Spark Therapeutics (ONCE) 10-K 27 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Feb. 22, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	Spark Therapeutics, Inc.
Entity Central Index Key	1,609,351
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Trading Symbol	ONCE
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		37,217,493
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,486,720

Consolidated balance sheets

Consolidated balance sheets - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 96,748,444	$ 58,923,097
Marketable securities	423,418,752	237,242,655
Other receivables	7,905,653	16,780,917
Prepaid expenses	5,092,877	1,647,008
Total current assets	533,165,726	314,593,677
Marketable securities	20,035,553	21,900,129
Property and equipment, net	61,712,793	19,794,306
Acquired in-process research and development	0	15,490,000
Goodwill	1,254,005	1,160,104
Other assets	628,235	924,579
Total assets	616,796,312	373,862,795
Current liabilities:
Accounts payable	14,182,804	9,928,737
Accrued expenses	24,697,225	13,826,920
Current portion of long-term debt	311,976	302,013
Current portion of deferred rent	968,534	771,196
Current portion of deferred revenue	11,968,915	5,168,674
Current other liabilities	1,557,062	0
Total current liabilities	53,686,516	29,997,540
Long-term debt	912,027	1,224,003
Long-term deferred rent	8,317,952	7,498,419
Long-term deferred revenue	0	3,865,885
Deferred tax liability	0	1,000,235
Other liabilities	40,255,605	0
Total liabilities	103,172,100	43,586,082
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; 30,873,430 shares issued and 30,864,224 shares outstanding as of December 31, 2016; 37,131,626 shares issued and 37,111,404 shares outstanding as of December 31, 2017	37,132	30,874
Additional paid-in capital	1,026,589,507	583,973,682
Accumulated other comprehensive loss	(5,913,595)	(794,296)
Treasury stock, at cost, 9,206 shares as of December 31, 2016 and 20,222 shares as of December 31, 2017	(1,225,949)	(552,636)
Accumulated deficit	(505,862,883)	(252,380,911)
Total stockholders’ equity	513,624,212	330,276,713
Total liabilities and stockholders’ equity	$ 616,796,312	$ 373,862,795

Consolidated balance sheets (un

Consolidated balance sheets (unaudited) (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	37,131,626	30,873,430
Common stock, shares outstanding	37,111,404	30,864,224
Treasury stock, shares	20,222	9,206

Consolidated statements of oper

Consolidated statements of operations and other comprehensive income (loss) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues	$ 12,065,644	$ 20,182,835	$ 22,063,674
Operating expenses:
Research and development	135,160,047	86,379,405	46,029,314
Acquired in-process research and development	8,604,258	11,132,146	0
Impairment of in-process research and development	15,696,017	0	0
General and administrative	111,123,247	48,070,317	23,352,171
Total operating expenses	270,583,569	145,581,868	69,381,485
Loss from operations	(258,517,925)	(125,399,033)	(47,317,811)
Interest income, net	4,072,912	1,746,506	192,033
Loss before income taxes	(254,445,013)	(123,652,527)	(47,125,778)
Income tax benefit	963,041	0	0
Net loss	(253,481,972)	(123,652,527)	(47,125,778)
Preferred stock dividends	0	0	(634,794)
Net loss applicable to common stockholders	$ (253,481,972)	$ (123,652,527)	$ (47,760,572)
Basic and diluted net loss per common share (in usd per share)	$ (7.63)	$ (4.29)	$ (2.10)
Weighted average basic and diluted common shares outstanding (in shares)	33,242,072	28,804,133	22,710,105
Other comprehensive income (loss):
Unrealized loss on available-for-sale securities	$ (5,162,259)	$ (801,717)	$ 0
Foreign exchange translation adjustment	42,960	7,421	0
Total comprehensive loss	$ (258,601,271)	$ (124,446,823)	$ (47,125,778)

