Spark Therapeutics (ONCE) 10-Q 7 Nov 19 2019 Q3 Quarterly report Financial data

Cover page

Cover page - shares	9 Months Ended
	Sep. 30, 2019	Nov. 01, 2019
Cover page.
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Registrant Name	Spark Therapeutics, Inc.
Entity Central Index Key	0001609351
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		38,523,294
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Title of 12(b) Security	Common Stock, $0.001 par value per share
Trading Symbol	ONCE
Security Exchange Name	NASDAQ
Entity File Number	001-36819
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-2654405
Entity Address, Address Line One	3737 Market Street
Entity Address, Address Line Two	Suite 1300
Entity Address, City or Town	Philadelphia
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	19104
City Area Code	888
Local Phone Number	772-7560
Document Quarterly Report	true
Document Transition Report	false

Consolidated balance sheets (un

Consolidated balance sheets (unaudited) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 207,137	$ 95,247
Marketable securities	182,775	358,359
Trade and other receivables	35,482	47,385
Inventory, net	31,853	25,637
Prepaid expenses and other current assets	10,700	40,512
Total current assets	467,947	567,140
Restricted cash	51,500	53,000
Marketable securities	39,879	94,678
Property and equipment, net	72,001	95,998
Right of use asset	67,318	0
Goodwill	1,143	1,198
Other assets	3,712	2,338
Total assets	703,500	814,352
Current liabilities:
Accounts payable	18,129	19,492
Accrued expenses	41,695	34,790
Current portion of long-term debt	7,830	4,822
Current portion of deferred rent	0	1,365
Current portion of deferred revenue	22,120	0
Current portion of lease liability	8,367	0
Current other liabilities	2,158	1,885
Total current liabilities	100,299	62,354
Long-term debt	41,341	46,090
Long-term deferred rent	0	10,885
Long-term deferred revenue	132,719	160,000
Long-term lease liability	79,408	0
Other liabilities	3,724	38,510
Total liabilities	357,491	317,839
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; 37,764,213 shares issued and 37,686,301 shares outstanding as of December 31, 2018; 38,940,860 shares issued and 38,519,183 shares outstanding as of September 30, 2019	39	38
Additional paid-in capital	1,162,314	1,091,873
Accumulated other comprehensive loss	(1,281)	(1,050)
Treasury stock, at cost, 77,912 shares as of December 31, 2018 and 421,677 shares as of September 30, 2019	(39,878)	(4,661)
Accumulated deficit	(775,185)	(589,687)
Total stockholders’ equity	346,009	496,513
Total liabilities and stockholders’ equity	$ 703,500	$ 814,352

Consolidated balance sheets (_2

Consolidated balance sheets (unaudited) (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
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 (in shares)	0	0
Preferred stock, shares outstanding (in shares)	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 (in shares)	38,940,860	37,764,213
Common stock, shares outstanding (in shares)	38,519,183	37,686,301
Treasury stock, shares (in shares)	421,677	77,912

Consolidated statements of oper

Consolidated statements of operations and comprehensive income (loss) (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues	$ 22,447	$ 10,707	$ 56,721	$ 51,568
Operating expenses:
Research and development	44,655	32,829	122,730	88,462
Selling, general and administrative	41,407	29,305	116,105	92,543
Total operating expenses	91,105	62,452	248,670	186,824
Loss from operations	(68,658)	(51,745)	(191,949)	(135,256)
Unrealized gain (loss) on equity investments	(13)	1,672	(646)	4,291
Interest income, net	2,095	2,714	7,467	7,420
Other income	0	0	0	110,000
Loss before income taxes	(66,576)	(47,359)	(185,128)	(13,545)
Income tax benefit (expense)	(26)	2	(62)	(20)
Net loss	$ (66,602)	$ (47,357)	$ (185,190)	$ (13,565)
Basic and diluted net loss per common share (in dollars per share)	$ (1.72)	$ (1.26)	$ (4.82)	$ (0.36)
Weighted average basic and diluted common shares outstanding (in shares)	38,747,658	37,498,616	38,450,255	37,267,942
Net loss	$ (66,602)	$ (47,357)	$ (185,190)	$ (13,565)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	(210)	308	1,172	614
Unrealized gain (loss) on interest rate swap	(265)	122	(1,269)	122
Foreign exchange translation adjustment	(115)	(12)	(134)	(49)
Total comprehensive loss	(67,192)	(46,939)	(185,421)	(12,878)
Product sales, net
Revenues	6,918	8,866	28,160	15,599
Operating expenses:
Cost of goods sold	1,010	318	3,052	708
Contract revenue
Revenues	15,529	1,841	28,561	35,969
Operating expenses:
Cost of goods sold	$ 4,033	$ 0	$ 6,783	$ 5,111

