Spark Therapeutics (ONCE) 10-Q 6 Nov 18 2018 Q3 Quarterly report Financial data

Document and entity information

Document and entity information - shares	9 Months Ended
	Sep. 30, 2018	Nov. 01, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	Spark Therapeutics, Inc.
Entity Central Index Key	1,609,351
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Document Type	10-Q
Trading Symbol	ONCE
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus (Q1,Q2,Q3,FY)	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		37,747,117

Consolidated balance sheets (un

Consolidated balance sheets (unaudited) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 161,557	$ 96,748
Marketable securities	428,919	423,419
Trade and other receivables	13,506	7,906
Inventory	20,738	0
Prepaid expenses	10,492	5,093
Total current assets	635,212	533,166
Restricted cash	53,000	0
Marketable securities	27,964	20,035
Property and equipment, net	87,280	61,713
Goodwill	1,214	1,254
Other assets	2,732	628
Total assets	807,402	616,796
Current liabilities:
Accounts payable	18,048	14,183
Accrued expenses	35,092	24,697
Current portion of long-term debt	3,320	312
Current portion of deferred rent	1,051	969
Current portion of deferred revenue	0	11,969
Current other liabilities	1,808	1,557
Total current liabilities	59,319	53,687
Long-term debt	47,671	912
Long-term deferred rent	7,658	8,318
Long-term deferred revenue	105,000	0
Other liabilities	37,879	40,255
Total liabilities	257,527	103,172
Stockholders’ equity:
Preferred stock, $0.001 par value. Authorized, 5,000,000 shares; no shares issued or outstanding	0	0
Common stock, $0.001 par value. Authorized, 150,000,000 shares; 37,131,626 shares issued and 37,111,404 shares outstanding as of December 31, 2017; 37,684,255 shares issued and 37,610,295 shares outstanding as of September 30, 2018	38	37
Additional paid-in capital	1,078,989	1,026,590
Accumulated other comprehensive loss	(225)	(5,914)
Treasury stock, at cost, 20,222 shares as of December 31, 2017 and 73,960 shares as of September 30, 2018	(4,498)	(1,226)
Accumulated deficit	(524,429)	(505,863)
Total stockholders’ equity	549,875	513,624
Total liabilities and stockholders’ equity	$ 807,402	$ 616,796

Consolidated balance sheets (_2

Consolidated balance sheets (unaudited) (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock , par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common stock, shares issued	37,684,255	37,131,626
Common stock, shares outstanding	37,610,295	37,111,404
Treasury stock, shares	73,960	20,222

Consolidated statements of oper

Consolidated statements of operations and comprehensive income (loss) (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues [Abstract]
Product sales, net	$ 8,866	$ 0	$ 15,599	$ 0
Contract revenue	1,841	1,900	35,969	4,657
Total revenues	10,707	1,900	51,568	4,657
Operating expenses:
Cost of product sales	318	0	708	0
Cost of contract revenue	0	0	5,111	0
Research and development	32,829	39,341	88,462	104,679
Acquired in-process research and development	0	1,750	0	5,207
Impairment of acquired in-process research and development	0	0	0	15,696
Selling, general and administrative	29,305	26,641	92,543	74,783
Total operating expenses	62,452	67,732	186,824	200,365
Loss from operations	(51,745)	(65,832)	(135,256)	(195,708)
Unrealized gain on equity investments	1,672	0	4,291	0
Interest income, net	2,714	1,112	7,420	2,231
Other income	0	0	110,000	0
Loss before income taxes	(47,359)	(64,720)	(13,545)	(193,477)
Income tax benefit (expense)	2	(292)	(20)	1,816
Net loss	$ (47,357)	$ (65,012)	$ (13,565)	$ (191,661)
Basic and diluted net loss per common share (in dollars per share)	$ (1.26)	$ (1.90)	$ (0.36)	$ (5.89)
Weighted average basic and diluted common shares outstanding (in shares)	37,498,616	34,258,328	37,267,942	32,516,829
Net loss	$ (47,357)	$ (65,012)	$ (13,565)	$ (191,661)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities, net of tax	308	(359)	614	985
Unrealized gain on interest rate swap	122	0	122	0
Foreign exchange translation adjustment	(12)	(60)	(49)	42
Total comprehensive loss	$ (46,939)	$ (65,431)	$ (12,878)	$ (190,634)

Consolidated statements of cash

Consolidated statements of cash flows (unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (13,565)	$ (191,661)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash rent expense (income)	(1,686)	596
Depreciation and amortization expense	4,820	3,499
Loss on disposal of property and equipment	123	0
Acquired in-process research and development	0	5,207
Stock-based compensation expense	37,704	33,835
Impairment of acquired in-process research and development	0	15,696
Non-cash income tax benefit	0	(1,816)
Gain from sale of priority review voucher	(110,000)	0
Unrealized gain on equity investments	(4,291)	0
Non-cash interest income	(1,407)	0
Changes in operating assets and liabilities:
Inventory	(20,738)	0
Prepaid expenses and other assets	(5,564)	(2,501)
Trade and other receivables	(5,690)	10,690
Accounts payable and accrued expenses	(877)	839
Deferred rent	0	2,480
Deferred revenue	93,031	(4,657)
Other liabilities	104	0
Net cash used in operating activities	(28,036)	(127,793)
Cash flows from investing activities:
Proceeds from sale of priority review voucher	110,000	0
Payment for license agreement	(2,000)	0
Purchase of acquired in-process research and development	0	(4,820)
Purchases of marketable securities	(360,569)	(374,747)
Proceeds from maturities of marketable securities	352,167	206,825
Purchases of property and equipment	(14,920)	(8,895)
Net cash (used in) provided by investing activities	84,678	(181,637)
Cash flows from financing activities:
Proceeds from exercise of options	13,129	17,044
Purchase of treasury stock	(3,272)	(633)
Proceeds from public offerings of common stock, net	0	380,008
Proceeds from issuance of common stock under ESPP	1,567	646
Proceeds from long-term debt	50,000	0
Payments on long-term debt	(233)	(226)
Net cash provided by financing activities	61,191	396,839
Effect of exchange rate changes on cash and cash equivalents and restricted cash	(24)	102
Net increase in cash and cash equivalents and restricted cash	117,809	87,511
Cash and cash equivalents and restricted cash, beginning of period	96,748	58,923
Cash and cash equivalents and restricted cash, end of period	214,557	146,434
Supplemental disclosure of cash flow information:
Deferred financing costs included in accounts payable and accrued expenses	0	137
Property and equipment purchases included in accounts payable and accrued expenses	17,946	856
One Drexel Plaza lease cost included in other liabilities	$ 1,077	$ 0

The Company

The Company	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
The Company	The Company Spark Therapeutics, Inc. was formed on March 13, 2013, in the state of Delaware as AAVenue Therapeutics, LLC and amended its Certificate of Formation in October 2013 to change its name to Spark Therapeutics LLC. In May 2014, it converted from a limited liability company to a C corporation, Spark Therapeutics, Inc. (the Company). The Company is a gene therapy company, seeking to transform the lives of patients suffering from debilitating genetic diseases by developing potentially one-time, life-altering treatments. The Company operates in one segment and has its principal offices in Philadelphia, Pennsylvania. In December 2017, the U.S. Food and Drug Administration (FDA) approved LUXTURNA TM (voretigene neparvovec-rzyl) for the treatment of patients with viable retinal cells and confirmed biallelic RPE65 mutation-associated retinal dystrophy.

Development-stage risks

Development-stage risks	9 Months Ended
	Sep. 30, 2018
Development-stage Risk [Abstract]
Development-stage risks	Development-stage risks The Company has incurred net losses since inception and expects to incur net losses for the foreseeable future. The Company had an accumulated deficit of $524.4 million as of September 30, 2018 . The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of LUXTURNA and its other product candidates in development. Additional financing may be needed by the Company to fund its operations and to commercially develop its other product candidates. The Company’s future operations are highly dependent on a combination of factors, including: (i) the success of its research and development; (ii) regulatory approval of the Company’s proposed future products; (iii) the success of the commercialization of LUXTURNA; (iv) the timely and successful completion of additional financing; and (v) the development of competitive therapies by other biotechnology and pharmaceutical companies.