Consolidated statement of stock

Consolidated statement of stockholders' equity - USD ($)	Total	Preferred stock [Member]Series A Convertible Preferred Stock [Member]	Preferred stock [Member]Series B Convertible Preferred Stock [Member]	Member units [Member]	Member units [Member]Series A Convertible Preferred Stock [Member]	Common stock in treasury [Member]	Common stock [Member]	Common stock [Member]Series A Convertible Preferred Stock [Member]	Common stock [Member]Series B Convertible Preferred Stock [Member]	Additional paid-in capital [Member]	Additional paid-in capital [Member]Series A Convertible Preferred Stock [Member]	Additional paid-in capital [Member]Series B Convertible Preferred Stock [Member]	Accumulated other comprehensive loss [Member]	Accumulated deficit [Member]
Beginning balance at Dec. 31, 2014	$ 55,205,720	$ 10,000,000	$ 72,437,203			$ 0	$ 6,290			$ 54,364,833				$ (81,602,606)
Beginning balance, preferred stock (in shares) at Dec. 31, 2014		5,000,000	45,186,334
Beginning balance (in shares) at Dec. 31, 2014						0	6,290,317
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of preferred stock and dividends to common stock upon initial public offering				$ (72,437,203)	$ (10,000,000)			$ 1,016	$ 9,184		$ 9,998,984	$ 72,428,019
Conversion of preferred stock and dividends to common stock upon initial public offering (in shares)				(45,186,334)	(5,000,000)			1,016,219	9,183,831
Issuance of common stock, net of issuance costs	268,321,943						$ 10,317			268,311,626
Issuance of common stock, net of issuance costs (in shares)							10,316,995
Issuance of common stock for services	193,909						$ 4			193,905
Issuance of common stock for services (in shares)							3,556
Issuance of restricted stock	0						$ 50			(50)
Issuance of restricted stock (in shares)							49,750
Purchase of common stock in treasury	(552,636)					$ (552,636)
Purchase of common stock in treasury (in shares)						9,206
Exercise of stock options	1,116,043						$ 222			1,115,821
Exercise of stock options (in shares)							221,825
Unrealized gain on foreign currency translation	0
Stock-based compensation expense	13,378,594									13,378,594
Net loss	(47,125,778)													(47,125,778)
Ending balance at Dec. 31, 2015	290,537,795	$ 0	$ 0			$ (552,636)	$ 27,083			419,791,732			$ 0	(128,728,384)
Ending balance, preferred stock (in shares) at Dec. 31, 2015		0	0
Ending balance (in shares) at Dec. 31, 2015						9,206	27,082,493
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs	127,566,064						$ 3,025			127,563,039
Issuance of common stock, net of issuance costs (in shares)							3,025,000
Issuance of restricted stock	0						$ 40			(40)
Issuance of restricted stock (in shares)							40,000
Restricted stock canceled	0						$ (2)			2
Restricted stock canceled (in shares)							(2,213)
Purchase of common stock under ESPP	339,869						$ 8			339,861
Purchase of common stock under ESPP (in shares)							8,012
Issuance of stock for purchase/acquisition	9,150,450						$ 265			9,150,185
Issuance of stock for purchase/acquisition (in shares)							265,000
Exercise of stock options	$ 2,591,922						$ 455			2,591,467
Exercise of stock options (in shares)	455,138						455,138
Unrealized loss on investments	$ (801,717)												(801,717)
Unrealized gain on foreign currency translation	7,421												7,421
Stock-based compensation expense	24,537,436									24,537,436
Net loss	(123,652,527)													(123,652,527)
Ending balance at Dec. 31, 2016	330,276,713					$ (552,636)	$ 30,874			583,973,682			(794,296)	(252,380,911)
Ending balance (in shares) at Dec. 31, 2016						9,206	30,873,430
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs	379,870,085						$ 5,296			379,864,789
Issuance of common stock, net of issuance costs (in shares)							5,296,053
Issuance of restricted stock	0						$ 1			(1)
Issuance of restricted stock (in shares)							1,193
Purchase of common stock in treasury	(673,313)					$ (673,313)
Purchase of common stock in treasury (in shares)						11,016
Restricted stock canceled	0						$ (4)			4
Restricted stock canceled (in shares)							(4,107)
Purchase of common stock under ESPP	645,915						$ 12			645,903
Purchase of common stock under ESPP (in shares)							11,804
Exercise of stock options	$ 20,733,440						$ 953			20,732,487
Exercise of stock options (in shares)	953,253						953,253
Unrealized loss on investments	$ (5,162,259)												(5,162,259)
Unrealized gain on foreign currency translation	42,960												42,960
Stock-based compensation expense	41,372,643									41,372,643
Net loss	(253,481,972)													(253,481,972)
Ending balance at Dec. 31, 2017	$ 513,624,212					$ (1,225,949)	$ 37,132			$ 1,026,589,507			$ (5,913,595)	$ (505,862,883)
Ending balance (in shares) at Dec. 31, 2017						20,222	37,131,626

