
One-time option fee					$ 10,000,000
Percentage of budgeted research and development cost allocation					55.00%
Collaboration expense																8,600,000	0	0
Development milestone and up-front payments																$ 0	$ 22,000,000	$ 40,000,000
Collaboration and License Agreement [Member] \| Summit [Member] \| Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional payments upon successful achievement					$ 192,000,000
Development and regulatory milestone payments					290,000,000
Sales milestone payments					330,000,000
Regulatory milestone payments upon exercise of option					7,000,000
Sales milestone payments upon exercise of option					$ 82,500,000

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

Assets Measured and Carried at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 1,005,270	$ 909,059
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	42,920	352,370
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	125,907	133,368
Government and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	760,235	294,717
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	43,468	127,956
Strategic Equity Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	31,739
Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1,001	648
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	849,363	765,719
Level 1 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	42,920	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	760,235	284,745
Level 1 [Member] \| Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	43,468	127,956
Level 1 [Member] \| Strategic Equity Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1,739
Level 1 [Member] \| Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	1,001	648
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	125,907	143,340
Level 2 [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	125,907	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
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	30,000
Level 3 [Member] \| Strategic Equity Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 30,000

Summary of Company Financial Assets with Maturities of Less Than 90 Days Included in Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 169,447	$ 419,062
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	42,920	352,370
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents		16,720
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	111,587	$ 49,972
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 14,940

Cash, Cash Equivalents and Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Weighted average maturity period of available-for-sale securities	2 months	7 months

Summary of Company Cash, Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	$ 370,827	$ 599,698
Cash and cash equivalents, Gross unrealized gains	3	2
Cash and cash equivalents, Gross unrealized losses	(1)	(9)
Cash and cash equivalents	370,829	599,691	$ 122,420
Cash, cash equivalents and investments, Amortized cost	1,174,011	1,089,419
Cash, cash equivalents and investments, Gross unrealized gains	12	2
Cash, cash equivalents and investments, Gross unrealized losses	(111)	(381)
Cash, cash equivalents and investments, Fair market value	1,173,912	1,089,040
Available for sale debt securities current, Amortized cost	803,184	479,705
Available for sale debt securities current, Gross unrealized gains	9
Available for sale debt securities current, Gross unrealized losses	(110)	(336)
Available for sale debt securities current, Fair market value	803,083	479,369
Available for sale debt securities noncurrent, Amortized cost		10,016
Available for sale debt securities noncurrent, Gross unrealized losses		(36)
Available for sale debt securities noncurrent, Fair market value		9,980
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	244,302	532,999
Cash and cash equivalents	244,302	532,999
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	125,907	133,368
Available for sale debt securities, Fair market value	125,907	133,368
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	760,258	294,915
Available for sale debt securities, Gross unrealized gains	12	2
Available for sale debt securities, Gross unrealized losses	(35)	(200)
Available for sale debt securities, Fair market value	760,235	294,717
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Available for sale debt securities, Amortized cost	43,544
Available for sale debt securities, Gross unrealized losses	(76)
Available for sale debt securities, Fair market value	$ 43,468
Available for sale debt securities current, Amortized cost		118,121
Available for sale debt securities current, Gross unrealized losses		(145)
Available for sale debt securities current, Fair market value		117,976
Available for sale debt securities noncurrent, Amortized cost		10,016
Available for sale debt securities noncurrent, Gross unrealized losses		(36)
Available for sale debt securities noncurrent, Fair market value		$ 9,980

Summary of Components of Accounts Receivable (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Receivables Net Current [Abstract]
Product sales, net of discounts and allowances	$ 48,252	$ 28,539
Government contract receivables	792	929
Total accounts receivable, net	$ 49,044	$ 29,468

Summary of Change in Reserves for Discounts and Allowances (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	$ 8,587
Provision	48,614
Payments/credits	(28,691)
Balance, as of December 31, 2018	28,510
Chargebacks [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	995
Provision	12,284
Payments/credits	(11,901)
Balance, as of December 31, 2018	1,378
Rebates [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	6,959
Provision	28,420
Payments/credits	(11,103)
Balance, as of December 31, 2018	24,276
Prompt Pay [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	169
Provision	2,624
Payments/credits	(2,255)
Balance, as of December 31, 2018	538
Other Accruals [Member]
Accounts Notes And Loans Receivable [Line Items]
Balance, as of December 31, 2017	464
Provision	5,286
Payments/credits	(3,432)
Balance, as of December 31, 2018	$ 2,318

Summary of Total Reserves Included in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Receivables Net Current [Abstract]
Reduction to accounts receivable	$ 2,364	$ 1,285
Component of accrued expenses	26,146	7,302
Total reserves	$ 28,510	$ 8,587