Consolidated statement of stock

Consolidated statement of stockholders' equity (unaudited) - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Common stock in treasury	Accumulated deficit
Balance, (in shares) at Dec. 31, 2017		37,131,626			20,222
Balance, at Dec. 31, 2017	$ 513,624	$ 37	$ 1,026,590	$ (5,914)	$ (1,226)	$ (505,863)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted stock (in shares)		161,218
Issuance of restricted stock	0
Restricted stock canceled (in shares)		(25,000)
Restricted stock canceled	0
Purchase of common stock in treasury (in shares)					53,738
Purchase of common stock in treasury	(3,272)				$ (3,272)
Purchase of common stock under ESPP (in shares)		30,814
Purchase of common stock under ESPP	1,566		1,566
Exercise of stock options (in shares)		385,597
Exercise of stock options	13,130	$ 1	13,129
Unrealized gain (loss) on investments	614			614
Unrealized gain (loss) on interest rate swap	122
Unrealized loss on foreign currency translation	(49)			(49)
Stock-based compensation expense	37,704		37,704
Net loss	(13,565)					(13,565)
Balance, at Sep. 30, 2018	549,874	$ 38	1,078,989	(225)	$ (4,498)	(524,430)
Balance, (in shares) at Sep. 30, 2018		37,684,255			73,960
Balance, (in shares) at Jun. 30, 2018		37,543,411			66,433
Balance, at Jun. 30, 2018	583,740	$ 38	1,065,438	(643)	$ (4,020)	(477,073)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted stock (in shares)		22,350
Issuance of restricted stock	0
Purchase of common stock in treasury (in shares)					7,527
Purchase of common stock in treasury	(478)				$ (478)
Purchase of common stock under ESPP (in shares)		17,351
Purchase of common stock under ESPP	804		804
Exercise of stock options (in shares)		101,143
Exercise of stock options	1,383		1,383
Unrealized gain (loss) on investments	308			308
Unrealized gain (loss) on interest rate swap	122			122
Unrealized loss on foreign currency translation	(12)			(12)
Stock-based compensation expense	11,364		11,364
Net loss	(47,357)					(47,357)
Balance, at Sep. 30, 2018	549,874	$ 38	1,078,989	(225)	$ (4,498)	(524,430)
Balance, (in shares) at Sep. 30, 2018		37,684,255			73,960
Balance, (in shares) at Dec. 31, 2018		37,764,213			77,912
Balance, at Dec. 31, 2018	496,513	$ 38	1,091,873	(1,050)	$ (4,661)	(589,687)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted stock (in shares)		252,886
Issuance of restricted stock	0
Purchase of common stock in treasury (in shares)					82,273
Purchase of common stock in treasury	(5,583)				$ (5,583)
Purchase of common stock under ESPP (in shares)		15,829
Purchase of common stock under ESPP	$ 563		563
Exercise of stock options (in shares)	907,932	907,932			261,492
Exercise of stock options	$ 5,698	$ 1	35,331		$ (29,634)
Unrealized gain (loss) on investments	1,172			1,172
Unrealized gain (loss) on interest rate swap	(1,269)			(1,269)
Unrealized loss on foreign currency translation	(134)			(134)
Stock-based compensation expense	34,547		34,547
Net loss	(185,190)					(185,190)
Balance, at Sep. 30, 2019	346,009	$ 39	1,162,314	(1,281)	$ (39,878)	(775,185)
Balance, (in shares) at Sep. 30, 2019		38,940,860			421,677
Balance, (in shares) at Jun. 30, 2019		38,893,404			413,490
Balance, at Jun. 30, 2019	401,604	$ 39	1,149,906	(691)	$ (39,068)	(708,582)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted stock (in shares)		23,025
Issuance of restricted stock	0
Purchase of common stock in treasury (in shares)					8,187
Purchase of common stock in treasury	(810)				$ (810)
Exercise of stock options (in shares)		24,431
Exercise of stock options	1,374		1,374
Unrealized gain (loss) on investments	(210)			(210)
Unrealized gain (loss) on interest rate swap	(265)			(265)
Unrealized loss on foreign currency translation	(115)			(115)
Stock-based compensation expense	11,034		11,034
Net loss	(66,602)					(66,602)
Balance, at Sep. 30, 2019	$ 346,009	$ 39	$ 1,162,314	$ (1,281)	$ (39,878)	$ (775,185)
Balance, (in shares) at Sep. 30, 2019		38,940,860			421,677

Consolidated statements of cash

Consolidated statements of cash flows (unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (185,190)	$ (13,565)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash income	(738)	(1,686)
Depreciation and amortization expense	7,540	4,820
Loss on disposal of property and equipment	1,626	123
Stock-based compensation expense	34,547	37,704
Unrealized (gain) loss on equity investments	646	(4,291)
Gain from sale of priority review voucher	0	(110,000)
Non-cash interest (income) expense	269	(1,407)
Changes in operating assets and liabilities:
Inventory	(6,216)	(20,738)
Prepaid expenses and other assets	28,061	(5,564)
Trade and other receivables	19,226	(5,690)
Accounts payable and accrued expenses	3,669	(877)
Deferred revenue	(5,161)	93,031
Other liabilities	1,273	104
Net cash used in operating activities	(100,448)	(28,036)
Cash flows from investing activities:
Gain from sale of priority review voucher	0	110,000
Payment for license agreement	0	(2,000)
Purchases of marketable securities	(76,023)	(360,569)
Proceeds from maturities of marketable securities	305,664	352,167
Purchases of property and equipment	(17,677)	(14,920)
Net cash provided by investing activities	211,964	84,678
Cash flows from financing activities:
Proceeds from exercise of options	17,910	13,129
Purchase of treasury stock	(17,795)	(3,272)
Proceeds from issuance of common stock under ESPP	563	1,567
Proceeds from long-term debt	0	50,000
Payments on long-term debt	(1,741)	(233)
Net cash provided by (used in) financing activities	(1,063)	61,191
Effect of exchange rate changes on cash and cash equivalents and restricted cash	(63)	(24)
Net increase in cash and cash equivalents and restricted cash	110,390	117,809
Cash and cash equivalents and restricted cash, beginning of period	148,247	96,748
Cash and cash equivalents and restricted cash, end of period	258,637	214,557
Supplemental disclosure of cash flow information:
Property and equipment purchases included in accounts payable and accrued expenses	$ 11,409	17,946
One Drexel Plaza lease cost included in other liabilities		$ 1,077

The Company

The Company	9 Months Ended
	Sep. 30, 2019
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 (LLC) 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. On February 22, 2019, the Company entered into an Agreement and Plan of Merger (the Merger Agreement) with Roche Holdings, Inc. (Roche) and 022019 Merger Subsidiary, Inc. (Merger Sub). Pursuant to the Merger Agreement, and upon the terms and subject to the conditions thereof, Merger Sub has commenced a cash tender offer (the Tender Offer) to acquire all of the issued and outstanding shares of the Company's common stock at a price per share of $114.50 , net to the seller of such shares in cash, without interest, subject to any withholding of taxes required by applicable law. The completion of the Tender Offer is conditioned on at least a majority of the shares of the Company's outstanding common stock having been validly tendered into and not withdrawn from the offer, receipt of certain regulatory approvals, and other customary conditions. Following the completion of the Tender Offer, Merger Sub will merge with and into the Company, with the Company surviving as a wholly owned subsidiary of Roche. The merger will be governed by Section 251(h) of the General Corporation Law of the State of Delaware, with no stockholder vote required to consummate the merger. In the merger, each outstanding share of the Company's common stock (other than shares of common stock held by the Company as treasury stock, or owned by Roche or Merger Sub or held by stockholders who are entitled to demand, and who properly demand, appraisal rights under Delaware law) will be converted into the right to receive $114.50 per share in cash, without interest, subject to any withholding of taxes required by applicable law. The transaction is expected to close in 2019.