Summary of significant accounti

Summary of significant accounting policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies (a) Basis of presentation The accompanying unaudited interim consolidated financial statements of the Company and its wholly-owned subsidiaries have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying consolidated financial statements include all normal and recurring adjustments (which consist primarily of accruals, estimates and assumptions that impact the consolidated financial statements) considered necessary to present fairly the Company’s financial position as of September 30, 2018 , its results of operations for the three and nine months ended September 30, 2017 and 2018 and cash flows for the nine months ended September 30, 2017 and 2018 . Operating results for the three and nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the year ending December 31, 2018 or any other period. The interim consolidated financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual consolidated financial statements. The accompanying unaudited interim consolidated financial statements should be read in conjunction with the annual audited consolidated financial statements and related notes as of and for the year ended December 31, 2017 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017 . (b) Use of estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates. (c) Trade and other receivables Trade accounts receivable are recorded at gross value, and reserves for other sales-related allowances, such as discounts, rebates, services and insurance co-pay assistance, are included in accrued expenses on the Company's consolidated balance sheets. (d) Inventory Inventory is stated at the lower of cost or net realizable value and consists of those costs incurred following FDA approval of LUXTURNA. Cost is determined using the first-expired, first-out (FEFO) method. The Company reserves for potentially excess, dated or obsolete inventories based on an analysis of inventory on hand compared to forecasts of future sales. Based on management's assessment, no such inventory reserve is necessary as of September 30, 2018 . Inventory consisted of the following (in thousands): September 30, Raw materials $ 2,009 Work in process 18,697 Finished goods 32 $ 20,738 (e) Fair value of financial instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, trade and other receivables, accounts payable and accrued expenses, approximate fair value due to the short-term nature of those instruments. Management believes the carrying value of debt approximates fair value as the interest rates are reflective of the rate the Company could obtain on debt with similar terms and conditions. (f) Net product sales LUXTURNA is distributed in the United States through two distribution models: (1) the traditional buy-and-bill model where the treatment center purchases and pays for the product and then submits a claim to the payer; and (2) the Company's innovative contracting and distribution model, branded Spark PATH (Pioneering Access to Healthcare), which includes options for direct-to-payer contracting and outcomes-based rebates. During the quarter, the Company made the first sale of LUXTURNA under its Spark PATH outcomes-based rebate model. The Company evaluated the variable consideration under Accounting Standards Codification (ASC) 606, Revenue from Contracts with Customers, as it relates to the outcomes-based method and determined that based on historical clinical data, no additional reserves were required as of September 30, 2018. The Company’s net product sales represent total gross product sales in the United States less allowances for estimated prompt-payment discounts, service fees, rebates and insurance co-payment assistance. Allowances are established based on contractual terms and management’s reasonable estimates, as well as the expectation that 100% of the prompt-payment discounts will be earned. Product shipping and handling costs and distributor reporting fees are included in cost of product sales. All sales are recognized when control is transferred, which follows the Company’s verification of a scheduled LUXTURNA treatment. The Company’s product return policy is to provide non-monetary credit or product replacement. As the product is sold in direct relation to a scheduled treatment, Company management estimates that there is minimal risk of product return, including the risk of product expiration. (g) Contract revenue Under certain of the Company’s licensing, supply and collaboration agreements, it is entitled to receive payment upon the achievement of contingent milestone events or the performance of obligations. The Company recognizes revenue based on guidance in ASC 606. Prior to 2018, the Company generated revenue solely through license and collaborative arrangements. In the first quarter of 2018, the Company adopted ASC 606. The adoption of this standard resulted in no cumulative adjustment to the Company’s consolidated financial statements. ASC 606 applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services in an amount that reflects the consideration the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. At contract inception, once the contract is determined to be within the scope of ASC 606, 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. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the Company’s consolidated balance sheet. Amounts expected to be recognized as revenue in the next 12 months following the balance sheet date are classified as current liabilities. (h) Net loss per common share Basic and diluted net loss per common share is determined by dividing net loss by the weighted average number of common shares outstanding during the period. For all periods presented, unvested restricted shares and common stock options have been excluded from the calculation because their effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same. The following potentially dilutive securities have been excluded from the computations of diluted weighted average shares outstanding as of September 30, 2017 and 2018 as they would be anti-dilutive: September 30, 2017 September 30, 2018 Unvested restricted common shares 781,703 949,925 Stock options issued and outstanding 3,771,059 3,623,311 (i) Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property. Tenant improvement allowances from the lessor are included in the accompanying consolidated balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of September 30, 2018 represents the net excess of rent expense over the actual cash paid for rent plus tenant improvement allowances received. (j) Other comprehensive loss The Company follows the provisions of ASC 220, Comprehensive Income , which establishes standards for the reporting and display of comprehensive income and its components. Comprehensive income (loss) is defined to include all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) includes gains and losses related to changes in the fair value of available-for-sale securities, changes in the fair value of an interest rate swap and foreign currency translation. The accumulated balances related to each component of other comprehensive income (loss) are summarized as follows (in thousands): Net unrealized (loss) gain on available for sale securities Unrealized gain on interest rate swap Foreign currency translation adjustments Accumulated other comprehensive loss Balance as of December 31, 2017 $ (5,964 ) $ — $ 50 $ (5,914 ) Cumulative adjustment to prior accumulated deficit 5,002 — — 5,002 Current period other comprehensive income (loss) 614 122 (49 ) 687 Balance as of September 30, 2018 $ (348 ) $ 122 $ 1 $ (225 ) (k) Recent accounting pronouncements In February 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-02, “Leases.” ASU 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. ASU 2016-02 is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company plans to elect the effective date method using a modified retrospective transition approach. The Company currently is evaluating the impact of the updated guidance on the Company's consolidated financial statements. In May 2014, the FASB issued a new standard, ASC 606, regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. ASC 606 provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflect the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. The allowable adoption methods are the full retrospective method, which requires the standard to be applied to each prior period presented, or the modified retrospective method which requires the cumulative effect of adoption to be recognized as an adjustment to opening retained earnings in the period of adoption. The Company adopted ASC 606 using the modified retrospective method and the adoption had no cumulative adjustment to its consolidated financial statements as it relates to the Pfizer Inc. (Pfizer) collaboration agreement discussed in note 14. For the three months ended September 30, 2018 , $14.0 million and $5.1 million in contract revenue and cost of contract revenue, respectively, would have been recognized under revenue recognition guidance in effect during 2017 prior to the adoption of ASC 606. For the nine months ended September 30, 2018, there was no impact from the adoption of ASC 606 that resulted in a different revenue recognition than under the prior revenue recognition guidance in effect during 2017. In January 2016, the FASB issued ASU No. 2016-01, "Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities." ASU 2016-01 changes accounting for equity investments, financial liabilities under the fair value option, and presentation and disclosure requirements for financial instruments. ASU 2016-01 does not apply to equity investments in consolidated subsidiaries or those accounted for under the equity method of accounting. In addition, the FASB clarified guidance related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale debt securities. Equity investments with readily determinable fair values will be measured at fair value with changes in fair value recognized in net income. Companies have the option to either measure equity investments without readily determinable fair values at fair value or at cost, adjusted for changes in observable prices, minus impairment. Changes in measurement under either alternative will be recognized in net income. Companies that elect the fair value option for financial liabilities must recognize changes in fair value related to instrument-specific credit risk in other comprehensive income. Companies must assess valuation allowances for deferred tax assets related to available-for-sale debt securities in combination with their other deferred tax assets. The Company adopted this guidance effective January 1, 2018, and the adoption required an adjustment of $5.0 million to accumulated deficit, which was related to an equity security investment on the consolidated balance sheet. In August 2017, the FASB issued ASU 2017-12, "Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities", which improves the financial reporting of hedging relationships to better portray the economic results of an entity's risk management activities in its financial statements and make certain targeted improvements to simplify the application of the hedge accounting guidance in current U.S. GAAP. The amendments in this update better align an entity's risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. The effective date for the standard is for fiscal years beginning after December 15, 2018. The Company elected to early adopt this ASU in the third quarter of 2018. The adoption of this guidance did not have a material impact on the Company’s consolidated financial statements. See Note 9 for a discussion of the Company’s derivatives.

Marketable securities

Marketable securities	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable securities The following table summarizes the available-for-sale securities held at December 31, 2017 and September 30, 2018 (in thousands): Description Amortized cost Unrealized gains Unrealized losses Fair value December 31, 2017 U.S. government agency $ 222,179 $ — $ (640 ) $ 221,539 Corporate securities $ 227,238 $ — $ (5,323 ) $ 221,915 September 30, 2018 U.S. government agency $ 240,749 $ — $ (258 ) $ 240,491 Corporate securities $ 217,193 $ 11 $ (812 ) $ 216,392 No available-for-sale securities held as of September 30, 2018 had remaining maturities greater than two years.