Inventory - Summary of Components of Inventory (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 71,313	$ 53,875
Work in progress	47,279	27,442
Finished goods	6,853	2,288
Total inventory	$ 125,445	$ 83,605

Summary of Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Manufacturing-related deposits and prepaids	$ 39,036	$ 18,650
Leasehold improvement receivable	13,474
Prepaid clinical and pre-clinical expenses	9,706	5,175
Prepaid maintenance services	2,994	1,711
Prepaid income tax	2,130
Prepaid research expenses	1,932	2,896
Interest receivable	1,918	709
Prepaid commercial expenses	1,573	1,589
Asset held for sale		1,501
Other prepaids	3,671	2,726
Other	1,348	1,554
Total other current assets	$ 77,782	$ 36,511

Summary of Investments and Other Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Noncurrent [Abstract]
Manufacturing-related deposits	$ 62,821
Strategic investments	31,739
Prepaid clinical expenses	7,541	$ 7,488
Alternative minimum tax credit	3,367	3,315
Restricted investments	1,001	784
Available for sale debt securities noncurrent, Fair market value		9,980
Other	825	242
Total investments and other assets	$ 107,294	$ 21,809

Summarizes Components of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 125,173	$ 61,178
Less: accumulated depreciation	(28,149)	(18,022)
Property and equipment, net	97,024	43,156
Land [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,158	4,158
Building and Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	22,972	14,543
Software and Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	15,774	8,965
Lab Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	17,659	9,860
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	3,663	2,211
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	20,937	13,633
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 40,010	$ 7,808

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation expense	$ 10.2	$ 6.4	$ 5.0

Intangible Assets - Summary of Components of Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite Lived Intangible Assets [Line Items]
Total intangible assets	$ 11,574	$ 14,355
Patents [Member]
Finite Lived Intangible Assets [Line Items]
Total intangible assets	5,773	7,660
In-Licensed Rights [Member]
Finite Lived Intangible Assets [Line Items]
Total intangible assets	5,626	6,491
Software Licenses [Member]
Finite Lived Intangible Assets [Line Items]
Total intangible assets	$ 175	$ 204

Intangible Assets - Additional Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2017
Finite Lived Intangible Assets [Line Items]
Amortization of in-licensed rights	$ 216,000	$ 216,000	$ 217,000	$ 216,000	$ 216,000	$ 780,000	$ 29,000	$ 28,000	$ 865,000	$ 1,053,000	$ 29,000
Patents [Member]
Finite Lived Intangible Assets [Line Items]
Amortization of in-licensed rights									$ 700,000	600,000	600,000
Initial term of patents									20 years
Patents [Member] \| Research and Development [Member]
Finite Lived Intangible Assets [Line Items]
Book value of patent abandoned									$ 100,000	600,000	300,000
Impairment loss on patent assets									3,800,000
Maximum [Member]
Finite Lived Intangible Assets [Line Items]
Amortization of in-licensed rights											100,000
EXONDYS 51 [Member]
Finite Lived Intangible Assets [Line Items]
Sales milestone recorded as an in-license right	0				$ 6,600,000				0	$ 6,600,000	$ 1,000,000
EXONDYS 51 [Member] \| Bio Marin [Member]
Finite Lived Intangible Assets [Line Items]
Sales milestone recorded as an in-license right												$ 6,600,000
EXONDYS 51 [Member] \| University of Western Australia [Member]
Finite Lived Intangible Assets [Line Items]
Sales milestone recorded as an in-license right	$ 1,000,000								$ 1,000,000			$ 1,000,000

Intangible Assets - Summary of Estimated Future Amortization for Intangible Assets (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Goodwill And Intangible Assets Disclosure [Abstract]
2019	$ 1,490
2020	1,300
2021	1,299
2022	1,298
2023	1,297
Total	$ 6,684

Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Product revenue related reserves	$ 26,146	$ 7,302
Accrued employee compensation costs	24,692	14,402
Accrued milestone expense	24,020
Accrued contract manufacturing costs	15,794	14,019
Accrued clinical and pre-clinical costs	11,396	15,975
Accrued professional fees	11,319	6,794
Accrued BioMarin royalties	8,254	2,846
Accrued property and equipment	5,421	2,525
Accrued collaboration cost sharing	2,167
Accrued research costs	1,070	401
Accrued interest expenses	1,045	1,291
Accrued income taxes	301	943
Other	2,470	2,484
Total accrued expenses	$ 134,095	$ 68,982