Development-stage risks

Development-stage risks	9 Months Ended
	Sep. 30, 2019
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 $775.2 million as of September 30, 2019 . 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 continued 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, 2019
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 financial statements) considered necessary to present fairly the Company’s financial position as of September 30, 2019 , its results of operations for the three and nine months ended September 30, 2018 and 2019 , its stockholders' equity for the three and nine months ended September 30, 2018 and 2019, and cash flows for the nine months ended September 30, 2018 and 2019 . Operating results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results that may be expected for the year ending December 31, 2019 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, 2018 , included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018 . (b) Use of estimates The preparation of consolidated 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 realized following the U.S. Food and Drug Administration (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 was necessary as of December 31, 2018. As of September 30, 2019 , the Company has recorded an inventory obsolescence reserve of $4.0 million against work in process. Inventory consisted of the following (in thousands): December 31, September 30, Raw materials $ 3,843 $ 3,427 Work in process 19,843 27,716 Finished goods 1,951 710 $ 25,637 $ 31,853 (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. 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. The Company evaluated the variable consideration under Accounting Standards Codification (ASC) Topic 606, Revenue from Contracts with Customers (ASC 606), as it relates to the outcomes-based method and determined that based on historical clinical data, no rebate reserves were required for the nine months ended September 30, 2018 and 2019 . 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. 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. 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, 2018 and 2019 as they would be anti-dilutive: September 30, 2018 September 30, 2019 Unvested restricted common shares 949,925 1,151,331 Stock options issued and outstanding 3,623,311 3,253,241 (i) 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 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 loss on interest rate swap Foreign currency translation adjustments Accumulated other comprehensive loss Balance as of December 31, 2018 $ (376 ) $ (641 ) $ (33 ) $ (1,050 ) Current period other comprehensive income (loss) 1,172 (1,269 ) (134 ) (231 ) Balance as of September 30, 2019 $ 796 $ (1,910 ) $ (167 ) $ (1,281 ) (j) Recent accounting pronouncements In February 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-02, Leases . ASU 2016-02 requires lessees to apply a two-method approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight line basis over the term of the lease, respectively. Lessees are also required to record a right-of-use (ROU) asset and a lease liability for all leases with a term greater than 12 months. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. ASU 2016-02 is effective for interim and annual periods beginning after December 15, 2018. The Company adopted this standard on January 1, 2019, the effective date, using the effective date method under the modified retrospective transition approach and elected to apply the package of practical expediants. The adoption of the standard had a material impact on the consolidated balance sheet, which resulted in the derecognition of the Company’s built-to-suit asset of $34.8 million and corresponding liability of $34.5 million , and an adjustment to beginning accumulated deficit of $0.3 million in the first quarter of 2019. Upon adoption, the Company also recognized ROU assets and lease liabilities of $31.7 million and $43.9 million

Marketable securities

Marketable securities	9 Months Ended
	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable securities The following table summarizes the available-for-sale securities held at December 31, 2018 and September 30, 2019 (in thousands): Description Amortized cost Unrealized gains Unrealized losses Fair value December 31, 2018 U.S. government agency $ 223,553 $ 54 $ (123 ) $ 223,484 Corporate securities $ 239,940 $ 34 $ (10,421 ) $ 229,553 September 30, 2019 U.S. government agency $ 117,915 $ 452 $ 118,367 Corporate securities $ 114,684 $ 344 $ (10,741 ) $ 104,287 No available-for-sale securities held as of September 30, 2019 had remaining maturities greater than two years.

Fair value of financial instrum

Fair value of financial instruments	9 Months Ended
	Sep. 30, 2019
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, 2018: Assets: Money market funds (included in cash and cash equivalents) $ 94,216 — — Certificates of deposit - restricted cash $ 53,000 — — Marketable securities - U.S. government agencies $ 223,484 — — Marketable securities - corporate securities $ 229,553 — — Liabilities: Interest rate swap — $ 641 — At September 30, 2019: Assets: Money market funds (included in cash and cash equivalents) $ 205,710 — — Certificates of deposit - restricted cash $ 51,500 — — Marketable securities - U.S. government agencies $ 118,367 — — Marketable securities - corporate securities $ 104,287 — — Liabilities: Interest rate swap — $ 1,910 — (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, 2019 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, 2019 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 13), 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, 2019
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 U.S. 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.

Accrued expenses

Accrued expenses	9 Months Ended
	Sep. 30, 2019
Payables and Accruals [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following (in thousands): December 31, September 30, Compensation and benefits $ 15,575 $ 14,740 Consulting and professional fees 11,499 17,233 Commercial costs 2,405 2,909 Research and development 2,706 3,788 Other 2,605 3,025 $ 34,790 $ 41,695

Debt

Debt	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Debt	Debt The Company's debt consists of the following (in thousands): December 31, September 30, Term loan outstanding $ 50,000 $ 48,500 MELF loan outstanding 912 671 Less: current portion of long-term debt (4,822 ) (7,830 ) Total long-term debt due after one year $ 46,090 $ 41,341 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 was only 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, 2019 , the Company recorded interest expense of $0.4 million and $1.1 million , respectively, related to the Credit Agreement. 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, 2019 , the fair value of the derivative liability was $1.9 million , of which $0.5 million was recognized within other current liabilities and $1.4 million was recognized within other long-term liabilities. The effective portion of the swap is reported as a component of accumulated other comprehensive loss. There was no hedge ineffectiveness as of September 30, 2019 . 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 nine months ended September 30, 2019 , the Company reclassified $0.2 million from accumulated other comprehensive loss into interest expense related to the effective portion of the swap. Over the next 12 months, $1.3 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 $29 thousand of principal and interest at an annual interest rate of 3.25% . For the three and nine months ended September 30, 2019, the Company recorded interest expense of $6 thousand and $19 thousand , respectively, related to the MELF loan. For the three and nine months ended September 30, 2018, the Company recorded interest expense of $8 thousand and $27 thousand , respectively, related to the MELF loan.