Fair value of financial instrum

Fair value of financial instruments	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair value of financial instruments	Fair value of financial instruments The Company follows FASB accounting guidance on fair value measurements for financial assets and liabilities measured on a recurring basis. The guidance requires fair value measurements to maximize the use of “observable inputs.” The three-level hierarchy of inputs to measure fair value are as follows: • Level 1: Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities • Level 2: Significant other observable inputs other than Level 1 prices such as quoted prices in markets that are not active, or inputs that are observable, either directly or indirectly, for substantially the full term of the asset or liability • Level 3: Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (i.e., supported by little or no market activity) The Company has classified assets and liabilities measured at fair value on a recurring basis as follows (in thousands): Fair value measurements at reporting date using Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) At December 31, 2017: Assets: Money market funds (included in cash and cash equivalents) $ 90,348 — — Corporate securities (included in cash and cash equivalents) $ 5,548 — — Marketable securities - U.S. government agencies $ 221,539 — — Marketable securities - corporate securities $ 220,020 $ 1,895 — At September 30, 2018: Assets: Money market funds (included in cash and cash equivalents) $ 160,857 — — Certificates of deposit - restricted cash $ 53,000 Marketable securities - U.S. government agencies $ 240,491 — — Marketable securities - corporate securities $ 216,392 — — (a) Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of September 30, 2018 consisted of money market funds. (b) Marketable securities The Company classifies its marketable security investments as available-for-sale securities and the securities are stated at fair value. There were no material realized gains or losses recognized on the maturity of available-for-sale securities during the nine months ended September 30, 2018 and, as a result, the Company did not reclassify any amount out of accumulated other comprehensive loss for the same period. In addition, as part of the license and stock purchase agreements entered into with Selecta Biosciences, Inc. (Selecta) (note 14), the Company purchased restricted common shares of Selecta. The investment is classified as available-for-sale and is stated at fair value, and changes in fair value are recognized in the consolidated statements of operations and comprehensive income (loss).

Sale of priority review voucher

Sale of priority review voucher	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Sale of priority review voucher	Sale of priority review voucher In May 2018, the Company sold the rare pediatric disease priority review voucher (PRV) it received from FDA in connection with the United States approval of LUXTURNA to Jazz Pharmaceuticals Ireland Limited for consideration of $110.0 million . The proceeds from the sale of the PRV were recognized as a gain on the sale of an intangible asset, within other income on the consolidated statements of operations and comprehensive income (loss), as the PRV did not have a carrying value on the Company’s consolidated balance sheet at the time of sale.

Impairment of acquired in-proce

Impairment of acquired in-process research and development	9 Months Ended
	Sep. 30, 2018
Research and Development [Abstract]
Impairment of acquired in-process research and development	Impairment of acquired in-process research and development The acquired in-process research and development (IPR&D) asset was an indefinite-lived intangible asset and was assessed for impairment annually, or more frequently if impairment indicators existed. During the nine months ended September 30, 2017, the Company determined that it would no longer pursue product candidates utilizing the technology acquired from Genable Technologies, Ltd. in March 2016 and, accordingly, recorded a non-cash impairment charge of $15.7 million within its consolidated statements of operations and comprehensive income (loss). Additionally, the Company recognized an income tax benefit of $1.0 million related to the reversal of the deferred tax liability associated with the IPR&D during the nine months ended September 30, 2017.

Accrued expenses

Accrued expenses	9 Months Ended
	Sep. 30, 2018
Payables and Accruals [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following (in thousands): December 31, September 30, Compensation and benefits $ 15,012 $ 11,126 Consulting and professional fees 4,846 18,629 Research and development 2,809 3,324 Other 2,030 2,013 Total accrued expenses $ 24,697 $ 35,092

Debt

Debt	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Debt	Debt The Company's debt consists of the following (in thousands): December 31, September 30, Term loan outstanding $ — $ 50,000 MELF loan outstanding 1,224 991 Less: current portion of long-term debt 312 3,320 Total long-term debt due after one year $ 912 $ 47,671 In July 2018, the Company entered into a credit agreement with Wells Fargo Bank, National Association (Wells Fargo) as lender (the Credit Agreement), pursuant to which Wells Fargo extended a $50.0 million term loan to the Company, which bears interest at one-month LIBOR plus 0.65% , which rate is converted to a fixed rate per annum of 3.463% under the swap agreement discussed below. The term loan was fully drawn on the date of the Credit Agreement and matures on July 3, 2023 (Maturity Date). Through June 2019, the Company will only be obligated to make monthly interest payments with respect to the term loan. Thereafter, the term loan will amortize in equal monthly installments through the maturity date. For the three and nine months ended September 30, 2018, the Company recorded interest expense of $0.3 million related to the Credit Agreement. In order to manage the interest risk associated with the term loan, the Company entered into an interest rate swap with Wells Fargo on an initial notional amount of $50.0 million . Under this interest rate swap agreement, which expires on July 3, 2023, the Company receives a floating rate based on 1-month LIBOR plus 0.65% and pays a fixed rate of 3.463% . The Company designated this interest rate swap as a cash flow hedge of the forecasted interest payments related to the debt issuance. To maintain this rate, the Company entered into a cash collateral agreement with Wells Fargo that requires the Company to maintain a restricted cash balance equal to at least the principal balance of the term loan. At September 30, 2018, the fair value of the derivative asset was $0.2 million and the fair value of the derivative liability was $0.1 million and are recognized within long-term assets and other current liabilities, respectively. The effective portion of the swap is reported as a component of accumulated other comprehensive loss. There was no hedge ineffectiveness at September 30, 2018. Changes in the fair value are reclassified from accumulated other comprehensive loss into operations in the same period that the hedged item affects earnings. During the three months ended September 30, 2018, the Company reclassified $86 thousand from accumulated other comprehensive loss into interest expense related to the effective portion of the swap. Over the next 12 months, $0.1 million of the effective portion of the interest rate swap is expected to be reclassified from accumulated other comprehensive loss into interest expense. If, at any time, the interest rate swap is determined to be ineffective, in whole or in part, due to changes in the interest rate swap or underlying debt agreements, the fair value of the portion of the interest rate swap determined to be ineffective will be recognized as a gain or loss in the statement of operations and comprehensive income (loss) for the applicable period. In August 2016, the Company executed an agreement with the Commonwealth of Pennsylvania to fund machinery and equipment and other assets purchased (MELF Loan) in the amount of $1.6 million . Borrowings under the MELF Loan are secured by equipment, as defined in the loan agreement. Under the terms of the MELF Loan, the Company has a five -year period of monthly payments of approximately $29 thousand of principal and interest at an annual interest rate of 3.25% . For the three and nine months ended September 30, 2018 , the Company recorded interest expense of approximately $8 thousand and $27 thousand , respectively, related to the MELF Loan.

Stockholders' equity

Stockholders' equity	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Stockholders' equity	Stockholders’ equity The Company’s certificate of incorporation and bylaws contain the rights, preferences and privileges of the Company’s stockholders and their respective shares. The Company has authorized 150,000,000 shares of common stock and 5,000,000 shares of preferred stock. In 2013 and 2014, the Company issued restricted stock to various employees, directors and consultants of the Company. The vesting terms of the restricted stock issued varied, but primarily, shares vested 25% on the first anniversary of the vesting commencement date and then quarterly over the next three years , with accelerated vesting in the event of a change in control, as defined. Any unvested shares are forfeited in the event that the individual ceases to provide services to the Company. Additionally, in 2014, the Company issued 200,000 restricted shares of common stock to The Trustees of the University of Pennsylvania (Penn) in connection with a license agreement, of which 175,000 shares have vested. The remaining shares were canceled during the nine months ended September 30, 2018 . For the nine months ended September 30, 2017 , the Company recorded compensation expense of $30 thousand and $6.7 million in selling, general and administrative expense and research and development expense, respectively, related to the restricted shares. For the nine months ended September 30, 2018 , the Company recorded an immaterial amount of compensation expense in selling, general and administrative expense and $2.2 million of expense in research and development expense related to the restricted shares. The following table summarizes restricted stock activity: Number of shares Nonvested shares at December 31, 2017 100,834 Shares canceled (25,000 ) Shares vested (75,834 ) Nonvested shares at September 30, 2018 —