Restructuring - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Jun. 30, 2018	Apr. 30, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost And Reserve [Line Items]
Restructuring expenses				$ (2,222)	$ 3,020	$ 4,562
Relieved cease-use liability				(2,222)
Selling, General And Administrative Expenses [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses				$ (2,222)	$ 2,832	$ 2,549
Corvallis Plan [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring, initiation date				2016-03
Restructuring, completion date				Dec. 31, 2017
Corvallis Plan [Member] \| Oregon [Member] \| Selling, General And Administrative Expenses [Member]
Restructuring Cost And Reserve [Line Items]
Relieved cease-use liability	$ (2,200)
Corvallis Plan [Member] \| Oregon [Member] \| Facility Closing [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring expenses		$ 2,300	$ 1,500

Summary of Restructuring Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses	$ (2,222)	$ 3,020	$ 4,562
Research and Development [Member]
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses		188	2,013
Selling, General and Administrative [Member]
Restructuring Cost And Reserve [Line Items]
Total restructuring expenses	$ (2,222)	$ 2,832	$ 2,549

Summary of Restructuring Reserve (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 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	(711)	(1,675)
Reversal of cease-use liability	$ (2,222)
Restructuring reserve ending balance		$ 2,933

Indebtedness - Additional Information (Detail)		1 Months Ended		12 Months Ended
Indebtedness - Additional Information (Detail)	Nov. 14, 2017 USD ($) shares $ / shares	Sep. 30, 2018 USD ($)	Jul. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) Loan	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Debt instrument, face amount				$ 570,000,000
Proceeds for issuance of notes allocated to equity					$ 570,000,000
Amount paid for capped calls transactions					50,901,000
Debt extinguishment loss				(2,322,000)
Revolving line-of-credit outstanding balance				420,554,000	431,051,000
Total indebtedness of unpaid principal balance					$ 1,300,000
Sale of Airport Facility date					2018-01
Long-term debt				420,554,000	$ 424,876,000
Mortgage Loan One [Member]
Debt Instrument [Line Items]
Revolving line-of-credit outstanding balance					800,000
Mortgage Loan Two [Member]
Debt Instrument [Line Items]
Revolving line-of-credit outstanding balance					500,000
Convertible Debt, The Term Loan, The Revolver and The Mortgage Loans [Member]
Debt Instrument [Line Items]
Long-term debt				420,600,000
Interest expense related to debt facilities				33,700,000	5,800,000	$ 1,900,000
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			$ 73.42
Proceeds for issuance of notes allocated to equity	$ 161,100,000
Offering costs	4,200,000
Debt issuance costs	$ 10,600,000
Effective interest rate percentage				6.90%	6.90%
Capped calls strike price \| $ / shares	73.42
Capped calls cap price \| $ / shares	104.88
Amount paid for capped calls transactions	$ 50,900,000
2024 Convertible Notes [Member] \| Fundamental Change [Member]
Debt Instrument [Line Items]
Debt instrument, repurchase price percentage	100.00%
2024 Convertible Notes [Member] \| Minimum [Member] \| Beneficial Owner [Member] \| Fundamental Change [Member]
Debt Instrument [Line Items]
Voting percentage	50.00%
2024 Convertible Notes [Member] \| Common Stock [Member]
Debt Instrument [Line Items]
Debt instrument, convertible into shares \| shares	7,763,552
MidCap Financial Amended and Restated Credit and Security Agreement [Member] \| July 2017 Term Loan [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate per annum			6.25%
Current borrowing capacity			$ 60,000,000
Variable interest rate description			one-month London Interbank Offered Rate
Origination fee, percentage			0.50%
Final payment fee, percentage			2.00%
Monthly principal payments commencement date			Jul. 01, 2018
Periodic principal payment frequency			monthly
Monthly principal payments			$ 800,000
Credit facility maturity date			2021-07
Midcap Financial Revolving Credit and Security Agreement [Member] \| Revolving Loan [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate per annum			3.95%
Current borrowing capacity			$ 40,000,000
Variable interest rate description				one-month LIBOR
Origination fee, percentage			0.50%
Credit facility maturity date			2021-07
Line of credit facility additional tranche that may be increased			$ 20,000,000
Origination fee			$ 200,000
MidCap Financial Amended and Restated Credit and Security Agreement and Revolving Credit and Security Agreement [Member]
Debt Instrument [Line Items]
Agreement termination date				2018-09
MidCap Financial Amended and Restated Credit and Security Agreement and Revolving Credit and Security Agreement [Member] \| July 2017 Term Loan [Member]
Debt Instrument [Line Items]
Debt extinguishment loss		$ (2,300,000)
Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate per annum					4.75%
Credit facility maturity date					2027-02
Number of mortgage loans outstanding \| Loan					2