Stock-based compensation

Stock-based compensation	9 Months Ended
	Sep. 30, 2019
Share-based Payment Arrangement [Abstract]
Stock-based compensation	Stock-based compensation 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 2019, 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 1,510,533 shares. As of September 30, 2019 , 2,137,500 shares were available for future grants under the 2015 Plan. In January 2019, 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 377,633 shares. The 2015 ESPP provides participating employees with the opportunity to purchase an aggregate of 1,481,992 shares of common stock. For the nine months ended September 30, 2019 , 15,829 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, 2018 2019 Stock options $ 22,816 $ 18,645 Restricted stock 14,344 15,595 Employee stock purchase plan 544 307 $ 37,704 $ 34,547 Of the $34.5 million of stock-based compensation expense incurred during the nine months ended September 30, 2019 , $13.5 million is classified as research and development expense and $21.0 million is classified as selling, general and administrative expense in the consolidated statements of operations and comprehensive income (loss). Of the $37.7 million of stock-based compensation expense incurred during the nine months ended September 30, 2018 , $16.4 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). Stock options The following table summarizes stock option activity: Number of options Weighted- average exercise price Outstanding at December 31, 2018 3,487,376 $ 42.40 Granted 847,500 $ 43.62 Exercised (907,932 ) $ 38.92 Forfeited (173,703 ) $ 50.44 Outstanding at September 30, 2019 3,253,241 $ 43.26 Vested at September 30, 2019 1,601,470 $ 38.99 During the nine months ended September 30, 2019, the Company withheld 153,727 shares to satisfy the exercise price of options exercised and 107,765 shares to satisfy minimum tax withholding obligations, which have been reflected as the acquisition of treasury shares. Restricted stock The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2018 942,776 $ 58.71 Shares granted 546,700 $ 47.57 Shares vested (256,636 ) $ 60.03 Shares canceled (81,509 ) $ 55.63 Nonvested shares at September 30, 2019 1,151,331 $ 53.34

Related-party transactions

Related-party transactions	9 Months Ended
	Sep. 30, 2019
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions As of December 31, 2018 , and September 30, 2019 , 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 each of the nine months ended September 30, 2018 and 2019 , the Company recorded $0.9 million 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. For the nine months ended September 30, 2018 and 2019 , the Company recorded $4.3 million and $1.0 million , respectively, as research and development expense. As of December 31, 2018 , $0.7 million and $0.5 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP. As of September 30, 2019 , $0.3 million and $0.4 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP.

Leases

Leases	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Leases	Leases The Company has operating leases for its corporate headquarters and research facilities in Philadelphia, Pennsylvania and leases for supplemental office facilities and certain equipment with original lease terms ranging from one to fifteen years , some of which include options to extend. These options to extend were not recognized as part of the Company's measurement of the ROU asset and operating lease liability. As of September 30, 2019, the weighted average remaining lease term of the Company’s operating leases was 11.93 years. As of September 30, 2019, the Company has ROU assets of $67.3 million and current and noncurrent operating lease liabilities of $8.4 million and $79.4 million , respectively. These lease liabilities are based on the present value of the remaining minimum lease payments related to the Company’s operating leases as of September 30, 2019 , discounted using the Company’s estimated incremental borrowing rate commensurate with the corresponding lease terms. As of September 30, 2019 , the weighted average discount rate used to present value the Company’s operating lease liabilities was 5.65% . Rent expense related to the Company’s operating leases was $5.4 million and $3.1 million for the nine months ended September 30, 2019 and 2018, respectively. During the nine months ended September 30, 2019, the Company paid cash of $5.0 million related to its operating leases. Future minimum rental payments under these leases as of September 30, 2019 are as follows (in thousands): Year ending December 31, 2019 $ 1,829 2020 9,468 2021 10,457 2022 10,445 2023 10,711 2024 and thereafter 80,661 Total 123,571 Less: imputed interest (35,796 ) Current and long-term operating lease liability $ 87,775 Future minimum rental payments under our leases prior to adoption of ASU 2016-02, Leases, as of December 31, 2018 were as follows (in thousands): Year ending December 31, 2019 $ 8,102 2020 11,044 2021 10,073 2022 10,156 2023 9,228 2024 and thereafter 78,079 Total $ 126,682 In 2017, the Company exited office space prior to the end of the lease term and recognized a contract termination liability. As of September 30, 2019 , $2.4 million is recorded as long-term other liabilities and $1.6 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, 2018 Recognized during the nine months ended September 30, 2019 Balance as of September 30, 2019 Contract termination liability $5,270 $1,300 $3,970