Stock incentive plans

Stock incentive plans	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock incentive plans	Stock incentive plans The Company's 2015 Stock Incentive Plan (the 2015 Plan) provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to employees, officers, directors, consultants and advisors. In January 2018 , the number of shares of common stock authorized for issuance under the 2015 Plan automatically increased, pursuant to the terms of the 2015 Plan, by 1,485,322 shares. As of September 30, 2018 , 1,356,362 shares were available for future grants under the 2015 Plan. In January 2018 , the number of shares of common stock authorized for issuance under the 2015 Employee Stock Purchase Plan (the 2015 ESPP) automatically increased, pursuant to the terms of the 2015 ESPP, by 371,330 shares. The 2015 ESPP provides participating employees with the opportunity to purchase an aggregate of 1,120,188 shares of common stock as of September 30, 2018. For the nine months ended September 30, 2018, 30,814 shares were issued under the 2015 ESPP. Stock-based compensation expense Stock-based compensation expense by award type was as follows (in thousands): Nine Months Ended September 30, 2017 2018 Stock options $ 21,238 $ 22,816 Restricted stock 5,625 12,183 Employee stock purchase plan 259 544 $ 27,122 $ 35,543 Of the $35.5 million of stock-based compensation expense incurred during the nine months ended September 30, 2018 , $14.2 million is classified as research and development expense and $21.3 million is classified as selling, general and administrative expense in the consolidated statements of operations and comprehensive income (loss). Of the $27.1 million of stock-based compensation expense incurred during the nine months ended September 30, 2017 , $11.1 million is classified as research and development expense and $16.0 million is classified as selling, general and administrative expense in the consolidated statements of operations and comprehensive income (loss). Stock options The following table summarizes stock option activity: Number of options Weighted- average exercise price Outstanding at December 31, 2017 3,522,874 $ 40.11 Granted 671,500 $ 52.02 Exercised (385,597 ) $ 34.05 Canceled (185,466 ) $ 50.96 Outstanding at September 30, 2018 3,623,311 $ 42.41 Vested at September 30, 2018 1,891,994 $ 34.63 Restricted stock The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2017 697,667 $ 63.40 Shares granted 517,300 $ 55.13 Shares vested (171,635 ) $ 61.38 Shares canceled (93,407 ) $ 59.25 Nonvested shares at September 30, 2018 949,925 $ 59.67

Related-party transactions

Related-party transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions As of December 31, 2017 , and September 30, 2018 , the Children's Hospital of Philadelphia (CHOP) was considered a significant equity holder. In October 2013, the Company entered into technology and license agreements with CHOP for certain commercialization licenses to be provided to the Company in order to develop services, methods and marketable products for commercialization. The license agreement requires the Company to reimburse CHOP for the patent costs related to the underlying licensed rights incurred after the effective date. For the nine months ended September 30, 2017 , and 2018 , the Company recorded $0.6 million and $0.9 million , respectively, of selling, general and administrative expense related to the reimbursement of such patent costs in the accompanying consolidated statements of operations and comprehensive (income) loss. In 2013, the Company entered into a number of services agreements with CHOP. The Master Research Services Agreement provides for certain research, development and manufacturing services to be provided to the Company by CHOP. A separate Services Agreement provides for clinical, technical and administrative services to be provided by CHOP to the Company. The Company recorded $4.4 million and $4.3 million as research and development expense for the nine months ended September 30, 2017 , and 2018 . As of December 31, 2017 , $0.3 million and $1.4 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP. As of September 30, 2018 , $0.2 million and $1.0 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP.

Commitments and contingencies

Commitments and contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and contingencies (a) Leases In November 2016, the Company entered into a lease agreement for approximately 49,000 square feet of office space in Philadelphia, Pennsylvania, that will terminate on March 31, 2027. Under this lease, the Company received $1.6 million of tenant improvement allowances during 2017. In January 2017, the Company amended its lease for office space in Philadelphia to lease an additional 24,800 square feet, with the amendment commencing on January 1, 2018, and terminating on December 31, 2028. In November 2017, the Company amended this lease to accelerate the termination date of approximately 50,000 square feet of office space, with such termination to occur, at the latest, in December 2022. The Company recorded an expense of $6.9 million associated with the change in termination date in 2017. As of September 30, 2018 , $4.0 million is recorded as long-term other liabilities and $1.7 million is recorded as current other liabilities on the accompanying consolidated balance sheet related to the termination expense. The following table reconciles the termination cost discussed above (in thousands): Balance as of December 31, 2017 Recognized during the nine months ended September 30, 2018 Balance as of September 30, 2018 Contract termination liability $6,827 $1,153 $5,674 In November 2017, the Company entered into a lease for a new research facility at One Drexel Plaza in Philadelphia, Pennsylvania for approximately 108,000 square feet through June 2033. Based on the terms of the lease agreement for One Drexel Plaza, the Company has construction period risks during the construction period and the Company is deemed the owner of the building (for accounting purposes only) during the construction period. Accordingly, the Company recorded an asset of $35.0 million at December 31, 2017, representing the Company’s leased portion of the building, and recorded a corresponding liability. Upon completion of leasehold improvement construction, the Company may not meet the sale-leaseback criteria for de-recognition of the building asset and liability. Therefore, the lease may be accounted for as a financing obligation. The asset will be depreciated over the expected duration of the lease of 15.5 years , and rental payments will be treated as principal and interest payments on the lease financing obligation liability. The underlying accounting for this transaction has no impact on cash flows associated with the underlying lease or construction in process. At September 30, 2018 , the lease financing obligation balance was $33.9 million and was recorded as a long-term other liability on the consolidated balance sheets. (b) Manufacturing agreement In August 2018, the Company entered into a Dedicated Manufacturing and Commercial Supply Agreement (the Brammer Agreement) with Brammer Bio MA, LLC (Brammer), pursuant to which Brammer has agreed to manufacture and supply certain products for the Company for clinical trial and commercial purposes. The term of the agreement continues until March 2026 and shall automatically renew for successive three (3) year terms unless the Company notifies Brammer of its intention not to renew no less than two (2) years prior to the expiration of the original term. During the term of the agreement, the Company will have access to a dedicated, specified portion of the manufacturing capacity in Brammer’s manufacturing facility located in Cambridge, Massachusetts, as well as non-dedicated capacity at Brammer’s facilities for manufacturing and other supply-related activities. Under the Brammer Agreement, the Company made an upfront payment of $4.0 million to Brammer upon execution of the agreement, which is included as a prepaid asset on the consolidated balance sheet and will be credited towards future capacity access amounts owed under the Brammer Agreement. The Company is obligated to pay yearly capacity access fees and is required to purchase a minimum dollar amount of manufactured batches per year.