Indebtedness - Summary of Debt Facilities (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Nov. 14, 2017
Debt Instrument [Line Items]
Par value of the 2024 Notes	$ 570,000,000
Net carrying value of other debt facilities		$ 30,390,000
Net carrying value of total debt facilities	420,554,000	431,051,000
Fair value of convertible debt	952,681,000	619,641,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Par value of the 2024 Notes	570,000,000	570,000,000	$ 570,000,000
Unamortized discount - equity component	(140,206,000)	(158,890,000)
Unamortized discount - debt issuance costs	(9,240,000)	(10,449,000)
Net carrying value of convertible debt	$ 420,554,000	$ 400,661,000

Indebtedness - Summarizes Total Gross Payments Due under Company's Debt Arrangements (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Debt Disclosure [Abstract]
Thereafter	$ 570,000
Total Payments	$ 570,000

Equity Financing - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended
	Nov. 30, 2018	Jul. 31, 2017	Sep. 30, 2016	Jun. 30, 2016
Class Of Stock [Line Items]
Common stock issued			5.8	2.1
Common stock average price per share	$ 131.00	$ 42.50	$ 59.75	$ 17.84
Net proceeds from issue of common stock	$ 513.4	$ 354.0	$ 327.4	$ 37.3
Underwriting discounts, commission and offering expenses	$ 24.6	$ 20.0	$ 17.6	$ 0.2
Underwritten Public Offering [Member]
Class Of Stock [Line Items]
Common stock issued	4.1	8.8
Underwritten Offering [Member]
Class Of Stock [Line Items]
Common stock issued	0.3	1.2

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

1 Months Ended

12 Months Ended

Jul. 31, 2018

Dec. 31, 2017

Jun. 30, 2017

Mar. 31, 2017

Sep. 30, 2016

Jun. 30, 2016

Feb. 29, 2016

Oct. 31, 2015

Jun. 30, 2015

Jun. 30, 2013

Nov. 30, 2012

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Jun. 30, 2018

Dec. 31, 2015

Sep. 30, 2014

Jun. 30, 2011

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

Term of award

10 years

Share based compensation granted under plan vest period

4 years

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

$ 44.66

$ 14.78

$ 12.46

Stock option grants with performance conditions

2,152,439

4,805,722

^[1]

1,285,051

Former Chief Executive Officer [Member]

$ 50,127,000

$ 30,465,000

$ 29,962,000

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

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

2,100,000

Stock Options [Member]

Share based compensation granted under plan vest period

4 years

Stock option grants with performance conditions

300,000

400,000

Forfeited awards percentage

20.00%

Stock Options [Member] | EXONDYS 51 [Member]

37,671,000

23,416,000

$ 26,320,000

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

Vesting percentage

80.00%

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

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

Vesting percentage

50.00%

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

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

Vesting percentage

50.00%

Stock Options with Performance-based Conditions [Member]

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

Service-Vesting Conditions [Member]

200,000

$ 900,000

$ 5,000,000

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

Stock based compensation expense not yet recognized

73,000,000

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

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

Stock based compensation expense not yet recognized

200,000

Market Based Conditions [Member]

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

Stock based compensation expense not yet recognized

$ 31,000,000

Restricted Stock Awards (RSAs) [Member]

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

Grant date fair value, restricted stock units and awards

27,590

341,500

117,553

$ 98.57

$ 34.58

$ 41.22

Outstanding, Restricted stock units and awards

411,781

252,321

411,781

153,170

161,320

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

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

$ 0

71,925

Grant date fair value, restricted stock units and awards

Jun. 30, 2018

$ 48.94

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

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

$ 3,300,000

89,906

Grant date fair value, restricted stock units and awards

Jan. 01, 2019

$ 54.29

Restricted Stock Units (RSUs) [Member]

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

Grant date fair value, restricted stock units and awards

230,736

181,029

$ 87.95

$ 33.03

Outstanding, Restricted stock units and awards

66,552

251,298

66,552

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

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

$ 2,900,000

$ 200,000

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

156,029

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

Stock-based compensation related to grants

$ 700,000

Stock Appreciation Rights [Member]

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

Vested, Restricted stock units and awards

100,000

Outstanding, Restricted stock units and awards

100,000

Exercise price of SARs

$ 23.85

2011 Equity Incentive Plan [Member]

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

Number of additional shares approved under the plan

13,000,000

1,300,000

1,700,000

2013 Employee Stock Purchase Plan [Member]

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

200,000

Percentage of closing price of common stock

300,000

Term of award

24 months

85.00%

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

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

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

Feb. 28, 2018

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

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

Aug. 31, 2019

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

900,000

Number of additional shares approved under the plan

600,000

1,200,000

3,800,000

1,000,000

2018 Equity Incentive Plan [Member]

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

4,400,000