Commitments and contingencies

Commitments and contingencies	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and contingencies 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, 2019
Collaboration and License Agreements [Abstract]
Collaboration and license agreements	Collaboration and license agreements (a) 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, or 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, earned a $25.0 million milestone in the fourth quarter of 2018, earned a $30.0 million milestone in the second quarter of 2019, and is eligible to receive up to an additional $10.0 million in 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 Company evaluated whether or not the performance obligations granted under the License and Supply Agreements were distinct and concluded that they were not distinct as Novartis could not benefit from the Novartis License Agreement without the services under the Supply Agreement. As such, the performance obligations granted under the Novartis License Agreement and Supply Agreement are combined to constitute a single performance obligation and the Company accounts for them as a single contract. During the fourth quarter of 2018, and in conjunction with the European Commission granting marketing authorization for LUXTURNA, the Company earned an additional $25.0 million milestone payment. In the second quarter of 2019, the Company earned an additional $30.0 million milestone payment, which was determined to not be constrained in the fourth quarter of 2018. The $30.0 million , along with the $130.0 million of other payments, is included as deferred revenue on the accompanying consolidated balance sheet as of December 31, 2018. The Company allocated $160.0 million in license fees and milestone payments that were not constrained to the single performance obligation in the combined contracts. The Company will recognize the $160.0 million and variable amounts under the Supply Agreement over time using an outputs method based on vials supplied to Novartis. As of September 30, 2019 , the remaining $10.0 million in milestone payments under the Novartis License Agreement remains constrained, due to additional regulatory approvals, and will not begin to be recognized until such amount becomes unconstrained. For the nine months ended September 30, 2019 , the Company recognized $13.6 million of contract revenue related to the $160.0 million , and other variable amounts, including royalties, under the Supply Agreement. As of September 30, 2019 , there was $22.1 million of current deferred revenue and $132.7 million of long-term deferred revenue on the consolidated balance sheet related to the Novartis License Agreement. In determining the transaction price, the Company analyzed the variable consideration and whether or not such variable consideration was constrained. The Company will reassess this variable consideration at each reporting period and adjust the transaction price, if necessary. The total vials that the Company expects to manufacture for Novartis over the life of the Supply Agreement will be a significant judgment that will be relied upon using the outputs method to recognize revenue. (b) 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 $215.0 million in aggregate milestone payments, $95.0 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 contract revenue as the batch of the drug substance was completed. During the nine months ended September 30, 2018, the Company recognized $36.0 million of contract revenue related to the Company's agreements with Pfizer. During the nine months ended September 30, 2018, the Company recorded $3.3 million as a reduction to research and development expenses for the reimbursement of costs from Pfizer. During the nine months ended September 30, 2019, we recognized $15.0 million of contract revenue related to an achievement of a milestone in our agreement with Pfizer. In July 2018, Pfizer announced the initiation of a Phase 3 program following the Company's transfer of responsibility for the hemophilia B gene therapy program to Pfizer. (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 three 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 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, 2019

Summary of significant accoun_2

Summary of significant accounting policies (Policies)	9 Months Ended
	Sep. 30, 2019
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 financial statements) considered necessary to present fairly the Company’s financial position as of September 30, 2019 , its results of operations for the three and nine months ended September 30, 2018 and 2019 , its stockholders' equity for the three and nine months ended September 30, 2018 and 2019, and cash flows for the nine months ended September 30, 2018 and 2019 . Operating results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results that may be expected for the year ending December 31, 2019 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, 2018 , included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2018 .
Use of estimates	Use of estimates The preparation of consolidated 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 receivablesTrade 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	InventoryInventory is stated at the lower of cost or net realizable value and consists of those costs realized following the U.S. Food and Drug Administration (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.
Net products sales and contract revenue	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. 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. The Company evaluated the variable consideration under Accounting Standards Codification (ASC) Topic 606, Revenue from Contracts with Customers (ASC 606), as it relates to the outcomes-based method and determined that based on historical clinical data, no rebate reserves were required for the nine months ended September 30, 2018 and 2019 . 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. 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. 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 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.
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 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 lessees to apply a two-method approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight line basis over the term of the lease, respectively. Lessees are also required to record a right-of-use (ROU) asset and a lease liability for all leases with a term greater than 12 months. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. ASU 2016-02 is effective for interim and annual periods beginning after December 15, 2018. The Company adopted this standard on January 1, 2019, the effective date, using the effective date method under the modified retrospective transition approach and elected to apply the package of practical expediants. The adoption of the standard had a material impact on the consolidated balance sheet, which resulted in the derecognition of the Company’s built-to-suit asset of $34.8 million and corresponding liability of $34.5 million , and an adjustment to beginning accumulated deficit of $0.3 million in the first quarter of 2019. Upon adoption, the Company also recognized ROU assets and lease liabilities of $31.7 million and $43.9 million
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, 2019
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, 2019 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 13), 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).

Summary of significant accoun_3

Summary of significant accounting policies (Tables)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Schedule of inventory	Inventory consisted of the following (in thousands): December 31, September 30, Raw materials $ 3,843 $ 3,427 Work in process 19,843 27,716 Finished goods 1,951 710 $ 25,637 $ 31,853
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, 2018 and 2019 as they would be anti-dilutive: September 30, 2018 September 30, 2019 Unvested restricted common shares 949,925 1,151,331 Stock options issued and outstanding 3,623,311 3,253,241
Schedule of components 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 loss on interest rate swap Foreign currency translation adjustments Accumulated other comprehensive loss Balance as of December 31, 2018 $ (376 ) $ (641 ) $ (33 ) $ (1,050 ) Current period other comprehensive income (loss) 1,172 (1,269 ) (134 ) (231 ) Balance as of September 30, 2019 $ 796 $ (1,910 ) $ (167 ) $ (1,281 )

Marketable securities (Tables)

Marketable securities (Tables)	9 Months Ended
	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Summary of marketable securities	The following table summarizes the available-for-sale securities held at December 31, 2018 and September 30, 2019 (in thousands): Description Amortized cost Unrealized gains Unrealized losses Fair value December 31, 2018 U.S. government agency $ 223,553 $ 54 $ (123 ) $ 223,484 Corporate securities $ 239,940 $ 34 $ (10,421 ) $ 229,553 September 30, 2019 U.S. government agency $ 117,915 $ 452 $ 118,367 Corporate securities $ 114,684 $ 344 $ (10,741 ) $ 104,287

Fair value of financial instr_2

Fair value of financial instruments (Tables)	9 Months Ended
	Sep. 30, 2019
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, 2018: Assets: Money market funds (included in cash and cash equivalents) $ 94,216 — — Certificates of deposit - restricted cash $ 53,000 — — Marketable securities - U.S. government agencies $ 223,484 — — Marketable securities - corporate securities $ 229,553 — — Liabilities: Interest rate swap — $ 641 — At September 30, 2019: Assets: Money market funds (included in cash and cash equivalents) $ 205,710 — — Certificates of deposit - restricted cash $ 51,500 — — Marketable securities - U.S. government agencies $ 118,367 — — Marketable securities - corporate securities $ 104,287 — — Liabilities: Interest rate swap — $ 1,910 —