Collaboration and license agree

Collaboration and license agreements	9 Months Ended
	Sep. 30, 2018
Collaboration and License Agreements [Abstract]
Collaboration and license agreements	Collaboration and license agreements (a) Pfizer In December 2014, the Company entered into a global collaboration agreement with Pfizer for the development and commercialization of SPK-FIX product candidates for the treatment of hemophilia B. Under the agreement, the Company granted Pfizer an exclusive worldwide license to any factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024. The Company is primarily responsible for conducting all research and development activities through completion of Phase 1/2 clinical trials of hemophilia B product candidates. Pfizer and the Company will share development costs incurred under an agreed product development plan for each product candidate with the Company’s share of development costs under the agreement limited to $10.6 million . Following the completion of Phase 1/2 clinical trials, Pfizer will be primarily responsible for development, manufacture, regulatory approval and commercialization, including all costs associated therewith. In connection with this agreement, the Company received a $20.0 million upfront payment for the license in December 2014. As there was no stand-alone value for the license, the Company recognized revenue (through the estimated completion date of Phase 1/2 clinical trials). In November 2017, the Company amended its global collaboration agreement with Pfizer. Under the terms of this amendment, the Company received $15.0 million in payments upfront, and an additional $10.0 million upon completion of certain transition activities. The $25.0 million of consideration was recognized as revenue over the estimated performance period associated with the global collaboration agreement. The Company is eligible to receive up to an additional $230.0 million in aggregate milestone payments, $110.0 million of which relate to potential development, regulatory and commercial milestones for the first product candidate to achieve each milestone and $120.0 million of which relate to potential regulatory milestones for additional product candidates. In addition, the Company is entitled to receive royalties calculated as a low-teen percentage of net sales of licensed products. The royalties may be subject to certain reductions, including for a specified portion of royalty payments that Pfizer may become required to pay under any third-party license agreements, subject to a minimum royalty. Under the agreement, the Company remains solely responsible for the payment of license payments payable by the Company under specified license agreements. The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product; or (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements. In February 2018, the Company entered into a supply agreement with Pfizer for production in 2018, of one batch of drug substance expected to be used for Phase 3 development. The Company received $7.0 million upfront and received $7.0 million upon delivery in July 2018. The $14.0 million of consideration was recognized as revenue as the batch of the drug substance was completed. During the nine months ended September 30, 2017 , and 2018 , the Company recognized $4.7 million and $36.0 million of contract revenue, respectively, related to the Company's agreements with Pfizer. During the nine months ended September 30, 2017 and 2018 , the Company recorded $2.6 million and $3.3 million , respectively, as a reduction to research and development expenses for the reimbursement of costs from Pfizer. In July 2018, Pfizer announced they initiated a Phase 3 program following the Company's transfer of responsibility for the hemophilia B gene therapy program to Pfizer. (b) Novartis In January 2018, the Company entered into a licensing and commercialization agreement (Novartis License Agreement) with Novartis Pharma AG (Novartis) to develop and commercialize voretigene neparvovec (also known as LUXTURNA) outside the United States. Under the terms of the Novartis License Agreement, the Company has granted Novartis an exclusive right and license, with the right to grant certain sublicenses, under the Company’s intellectual property reasonably necessary or useful for the development or commercialization of LUXTURNA for the treatment, prevention, cure or control of RPE65 -mediated inherited retinal disease (IRD) in humans outside the United States. Under the terms of the Novartis License Agreement, the Company received a non-refundable, one-time payment of $105.0 million in the first quarter of 2018, which is included as deferred revenue on the accompanying consolidated balance sheet as of September 30, 2018 . The Company is eligible to receive up to an additional $65.0 million in aggregate milestone payments. The Company also is entitled to receive royalty payments at a percentage of net sales on a royalty-region by royalty-region basis, subject to reduction and extension in certain circumstances. In conjunction with the Novartis License Agreement, the Company and Novartis also entered into a Supply Agreement, under which the Company has agreed to supply all of the commercial supply of voretigene neparvovec required by Novartis, subject to certain conditions. The Supply Agreement continues until the expiration or early termination of the Novartis License Agreement. In addition, either party may terminate the Supply Agreement upon the other party’s uncured material breach of the Supply Agreement, insolvency or bankruptcy. The majority of the $170.0 million of consideration associated with the Novartis License Agreement will be recognized as revenue as the Company sells supply to Novartis. (c) Selecta In December 2016, the Company entered into a License and Option Agreement (Selecta License Agreement) with Selecta that provides the Company with exclusive worldwide rights to Selecta’s proprietary Synthetic Vaccine Particles (SVP™) platform technology for co-administration with gene therapy targets. Under the terms of the Selecta License Agreement, Selecta has granted the Company certain exclusive, worldwide, royalty-bearing licenses to Selecta’s intellectual property and know-how relating to its SVP technology to research, develop and commercialize gene therapies for factor VIII, an essential blood clotting protein relevant to the treatment of hemophilia A, which is the initial target under the license. In addition, for a specified period of time, the Company may exercise options to research, develop and commercialize gene therapies utilizing the SVP technology for up to four additional targets, subject to the Company’s payment of the applicable option exercise fee, in a range of $1.4 million to $2.0 million depending on the incidence of the applicable indication, to Selecta in each case. Pursuant to a letter agreement (Letter Agreement), entered into between the Company and Selecta on June 6, 2017, Selecta agreed to reimburse the Company for all costs and expenses related to research and development for any licensed products for a specified amount of time, up to an agreed upon cap. Additionally, the Company has agreed to reimburse Selecta in respect of full-time equivalents and out-of-pocket costs incurred in performing certain tasks or assistance specifically requested by the Company. Selecta retains the responsibility to manufacture the Company’s preclinical, clinical and commercial requirements for the SVP technology, subject to the terms of the Selecta License Agreement. In connection with the execution of the Selecta License Agreement, the Company paid Selecta an upfront payment of $10.0 million in December 2016. Additional payments in the aggregate of $5.0 million were paid in June 2017 and October 2017 pursuant to the terms of the Selecta License Agreement and the Letter Agreement. On a target-by-target basis, the Company will be responsible to pay up to an aggregate of $430.0 million in milestone payments for each target, with up to $65.0 million being based on the Company’s achievement of specified development and regulatory milestones and up to $365.0 million for commercial milestones, as well as tiered royalties on global net sales at percentages ranging from mid-single to low-double digits. For a period of 3 years , the Company has the right to fund up to 50% of any development or regulatory milestone payable to Selecta by issuing to Selecta shares of the Company’s common stock having a fair market value equal to the percentage of such development or regulatory milestone, as applicable. The Selecta License Agreement will continue on a country-by-country and product-by-product basis until the expiration of the Company's royalty payment obligations with respect to such product in such country unless earlier terminated by the parties. The Selecta License Agreement may be terminated by the Company for convenience upon 90 days’ notice and the Company will not be required to make any payments. Either party may terminate the Selecta License Agreement on a target-by-target basis for material breach with respect to such target. In connection with the Selecta License Agreement, the Company entered into a Stock Purchase Agreement (SPA) with Selecta pursuant to which the Company purchased 197,238 unregistered shares of Selecta's common stock for $5.0 million in December 2016. An additional 324,362 unregistered shares of Selecta's common stock were purchased for $5.0 million in June 2017, and 205,254 unregistered shares of Selecta's common stock were purchased for $5.0 million in October 2017. These shares are classified as available-for-sale securities as of September 30, 2018 . In December 2016, the Company accounted for the payments under the Selecta License Agreement and SPA as a basket transaction and allocated the $15.0 million in cash payments to the shares of Selecta’s common stock and the Selecta License Agreement in the amounts of $3.5 million and $11.5 million , respectively. The Company calculated the $3.5 million allocated for the Selecta shares acquired based on the closing market price on the date of purchase. The Company allocated the remaining $11.5 million to the Selecta License Agreement, which was expensed as acquired in-process research and development as the Company determined there was no alternative future use. In June 2017, the Company accounted for the payments under the Selecta License Agreement, Letter Agreement and SPA as a basket transaction and allocated the $7.5 million in cash payments to the shares of Selecta’s common stock and the Selecta License Agreement in the amounts of $4.4 million and $3.1 million , respectively. The Company calculated the $4.4 million allocated to the Selecta shares acquired based on the closing market price on the date of purchase. The Company allocated the remaining $3.1 million to the Selecta License Agreement, which was expensed as acquired in-process research and development as the Company determined there was no alternative future use. In October 2017, the Company accounted for the payments under the Selecta License Agreement, Letter Agreement and SPA as a basket transaction and allocated the $7.5 million in cash payments to the shares of Selecta’s common stock and the Selecta License Agreement in the amounts of $4.1 million and $3.4 million , respectively. The Company calculated the $4.1 million allocated to the Selecta shares acquired based on the closing market price on the date of purchase. The Company allocated the remaining $3.4 million to the Selecta License Agreement, which was expensed as acquired in-process research and development as the Company determined there was no alternative future use.