Accrued expenses (Tables)

Accrued expenses (Tables)	9 Months Ended
	Sep. 30, 2019
Payables and Accruals [Abstract]
Schedule of accrued expenses	Accrued expenses consist of the following (in thousands): December 31, September 30, Compensation and benefits $ 15,575 $ 14,740 Consulting and professional fees 11,499 17,233 Commercial costs 2,405 2,909 Research and development 2,706 3,788 Other 2,605 3,025 $ 34,790 $ 41,695

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Schedule of debt	The Company's debt consists of the following (in thousands): December 31, September 30, Term loan outstanding $ 50,000 $ 48,500 MELF loan outstanding 912 671 Less: current portion of long-term debt (4,822 ) (7,830 ) Total long-term debt due after one year $ 46,090 $ 41,341

Stock-based compensation (Table

Stock-based compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Share-based Payment Arrangement [Abstract]
Schedule of compensation expense by award type	Stock-based compensation expense by award type was as follows (in thousands): Nine months ended September 30, 2018 2019 Stock options $ 22,816 $ 18,645 Restricted stock 14,344 15,595 Employee stock purchase plan 544 307 $ 37,704 $ 34,547
Summary of stock option activity	The following table summarizes stock option activity: Number of options Weighted- average exercise price Outstanding at December 31, 2018 3,487,376 $ 42.40 Granted 847,500 $ 43.62 Exercised (907,932 ) $ 38.92 Forfeited (173,703 ) $ 50.44 Outstanding at September 30, 2019 3,253,241 $ 43.26 Vested at September 30, 2019 1,601,470 $ 38.99
Summary of restricted common stock activity	The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2018 942,776 $ 58.71 Shares granted 546,700 $ 47.57 Shares vested (256,636 ) $ 60.03 Shares canceled (81,509 ) $ 55.63 Nonvested shares at September 30, 2019 1,151,331 $ 53.34

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Future minimum rental payments	Future minimum rental payments under these leases as of September 30, 2019 are as follows (in thousands): Year ending December 31, 2019 $ 1,829 2020 9,468 2021 10,457 2022 10,445 2023 10,711 2024 and thereafter 80,661 Total 123,571 Less: imputed interest (35,796 ) Current and long-term operating lease liability $ 87,775
Future minimum rental payments prior to adoption of ASU 842	Future minimum rental payments under our leases prior to adoption of ASU 2016-02, Leases, as of December 31, 2018 were as follows (in thousands): Year ending December 31, 2019 $ 8,102 2020 11,044 2021 10,073 2022 10,156 2023 9,228 2024 and thereafter 78,079 Total $ 126,682
Reconciliation of termination Cost	The following table reconciles the termination cost discussed above (in thousands): Balance as of December 31, 2018 Recognized during the nine months ended September 30, 2019 Balance as of September 30, 2019 Contract termination liability $5,270 $1,300 $3,970

The Company - Additional Inform

The Company - Additional Information (Details)	9 Months Ended
	Sep. 30, 2019segment	Feb. 22, 2019$ / shares
Accounting Policies [Abstract]
Number of operating segment | segment	1
Roche Holdings Inc.
Business Acquisition [Line Items]
Merger agreement, share price | $ / shares		$ 114.50

Development-stage risks - Addit

Development-stage risks - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Development-stage Risk [Abstract]
Accumulated deficit	$ 775,185	$ 589,687

Summary of significant accoun_4

Summary of significant accounting policies - Schedule of Inventory (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Inventory obsolescence reserve	$ 4,000
Raw materials	3,427	$ 3,843
Work in process	27,716	19,843
Finished goods	710	1,951
Inventory, Net	$ 31,853	$ 25,637

Summary of significant accoun_5

Summary of significant accounting policies - Net product sales (Details)	Sep. 30, 2019distribution_model
Accounting Policies [Abstract]
Number of distribution methods	2

Summary of significant accoun_6

Summary of significant accounting policies - Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted Average Shares Outstanding (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Unvested restricted common shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding (in shares)	1,151,331	949,925
Stock options issued and outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding (in shares)	3,253,241	3,623,311

Summary of significant accoun_7

Summary of significant accounting policies - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Jul. 01, 2019	Jan. 01, 2019	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance,	$ 496,513
Reclassification of unrealized loss on equity investment to accumulated deficit		$ (1)	$ (308)	$ 0
Balance,	346,009
Net unrealized (loss) gain on available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance,	(376)
Current period other comprehensive income (loss)	1,172
Balance,	796
Unrealized loss on interest rate swap
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance,	(641)
Current period other comprehensive income (loss)	(1,269)
Balance,	(1,910)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance,	(33)
Current period other comprehensive income (loss)	(134)
Balance,	(167)
Accumulated other comprehensive loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance,	(1,050)
Reclassification of unrealized loss on equity investment to accumulated deficit				$ 5,002
Current period other comprehensive income (loss)	(231)
Balance,	$ (1,281)

Summary of significant accoun_8

Summary of significant accounting policies - Recent accounting pronouncements (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jul. 01, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Derecognition of built-to-suit asset	$ (72,001)			$ (95,998)
Adjustment to beginning accumulated deficit		$ (1)	$ (308)		$ 0
Right of use asset	67,318			$ 0
Lease liabilities	$ 87,775
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Derecognition of built-to-suit asset, corresponding liability			34,500
Adjustment to beginning accumulated deficit			(300)
Right of use asset			31,700
Lease liabilities			43,900
ASU 2016-02 | One Drexel Plaza Research Facility
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Derecognition of built-to-suit asset			$ 34,800

Marketable securities - Schedul

Marketable securities - Schedule of Available for Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
U.S. government agency
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 117,915	$ 223,553
Unrealized gains	452	54
Unrealized losses		(123)
Fair value	118,367	223,484
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	114,684	239,940
Unrealized gains	344	34
Unrealized losses	(10,741)	(10,421)
Fair value	$ 104,287	$ 229,553