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying unaudited interim consolidated financial statements of the Company and its wholly-owned subsidiaries have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying consolidated financial statements include all normal and recurring adjustments (which consist primarily of accruals, estimates and assumptions that impact the consolidated financial statements) considered necessary to present fairly the Company’s financial position as of September 30, 2018 , its results of operations for the three and nine months ended September 30, 2017 and 2018 and cash flows for the nine months ended September 30, 2017 and 2018 . Operating results for the three and nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the year ending December 31, 2018 or any other period. The interim consolidated financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual consolidated financial statements. The accompanying unaudited interim consolidated financial statements should be read in conjunction with the annual audited consolidated financial statements and related notes as of and for the year ended December 31, 2017 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017 .
Use of estimates	Use of estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates.
Trade and other receivables	Trade and other receivables Trade accounts receivable are recorded at gross value, and reserves for other sales-related allowances, such as discounts, rebates, services and insurance co-pay assistance, are included in accrued expenses on the Company's consolidated balance sheets.
Inventory	Inventory Inventory is stated at the lower of cost or net realizable value and consists of those costs incurred following FDA approval of LUXTURNA. Cost is determined using the first-expired, first-out (FEFO) method. The Company reserves for potentially excess, dated or obsolete inventories based on an analysis of inventory on hand compared to forecasts of future sales.
Fair value of financial instruments	Fair value of financial instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, trade and other receivables, accounts payable and accrued expenses, approximate fair value due to the short-term nature of those instruments. Management believes the carrying value of debt approximates fair value as the interest rates are reflective of the rate the Company could obtain on debt with similar terms and conditions.
Contract revenue	Contract revenue Under certain of the Company’s licensing, supply and collaboration agreements, it is entitled to receive payment upon the achievement of contingent milestone events or the performance of obligations. The Company recognizes revenue based on guidance in ASC 606. Prior to 2018, the Company generated revenue solely through license and collaborative arrangements. In the first quarter of 2018, the Company adopted ASC 606. The adoption of this standard resulted in no cumulative adjustment to the Company’s consolidated financial statements. ASC 606 applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under ASC 606, an entity recognizes revenue when its customer obtains control of promised goods or services in an amount that reflects the consideration the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. At contract inception, once the contract is determined to be within the scope of ASC 606, 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. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the Company’s consolidated balance sheet. Amounts expected to be recognized as revenue in the next 12 months following the balance sheet date are classified as current liabilities.
Net income (loss) per common share	Net loss per common share Basic and diluted net loss per common share is determined by dividing net loss by the weighted average number of common shares outstanding during the period. For all periods presented, unvested restricted shares and common stock options have been excluded from the calculation because their effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same.
Deferred rent	Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property. Tenant improvement allowances from the lessor are included in the accompanying consolidated balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of September 30, 2018 represents the net excess of rent expense over the actual cash paid for rent plus tenant improvement allowances received.
Other comprehensive loss	Other comprehensive loss The Company follows the provisions of ASC 220, Comprehensive Income , which establishes standards for the reporting and display of comprehensive income and its components. Comprehensive income (loss) is defined to include all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) includes gains and losses related to changes in the fair value of available-for-sale securities, changes in the fair value of an interest rate swap and foreign currency translation.
Recent accounting pronouncements	Recent accounting pronouncements In February 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-02, “Leases.” ASU 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. ASU 2016-02 is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company plans to elect the effective date method using a modified retrospective transition approach. The Company currently is evaluating the impact of the updated guidance on the Company's consolidated financial statements. In May 2014, the FASB issued a new standard, ASC 606, regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. ASC 606 provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflect the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. The allowable adoption methods are the full retrospective method, which requires the standard to be applied to each prior period presented, or the modified retrospective method which requires the cumulative effect of adoption to be recognized as an adjustment to opening retained earnings in the period of adoption. The Company adopted ASC 606 using the modified retrospective method and the adoption had no cumulative adjustment to its consolidated financial statements as it relates to the Pfizer Inc. (Pfizer) collaboration agreement discussed in note 14. For the three months ended September 30, 2018 , $14.0 million and $5.1 million in contract revenue and cost of contract revenue, respectively, would have been recognized under revenue recognition guidance in effect during 2017 prior to the adoption of ASC 606. For the nine months ended September 30, 2018, there was no impact from the adoption of ASC 606 that resulted in a different revenue recognition than under the prior revenue recognition guidance in effect during 2017. In January 2016, the FASB issued ASU No. 2016-01, "Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities." ASU 2016-01 changes accounting for equity investments, financial liabilities under the fair value option, and presentation and disclosure requirements for financial instruments. ASU 2016-01 does not apply to equity investments in consolidated subsidiaries or those accounted for under the equity method of accounting. In addition, the FASB clarified guidance related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale debt securities. Equity investments with readily determinable fair values will be measured at fair value with changes in fair value recognized in net income. Companies have the option to either measure equity investments without readily determinable fair values at fair value or at cost, adjusted for changes in observable prices, minus impairment. Changes in measurement under either alternative will be recognized in net income. Companies that elect the fair value option for financial liabilities must recognize changes in fair value related to instrument-specific credit risk in other comprehensive income. Companies must assess valuation allowances for deferred tax assets related to available-for-sale debt securities in combination with their other deferred tax assets. The Company adopted this guidance effective January 1, 2018, and the adoption required an adjustment of $5.0 million to accumulated deficit, which was related to an equity security investment on the consolidated balance sheet. In August 2017, the FASB issued ASU 2017-12, "Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities", which improves the financial reporting of hedging relationships to better portray the economic results of an entity's risk management activities in its financial statements and make certain targeted improvements to simplify the application of the hedge accounting guidance in current U.S. GAAP. The amendments in this update better align an entity's risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. The effective date for the standard is for fiscal years beginning after December 15, 2018. The Company elected to early adopt this ASU in the third quarter of 2018. The adoption of this guidance did not have a material impact on the Company’s consolidated financial statements. See Note 9 for a discussion of the Company’s derivatives.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of September 30, 2018 consisted of money market funds.
Marketable securities	Marketable securities The Company classifies its marketable security investments as available-for-sale securities and the securities are stated at fair value. There were no material realized gains or losses recognized on the maturity of available-for-sale securities during the nine months ended September 30, 2018 and, as a result, the Company did not reclassify any amount out of accumulated other comprehensive loss for the same period. In addition, as part of the license and stock purchase agreements entered into with Selecta Biosciences, Inc. (Selecta) (note 14), the Company purchased restricted common shares of Selecta. The investment is classified as available-for-sale and is stated at fair value, and changes in fair value are recognized in the consolidated statements of operations and comprehensive income (loss).
Impairment of acquired in-process research and development	The acquired in-process research and development (IPR&D) asset was an indefinite-lived intangible asset and was assessed for impairment annually, or more frequently if impairment indicators existed.

Summary of significant accoun_3

Summary of significant accounting policies (Tables)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Schedule of Inventory	Inventory consisted of the following (in thousands): September 30, Raw materials $ 2,009 Work in process 18,697 Finished goods 32 $ 20,738
Schedule of reconciliation of basic diluted net income (loss)
Schedule of potentially dilutive securities excluded from computation of diluted weighted average shares outstanding	The following potentially dilutive securities have been excluded from the computations of diluted weighted average shares outstanding as of September 30, 2017 and 2018 as they would be anti-dilutive: September 30, 2017 September 30, 2018 Unvested restricted common shares 781,703 949,925 Stock options issued and outstanding 3,771,059 3,623,311
Schedule of accumulated other comprehensive loss	The accumulated balances related to each component of other comprehensive income (loss) are summarized as follows (in thousands): Net unrealized (loss) gain on available for sale securities Unrealized gain on interest rate swap Foreign currency translation adjustments Accumulated other comprehensive loss Balance as of December 31, 2017 $ (5,964 ) $ — $ 50 $ (5,914 ) Cumulative adjustment to prior accumulated deficit 5,002 — — 5,002 Current period other comprehensive income (loss) 614 122 (49 ) 687 Balance as of September 30, 2018 $ (348 ) $ 122 $ 1 $ (225 )

Marketable securities (Tables)

Marketable securities (Tables)	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Schedule of available-for-sale securities	The following table summarizes the available-for-sale securities held at December 31, 2017 and September 30, 2018 (in thousands): Description Amortized cost Unrealized gains Unrealized losses Fair value December 31, 2017 U.S. government agency $ 222,179 $ — $ (640 ) $ 221,539 Corporate securities $ 227,238 $ — $ (5,323 ) $ 221,915 September 30, 2018 U.S. government agency $ 240,749 $ — $ (258 ) $ 240,491 Corporate securities $ 217,193 $ 11 $ (812 ) $ 216,392

Fair value of financial instr_2

Fair value of financial instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of assets and liabilities measured at fair value on recurring basis	The Company has classified assets and liabilities measured at fair value on a recurring basis as follows (in thousands): Fair value measurements at reporting date using Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) At December 31, 2017: Assets: Money market funds (included in cash and cash equivalents) $ 90,348 — — Corporate securities (included in cash and cash equivalents) $ 5,548 — — Marketable securities - U.S. government agencies $ 221,539 — — Marketable securities - corporate securities $ 220,020 $ 1,895 — At September 30, 2018: Assets: Money market funds (included in cash and cash equivalents) $ 160,857 — — Certificates of deposit - restricted cash $ 53,000 Marketable securities - U.S. government agencies $ 240,491 — — Marketable securities - corporate securities $ 216,392 — —

Accrued expenses (Tables)

Accrued expenses (Tables)	9 Months Ended
	Sep. 30, 2018
Payables and Accruals [Abstract]
Schedule of accrued expenses	Accrued expenses consist of the following (in thousands): December 31, September 30, Compensation and benefits $ 15,012 $ 11,126 Consulting and professional fees 4,846 18,629 Research and development 2,809 3,324 Other 2,030 2,013 Total accrued expenses $ 24,697 $ 35,092

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments [Table Text Block]	The Company's debt consists of the following (in thousands): December 31, September 30, Term loan outstanding $ — $ 50,000 MELF loan outstanding 1,224 991 Less: current portion of long-term debt 312 3,320 Total long-term debt due after one year $ 912 $ 47,671

Stockholders' equity (Tables)

Stockholders' equity (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Summary of restricted stock activity	The following table summarizes restricted stock activity: Number of shares Nonvested shares at December 31, 2017 100,834 Shares canceled (25,000 ) Shares vested (75,834 ) Nonvested shares at September 30, 2018 — The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2017 697,667 $ 63.40 Shares granted 517,300 $ 55.13 Shares vested (171,635 ) $ 61.38 Shares canceled (93,407 ) $ 59.25 Nonvested shares at September 30, 2018 949,925 $ 59.67

Stock incentive plans (Tables)