Fair value of financial instr_3

Fair value of financial instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Liabilities:
Realized gains or losses recognized on maturity of available-for-sale securities	$ 0
Recurring | Quoted prices in active markets for identical assets (Level 1)
Liabilities:
Interest rate swap	0	$ 0
Recurring | Significant other observable inputs (Level 2)
Liabilities:
Interest rate swap	1,910	641
Recurring | Significant unobservable inputs (Level 3)
Liabilities:
Interest rate swap	0	0
Money market funds (included in cash and cash equivalents) | Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets:
Money market funds (included in cash and cash equivalents)	205,710	94,216
Money market funds (included in cash and cash equivalents) | Recurring | Significant other observable inputs (Level 2)
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Money market funds (included in cash and cash equivalents) | Recurring | Significant unobservable inputs (Level 3)
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Certificates of deposit - restricted cash | Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets:
Money market funds (included in cash and cash equivalents)	51,500	53,000
Certificates of deposit - restricted cash | Recurring | Significant other observable inputs (Level 2)
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Certificates of deposit - restricted cash | Recurring | Significant unobservable inputs (Level 3)
Assets:
Money market funds (included in cash and cash equivalents)	0	0
U.S. government agency | Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets:
Marketable securities	118,367	223,484
U.S. government agency | Recurring | Significant other observable inputs (Level 2)
Assets:
Marketable securities	0	0
U.S. government agency | Recurring | Significant unobservable inputs (Level 3)
Assets:
Marketable securities	0	0
Corporate securities | Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets:
Marketable securities	104,287	229,553
Corporate securities | Recurring | Significant other observable inputs (Level 2)
Assets:
Marketable securities	0	0
Corporate securities | Recurring | Significant unobservable inputs (Level 3)
Assets:
Marketable securities	$ 0	$ 0

Sale of priority review vouch_2

Sale of priority review voucher (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	May 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Sale of priority review voucher, consideration	$ 110,000	$ 0	$ 110,000

Accrued expenses - Schedule of

Accrued expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Compensation and benefits	$ 14,740	$ 15,575
Consulting and professional fees	17,233	11,499
Commercial costs	2,909	2,405
Research and development	3,788	2,706
Other	3,025	2,605
Accrued expenses	$ 41,695	$ 34,790

Debt - Schedule of Debt (Detail

Debt - Schedule of Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Less: current portion of long-term debt	$ (7,830)	$ (4,822)
Total long-term debt due after one year	41,341	46,090
Secured Debt | Term loan outstanding
Debt Instrument [Line Items]
Loan amount	48,500	50,000
Secured Debt | MELF loan outstanding
Debt Instrument [Line Items]
Loan amount	$ 671	$ 912

Debt - Additional Information (

Debt - Additional Information (Details) - USD ($)	Jul. 03, 2018	Aug. 31, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Line of Credit Facility [Line Items]
Effective portion of interest rate expected to be reclassified from AOCL into interest expense over next 12 months			$ 1,300,000		$ 1,300,000
Proceeds from long-term debt					0	$ 50,000,000
Secured Debt | Term loan outstanding
Line of Credit Facility [Line Items]
Amount extended under term loan	$ 50,000,000
Debt interest expense			400,000	$ 300,000	1,100,000	300,000
Secured Debt | MELF loan outstanding
Line of Credit Facility [Line Items]
Debt interest expense			6,000	$ 8,000	19,000	$ 27,000
Proceeds from long-term debt		$ 1,600,000
Debt term		5 years
Principle and interest payment		$ 29,000
Stated percentage rate		3.25%
London Interbank Offered Rate (LIBOR) | Secured Debt | Term loan outstanding
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.65%
Swap agreement | Term loan outstanding
Line of Credit Facility [Line Items]
Fixed rate per annum under swap	3.463%
Designated as Hedging Instrument | Swap agreement
Line of Credit Facility [Line Items]
Notional amount	$ 50,000,000
Interest rate swap			1,900,000		1,900,000
Cash Flow Hedging | Swap agreement
Line of Credit Facility [Line Items]
Amount reclassified from AOCL into interest expense					200,000
Other current liabilities | Designated as Hedging Instrument | Swap agreement
Line of Credit Facility [Line Items]
Interest rate swap			500,000		500,000
Other long-term liabilities | Designated as Hedging Instrument | Swap agreement
Line of Credit Facility [Line Items]
Interest rate swap			$ 1,400,000		1,400,000
Hedge ineffectiveness					$ 0

Stock-based compensation - Addi

Stock-based compensation - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2019	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 34,547	$ 37,704
Shares withheld to satisfy exercise price of options (in shares)		153,727
Shares withheld to satisfy minimum tax withholding obligations (in shares)		107,765
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 13,500	16,400
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		21,000	21,300
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 307	$ 544
2015 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)	1,510,533
Shares available for future grants (in shares)		2,137,500
2015 Employee Stock Purchase Plan | Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in shares)	377,633
Common stock reserved for issuance (in shares)	1,481,992
Shares issued (in shares)		15,829

Stock-based compensation - Shar

Stock-based compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 34,547	$ 37,704
Stock options
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	18,645	22,816
Restricted stock
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	15,595	14,344
Employee stock purchase plan
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 307	$ 544

Stock-based compensation - Summ

Stock-based compensation - Summary of Stock Option Activity (Details) - $ / shares	9 Months Ended
	Sep. 30, 2019
Number of options (in shares):
Outstanding at beginning of period (in shares)	3,487,376
Granted (in shares)	847,500
Exercised (in shares)	(907,932)
Forfeited (in shares)	(173,703)
Outstanding at end of period (in shares)	3,253,241
Vested at end of period (in shares)	1,601,470
Weighted-average exercise price (in dollars per share):
Outstanding at beginning of period (in dollars per share)	$ 42.40
Granted (in dollars per share)	43.62
Exercised (in dollars per share)	38.92
Forfeited (in dollars per share)	50.44
Outstanding at end of period (in dollars per share)	43.26
Vested at period end (in dollars per share)	$ 38.99