Stock incentive plans (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of stock-based compensation expense	Stock-based compensation expense by award type was as follows (in thousands): Nine Months Ended September 30, 2017 2018 Stock options $ 21,238 $ 22,816 Restricted stock 5,625 12,183 Employee stock purchase plan 259 544 $ 27,122 $ 35,543
Summary of stock option activity	The following table summarizes stock option activity: Number of options Weighted- average exercise price Outstanding at December 31, 2017 3,522,874 $ 40.11 Granted 671,500 $ 52.02 Exercised (385,597 ) $ 34.05 Canceled (185,466 ) $ 50.96 Outstanding at September 30, 2018 3,623,311 $ 42.41 Vested at September 30, 2018 1,891,994 $ 34.63
Summary of restricted common stock activity	The following table summarizes restricted stock activity: Number of shares Nonvested shares at December 31, 2017 100,834 Shares canceled (25,000 ) Shares vested (75,834 ) Nonvested shares at September 30, 2018 — The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2017 697,667 $ 63.40 Shares granted 517,300 $ 55.13 Shares vested (171,635 ) $ 61.38 Shares canceled (93,407 ) $ 59.25 Nonvested shares at September 30, 2018 949,925 $ 59.67

Commitments and contingencies (

Commitments and contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Reconciliation of lease termination costs	The following table reconciles the termination cost discussed above (in thousands): Balance as of December 31, 2017 Recognized during the nine months ended September 30, 2018 Balance as of September 30, 2018 Contract termination liability $6,827 $1,153 $5,674

The Company (Details)

The Company (Details)	9 Months Ended
	Sep. 30, 2018segment
Accounting Policies [Abstract]
Number of operating segments	1

Development-stage risks - Addit

Development-stage risks - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Development-stage Risk [Abstract]
Retained earnings (accumulated deficit)	$ (524,429)	$ (505,863)

Summary of significant accoun_4

Summary of significant accounting policies - Schedule of Inventory (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Raw materials	$ 2,009
Work in process	18,697
Finished goods	32
Inventory	$ 20,738	$ 0

Summary of significant accoun_5

Summary of significant accounting policies - Potentially Dilutive Securities Excluded from Computations of Diluted Weighted Average Shares Outstanding (Detail) - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Unvested restricted common shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding (in shares)	949,925	781,703
Stock options issued and outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding (in shares)	3,623,311	3,771,059

Summary of significant accoun_6

Summary of significant accounting policies - Schedule of Other Comprehensive Income (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2017	$ 513,624
Cumulative adjustment to prior accumulated deficit		$ 5,002
Current period other comprehensive income (loss)	687
Balance as of September 30, 2018	549,875
Net unrealized (loss) gain on available for sale securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2017	(5,964)
Cumulative adjustment to prior accumulated deficit		5,002
Current period other comprehensive income (loss)	614
Balance as of September 30, 2018	(348)
Unrealized gain on interest rate swap [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2017	0
Cumulative adjustment to prior accumulated deficit		0
Current period other comprehensive income (loss)	122
Balance as of September 30, 2018	122
Foreign currency translation adjustments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2017	50
Cumulative adjustment to prior accumulated deficit		$ 0
Current period other comprehensive income (loss)	(49)
Balance as of September 30, 2018	1
Accumulated other comprehensive loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2017	(5,914)
Balance as of September 30, 2018	$ (225)

Summary of significant accoun_7

Summary of significant accounting policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract revenue	$ 1,841	$ 1,900	$ 35,969	$ 4,657
Cost of contract revenue	$ 0	$ 0	5,111	$ 0
Cumulative adjustment to prior accumulated deficit						$ 5,002
ASU 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment to prior accumulated deficit					$ 5,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Contract revenue			14,000
Cost of contract revenue			$ 5,100

Marketable securities - Availab

Marketable securities - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
US government agency [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 240,749	$ 222,179
Unrealized gains	0	0
Unrealized losses	(258)	(640)
Fair value	240,491	221,539
Corporate securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	217,193	227,238
Unrealized gains	11	0
Unrealized losses	(812)	(5,323)
Fair value	$ 216,392	$ 221,915

Fair value of financial instr_3

Fair value of financial instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Assets:
Realized gains or losses recognized on maturity of available-for-sale securities	$ 0
Fair Value Measurements Recurring [Member] | Money market funds (included in cash and cash equivalents) [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds and corporate securities (included in cash and cash equivalents)	160,857,000	$ 90,348,000
Fair Value Measurements Recurring [Member] | Money market funds (included in cash and cash equivalents) [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds and corporate securities (included in cash and cash equivalents)	0	0
Fair Value Measurements Recurring [Member] | Money market funds (included in cash and cash equivalents) [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds and corporate securities (included in cash and cash equivalents)	0	0
Fair Value Measurements Recurring [Member] | Certificates of deposit [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Restricted cash	53,000,000
Fair Value Measurements Recurring [Member] | Corporate securities [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds and corporate securities (included in cash and cash equivalents)		5,548,000
Fair Value Measurements Recurring [Member] | Corporate securities [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds and corporate securities (included in cash and cash equivalents)		0
Fair Value Measurements Recurring [Member] | Corporate securities [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds and corporate securities (included in cash and cash equivalents)		0
US government agency [Member] | Fair Value Measurements Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Marketable securities	240,491,000	221,539,000
US government agency [Member] | Fair Value Measurements Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Marketable securities	0	0
US government agency [Member] | Fair Value Measurements Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Marketable securities	0	0
Corporate securities [Member] | Fair Value Measurements Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Marketable securities	216,392,000	220,020,000
Corporate securities [Member] | Fair Value Measurements Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Marketable securities	0	1,895,000
Corporate securities [Member] | Fair Value Measurements Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Marketable securities	$ 0	$ 0

Sale of priority review vouch_2

Sale of priority review voucher - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	May 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Proceeds from sale of priority review voucher	$ 110,000	$ 110,000	$ 0

Impairment of acquired in-pro_2

Impairment of acquired in-process research and development - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Indefinite-lived Intangible Assets [Line Items]
Impairment of acquired in-process research and development	$ 0	$ 0	$ 0	$ 15,696
In-process research and development [Member]
Indefinite-lived Intangible Assets [Line Items]
Impairment of acquired in-process research and development				15,700
Tax benefit of impairment losses, amount				$ 1,000

Accrued expenses - Schedule of

Accrued expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Compensation and benefits	$ 11,126	$ 15,012
Consulting and professional fees	18,629	4,846
Research and development	3,324	2,809
Other	2,013	2,030
Total accrued expenses	$ 35,092	$ 24,697

Debt Schedule of Debt (Details)

Debt Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jul. 03, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Current portion of long-term debt	$ 3,320		$ 312
Long-term debt	47,671		912
Secured Debt | Term Loan
Debt Instrument [Line Items]
Long-term Debt	50,000	$ 50,000	0
Secured Debt | MELF Loan
Debt Instrument [Line Items]
Long-term Debt	$ 991		$ 1,224

Debt (Details)

Debt (Details) - USD ($)	Jul. 03, 2018	Aug. 31, 2016	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Effective portion of interest rate expected to be reclassified from AOCL into interest expense over next 12 months			$ 100,000	$ 100,000
Proceeds from long-term debt				50,000,000	$ 0
Swap Agreement | Designated as Hedging Instrument
Debt Instrument [Line Items]
Notional amount	$ 50,000,000
Derivative asset			200,000	200,000
Derivative liability			100,000	100,000
Swap Agreement | Designated as Hedging Instrument | Cash Flow Hedging
Debt Instrument [Line Items]
Amount reclassified from AOCL into interest expense			86,000
Term Loan | Swap Agreement
Debt Instrument [Line Items]
Fixed rate per annum under swap	3.463%
Secured Debt | Term Loan
Debt Instrument [Line Items]
Amount extended under term loan	$ 50,000,000
Interest Expense, Debt			300,000	300,000
Secured Debt | Term Loan | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	0.65%
Secured Debt | MELF Loan
Debt Instrument [Line Items]
Interest Expense, Debt			$ 8,000	$ 27,000
Proceeds from long-term debt		$ 1,600,000
Debt term		5 years
Monthly payments		$ 29,000
Annual interest rate		3.25%

Stockholders' equity - Addition

Stockholders' equity - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended	24 Months Ended	45 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2014	Dec. 31, 2014	Sep. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	150,000,000				150,000,000	150,000,000
Preferred stock, shares authorized (in shares)	5,000,000				5,000,000	5,000,000
Stock-based compensation expense	$ 35,543	$ 27,122
Research and Development Expense [Member]
Class of Stock [Line Items]
Stock-based compensation expense	$ 14,200	11,100
Restricted stock [Member]
Class of Stock [Line Items]
Stock vested in connection with license agreement (in shares)	75,834
Stock-based compensation expense	$ 12,183	5,625
Restricted stock [Member] | Selling, General and Administrative Expense [Member]
Class of Stock [Line Items]
Stock-based compensation expense	0	30
Restricted stock [Member] | Research and Development Expense [Member]
Class of Stock [Line Items]
Stock-based compensation expense	$ 2,200	$ 6,700
Restricted stock [Member] | University of Pennsylvania [Member] | License agreement [Member]
Class of Stock [Line Items]
Shares issued during period (in shares)			200,000
Stock vested in connection with license agreement (in shares)					175,000
Restricted stock [Member] | Share-based Compensation Award, Tranche One [Member]
Class of Stock [Line Items]
Percentage of vesting of units on first anniversary				25.00%
Shares, vesting period				3 years

Stockholders' equity - Summary

Stockholders' equity - Summary of Restricted Stock Activity (Detail) - Restricted stock [Member]	9 Months Ended
	Sep. 30, 2018shares
Number of shares (in shares):
Nonvested shares beginning balance (in shares)	100,834
Shares canceled (in shares)	(25,000)
Shares vested (in shares)	(75,834)
Nonvested shares ending balance (in shares)	0

Stock incentive plans - Additio

Stock incentive plans - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 35,543	$ 27,122
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		14,200	11,100
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		21,300	16,000
Employee stock purchase plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 544	$ 259
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)	1,485,322
Shares available for future grants (in shares)		1,356,362
2015 Employee Stock Purchase Plan [Member] | Employee stock purchase plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)	371,330
Common stock reserved for issuance (in shares)		1,120,188
Shares issued (in shares)		30,814

Stock incentive plans - Stock-b

Stock incentive plans - Stock-based Compensation Plan (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 35,543	$ 27,122
Stock option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	22,816	21,238
Restricted stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	12,183	5,625
Employee stock purchase plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 544	$ 259

Stock incentive plans - Summary

Stock incentive plans - Summary of Stock Option Activity (Detail)	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Number of options (in shares):
Outstanding at beginning of period (in shares) | shares	3,522,874
Granted (in shares) | shares	671,500
Exercised (in shares) | shares	(385,597)
Canceled (in shares) | shares	(185,466)
Outstanding at end of period (in shares) | shares	3,623,311
Vested at end of period (in shares) | shares	1,891,994
Weighted-average exercise price (in dollars per share):
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 40.11
Granted (in dollars per share) | $ / shares	52.02
Exercised (in dollars per share) | $ / shares	34.05
Canceled (in dollars per share) | $ / shares	50.96
Outstanding at end of period (in dollars per share) | $ / shares	42.41
Vested at end of period (in dollars per share) | $ / shares	$ 34.63

Stock incentive plans - Summa_2

Stock incentive plans - Summary of Restricted Common Stock Activity (Details) - Performance Shares [Member]	9 Months Ended
	Sep. 30, 2018$ / sharesshares
Number of shares (in shares):
Nonvested shares beginning balance (in shares) | shares	697,667
Shares granted (in shares) | shares	517,300
Shares vested (in shares) | shares	(171,635)
Shares canceled (in shares) | shares	(93,407)
Nonvested shares ending balance (in shares) | shares	949,925
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in dollars per share) | $ / shares	$ 63.40
Shares granted (in dollars per share) | $ / shares	55.13
Shares vested (in dollars per share) | $ / shares	61.38
Shares canceled (in dollars per share) | $ / shares	59.25
Nonvested shares ending balance (in dollars per share) | $ / shares	$ 59.67

Related-party transactions - Ad

Related-party transactions - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Research and development expense related to agreements	$ 32,829	$ 39,341	$ 88,462	$ 104,679
CHOP [Member]
Related Party Transaction [Line Items]
Accrued expenses due to related party	200		200		$ 300
Accounts payable due to related party	$ 1,000		1,000		$ 1,400
CHOP [Member] | Technology and License Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement of patent costs			900	600
CHOP [Member] | Master Research Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense related to agreements			$ 4,300	$ 4,400

Commitments and contingencies -

Commitments and contingencies - Additional Information (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Aug. 31, 2018USD ($)	Nov. 30, 2016USD ($)ft²	Dec. 31, 2017USD ($)	Sep. 30, 2018USD ($)	Nov. 30, 2017ft²	Jan. 31, 2017ft²
Operating Leased Assets [Line Items]
Long-term other liabilities			$ 40,255	$ 37,879
Current other liabilities			1,557	1,808
Contractual obligation				33,900
Brammer Bio Agreement [Member]
Operating Leased Assets [Line Items]
Product and supply manufacturing renewal term	3 years
Product manufacturing and supply, period prior to renewal term expiration to cancel renewal	2 years
Prepaid expense, current	$ 4,000
Lease of Office Space [Member]
Operating Leased Assets [Line Items]
Loss on contract termination			6,900
Long-term other liabilities				4,000
Current other liabilities				$ 1,700
One Drexel Plaza Research Facility [Member]
Operating Leased Assets [Line Items]
Area of leased property | ft²					108,000
Construction in progress			$ 35,000
Leased asset under construction, useful life					15 years 6 months
Corporate headquarters in Philadelphia [Member] | Lease of Office Space [Member]
Operating Leased Assets [Line Items]
Area of leased property | ft²		49,000				24,800
Terminated lease property | ft²					50,000
Corporate headquarters in Philadelphia [Member] | Sublease of Office Space [Member]
Operating Leased Assets [Line Items]
Allowance for leasehold improvements received		$ 1,600

Commitments and contingencies_2

Commitments and contingencies - Reconciliation of Termination Cost (Details) - Lease of Office Space [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Operating Leased Assets [Line Items]
Balance as of December 31, 2017	$ 6,827
Recognized during the nine months ended September 30, 2018	1,153
Balance as of September 30, 2018	$ 5,674

Collaboration and license agr_2

Collaboration and license agreements - Pfizer (Details) - USD ($)	1 Months Ended			3 Months Ended		7 Months Ended	8 Months Ended	9 Months Ended		12 Months Ended
	Feb. 28, 2018	Nov. 30, 2017	Dec. 31, 2014	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2014
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense related to agreements				$ 32,829,000	$ 39,341,000			$ 88,462,000	$ 104,679,000
Milestone payment, additional revenue potential to be earned								$ 230,000,000
Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement, rights and obligations								Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024.
Research and development expense related to agreements										$ 10,600,000
Upfront payment received for license	$ 7,000,000	$ 15,000,000	$ 20,000,000			$ 7,000,000	$ 14,000,000
Revenue recognition from aggregate milestone payments		$ 10,000,000						$ 110,000,000
Consideration recognized as revenue				$ 25,000,000		$ 25,000,000	$ 25,000,000	$ 25,000,000
Collaboration and license agreement expiration description								The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product or; (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements.
Collaboration agreement expiration, period								15 years
Licenses revenue								$ 36,000,000	4,700,000
Pfizer Collaboration and License Agreement [Member] | Research and Development Expense [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Reimbursed costs								3,300,000	$ 2,600,000
Additional Product Candidates [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment, additional revenue potential to be earned								$ 120,000,000

Collaboration and license agr_3

Collaboration and license agreements - Novartis (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment, additional revenue potential to be earned	$ 230
Novartis Licensing and Commercialization Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront payment received for license	105
Milestone payment, additional revenue potential to be earned	65
Consideration recognized as revenue	$ 170

Collaboration and license agr_4

Collaboration and license agreements - Selecta (Details) $ in Thousands	1 Months Ended			5 Months Ended	9 Months Ended
	Oct. 31, 2017USD ($)shares	Jun. 30, 2017USD ($)	Dec. 31, 2016USD ($)targetshares	Oct. 31, 2017USD ($)shares	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)shares
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments to acquire in process research and development					$ 0	$ 4,820
Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Prepaid expense, current			$ 10,000	$ 5,000
Milestone payment, option period			3 years
Shares purchased (in shares) | shares	205,254		197,238	205,254		324,362
Investment shares purchased, amount	$ 5,000	$ 5,000	$ 5,000
Payments to acquire project	7,500	7,500	15,000
Payments to acquire in process research and development	3,400	3,100	11,500
Selecta License Agreement [Member] | Licensing Agreements [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments to acquire project	3,400	3,100	11,500
Selecta License Agreement [Member] | Common Stock [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments to acquire project	$ 4,100	$ 4,400	3,500
Development and Regulatory Milestone [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments			$ 65,000
Collaboration agreement expiration, period			90 days
Commercial Milestone [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments			$ 365,000
Minimum [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of additional licensing targets | target			4
Option exercise fee			$ 1,400
Maximum [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option exercise fee			2,000
Milestone payments			$ 430,000
Milestone payment, rights to fund percentage			50.00%