Stock-based compensation - Su_2

Stock-based compensation - Summary of Restricted Common Stock Activity (Details) - Performance Shares	9 Months Ended
	Sep. 30, 2019$ / sharesshares
Number of shares (in shares):
Nonvested beginning balance (in shares) | shares	942,776
Shares granted (in shares) | shares	546,700
Shares vested (in shares) | shares	(256,636)
Shares canceled (in shares) | shares	(81,509)
Nonvested ending balance (in shares) | shares	1,151,331
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in dollars per share) | $ / shares	$ 58.71
Shares granted (in dollars per share) | $ / shares	47.57
Shares vested (in dollars per share) | $ / shares	60.03
Shares canceled (in dollars per share) | $ / shares	55.63
Nonvested shares ending balance (in dollars per share) | $ / shares	$ 53.34

Related-party transactions - Ad

Related-party transactions - Additional Information (Details) - CHOP - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Accrued expenses due to related party	$ 0.3		$ 0.7
Accounts payable due to related party	0.4		$ 0.5
Master Research Services Agreement
Related Party Transaction [Line Items]
Research and development expense related to agreements	1	$ 4.3
Technology and License Agreement | General and Administrative Expense
Related Party Transaction [Line Items]
Reimbursement of patent costs	$ 0.9	$ 0.9

Leases - Additional Information

Leases - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Weighted average remaining lease term, operating leases	11 years 11 months 4 days
ROU assets	$ 67,318		$ 0
Current operating lease liabilities	8,367		0
Noncurrent operating lease liabilities	$ 79,408		$ 0
Weighted average discount rate, operating leases	5.65%
Rent expense	$ 5,400
Rent expense		$ 3,100
Cash paid related to operating leases	5,000
Contract termination liability, long-term	2,400
Contract termination liability, current	$ 1,600
Minimum
Lessee, Lease, Description [Line Items]
Operating lease terms	1 year
Maximum
Lessee, Lease, Description [Line Items]
Operating lease terms	15 years

Leases - Future Minimum Rental

Leases - Future Minimum Rental Payments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Operating Leases, After Adoption of 842:
2019	$ 1,829
2020	9,468
2021	10,457
2022	10,445
2023	10,711
2024 and thereafter	80,661
Total	123,571
Less: imputed interest	(35,796)
Current and long-term operating lease liability	$ 87,775
Operating Leases, Before Adoption of 842:
2019		$ 8,102
2020		11,044
2021		10,073
2022		10,156
2023		9,228
2024 and thereafter		78,079
Total		$ 126,682

Leases - Contract Termination L

Leases - Contract Termination Liability (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019USD ($)
Movement In Contract Termination Liability [Roll Forward]
Balance as of December 31, 2018	$ 5,270
Recognized during the nine months ended September 30, 2019	1,300
Balance as of September 30, 2019	$ 3,970

Commitments and contingencies -

Commitments and contingencies - Additional information (Details) - Brammer agreement $ in Millions	1 Months Ended
	Aug. 31, 2018USD ($)
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
Upfront payment, prepaid expense, current	$ 4

Collaboration and license agr_2

Collaboration and license agreements - Novartis (Details) - USD ($) $ in Thousands	3 Months Ended					9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received	$ 22,447			$ 10,707		$ 56,721	$ 51,568
Milestone payment, additional revenue potential to be earned						215,000
Current deferred revenue	22,120		$ 0			22,120
Long-term deferred revenue	132,719		160,000			132,719
Novartis
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront payment received for license					$ 105,000
Milestone payments received		$ 30,000	25,000
Remaining milestone payments						10,000
Additional variable consideration		$ 30,000	30,000
Upfront payment			$ 130,000
License fees and milestone payments						160,000
Contract revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received	15,529			$ 1,841		28,561	$ 35,969
Contract revenue | Novartis
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received						13,600
License fees and milestone payments						160,000
Current deferred revenue	22,100					22,100
Long-term deferred revenue	$ 132,700					$ 132,700

Collaboration and license agr_3

Collaboration and license agreements - Pfizer (Details) - USD ($)	1 Months Ended				3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Jul. 31, 2018	Feb. 28, 2018	Nov. 30, 2017	Dec. 31, 2014	Sep. 30, 2019	Sep. 30, 2018	Jul. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2014
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment, additional revenue potential to be earned								$ 215,000,000
Milestone payments received					$ 22,447,000	$ 10,707,000		56,721,000	$ 51,568,000
Pfizer
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Research and development expense related to agreements										$ 10,600,000
Upfront payment received for license	$ 7,000,000	$ 7,000,000	$ 15,000,000	$ 20,000,000			$ 14,000,000
Revenue recognition from aggregate milestone payments			10,000,000					$ 95,000,000
Consideration recognized as revenue			$ 25,000,000
Collaboration agreement expiration, period								15 years
Contract revenue									36,000,000
Additional product candidates
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment, additional revenue potential to be earned								$ 120,000,000
Research and development expense | Pfizer
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Reimbursement of costs									3,300,000
Contract revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received					$ 15,529,000	$ 1,841,000		28,561,000	$ 35,969,000
Contract revenue | Pfizer
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received								$ 15,000,000

Collaboration and license agr_4

Collaboration and license agreements - Selecta (Details) $ in Millions	1 Months Ended			5 Months Ended
	Oct. 31, 2017USD ($)shares	Jun. 30, 2017USD ($)shares	Dec. 31, 2016USD ($)targetshares	Oct. 31, 2017USD ($)shares
Selecta license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Prepaid expense, current			$ 10	$ 5
Shares purchased (in shares) | shares	205,254	324,362	197,238	205,254
Investment shares purchased, amount	$ 5	$ 5	$ 5
Development and regulatory milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments			$ 65
Milestone payment, option period			3 years
Collaboration agreement expiration, period			90 days
Commercial milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments			$ 365
Minimum | Selecta license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of additional licensing targets | target			4
Option exercise fee			$ 1.4
Maximum | Selecta license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option exercise fee			2
Milestone payments			$ 430
Milestone payment, rights to fund percentage			50.00%

Uncategorized Items - spark-201

Label	Element	Value
Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (308,000)
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	(5,002,000)
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (1,000)