Consolidated statements of cash

Consolidated statements of cash flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (253,481,972)	$ (123,652,527)	$ (47,125,778)
Adjustments to reconcile net loss to net cash used in operating activities:
Noncash rent expense	(531,133)	(530,853)	181,979
Depreciation and amortization expense	4,860,428	3,634,349	1,732,983
Loss on disposal of property and equipment	32,176	101,490	0
Acquired in-process research and development	8,604,258	11,132,146	0
Stock-based compensation expense	41,372,643	24,537,436	13,572,503
Impairment of acquired in-process research and development	15,696,017	0	0
Non-cash income tax benefit	(1,013,539)	0	0
Non-cash interest income	(126,782)	0	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(3,157,214)	(264,230)	(1,627,602)
Other receivables	8,856,911	(234,803)	(16,700,175)
Accounts payable and accrued expenses	12,981,007	10,018,084	9,052,260
Deferred rent	1,610,115	0	0
Deferred revenue	2,934,356	(5,182,835)	(6,564,397)
Other liabilities	6,826,833	0	0
Net cash used in operating activities	(154,535,896)	(80,441,743)	(47,478,227)
Cash flows from investing activities:
Purchase of acquired in-process research and development	(8,217,474)	(11,132,146)	0
Payment for acquisition, net of cash acquired	0	(5,911,243)	0
Purchase of marketable securities	(441,485,816)	(279,944,501)	0
Proceeds from maturities of marketable securities	251,625,252	20,000,000	0
Purchases of property and equipment	(9,653,710)	(8,532,397)	(4,521,769)
Net cash used in investing activities	(207,731,748)	(285,520,287)	(4,521,769)
Cash flows from financing activities:
Proceeds from exercise of options	20,733,440	2,591,922	1,116,043
Purchase of treasury stock	(673,313)	0	(552,636)
Proceeds from public offerings of common stock, net	379,870,085	126,908,733	270,400,216
Proceeds from issuance of common stock under ESPP	645,915	339,869	0
Proceeds from long-term debt	0	1,550,610	0
Payments on long-term debt	(302,013)	(24,594)	0
Net cash provided by financing activities	400,274,114	131,366,540	270,963,623
Effect of exchange rate changes on cash and cash equivalents	(181,123)	(12,003)	0
Net increase (decrease) in cash and cash equivalents	37,825,347	(234,607,493)	218,963,627
Cash and cash equivalents, beginning of year	58,923,097	293,530,590	74,566,963
Cash and cash equivalents, end of year	96,748,444	58,923,097	293,530,590
Supplemental disclosure of cash flow information:
Property and equipment purchases included in accounts payable and accrued expenses	2,788,665	681,790	2,683,487
Deferred financing costs included in accounts payable and accrued expenses	0	0	657,331
One Drexel Plaza lease cost included in other liabilities	$ 34,985,836	$ 0	$ 0

Background

Background	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Background	Background 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, the Company 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 one-time, life-altering treatments. The Company operates in one segment and has its principal offices in Philadelphia, Pennsylvania. In December 2017, the U.S. Food and Drug Administration, or FDA, approved LUXTURNA TM (voretigene neparvovec-rzyl) for the treatment of patients with confirmed biallelic RPE65 mutation-associated retinal dystrophy and viable retinal cells. (a) Public Offerings On February 4, 2015 , the Company closed its initial public offering, or IPO, having sold 8,050,000 shares of common stock at an IPO price of $23.00 per share, for aggregate gross proceeds of $185.2 million . The Company received net proceeds from the IPO of $168.9 million , after deducting underwriting discounts and commissions and other offering costs. As part of the IPO, all of the outstanding shares of preferred stock, including shares of preferred stock issued as accrued dividends, were converted into an aggregate of 10,200,050 shares of common stock. On December 28, 2015, the Company completed its follow-on public offering, having sold 2,266,995 shares of common stock at a offering price of $47.00 per shares, for aggregate gross proceeds of $106.5 million . The Company received net proceeds from the public offering of $99.4 million , after deducting underwriting discounts and commissions and other offering costs. On June 20, 2016, the Company completed a follow-on public offering, having sold 3,025,000 shares of common stock at an offering price of $45 .00 per share, for aggregate gross proceeds of $136.1 million . The Company received net proceeds from the public offering of $127.6 million , after deducting underwriting discounts and commissions and other offering costs. On August 9, 2017, the Company completed a follow-on public offering, having sold 5,296,053 shares of common stock at an offering price of $76.00 per share, for aggregate gross proceeds of $402.5 million . The Company received net proceeds from the public offering of $379.9 million , after deducting underwriting discounts and commissions and other offering costs.

Development-stage risks

Development-stage risks	12 Months Ended
	Dec. 31, 2017
Development-stage Risk [Abstract]
Development-stage risks	Development-stage risks The Company has incurred losses and negative cash flows from operations since inception and had an accumulated deficit of $505.9 million at December 31, 2017 . 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 and market acceptance of LUXTURNA and the Company’s proposed future products; (iii) the timely and successful completion of additional financing; and (iv) the development of competitive therapies by other biotechnology and pharmaceutical companies.

Summary of significant accounti

Summary of significant accounting policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies (a) Use of estimates The preparation of consolidated financial statements in conformity with U.S. generally accepted accounting principles (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. (b) Principles of consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Spark Therapeutics, Inc., Spark Therapeutics Ireland Limited, Spark Therapeutics UK Limited, Spark Therapeutics Argentina Limited, Spark Therapeutics Switzerland Limited, Spark Therapeutics Germany Limited and Spark Therapeutics France Limited. All intercompany balances and transactions have been eliminated in consolidation. (c) Fair value of financial instruments Management believes that the carrying amounts of the Company’s financial instruments, including cash equivalents, other receivables and 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. (d) 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 December 31, 2016 and 2017 consisted primarily of money market funds. (e) Marketable securities The Company classifies its marketable security investments as available-for-sale securities and the securities are stated at fair value. At December 31, 2017 , the balance in the Company’s accumulated other comprehensive loss included activity related to the Company’s available-for-sale marketable securities. There were no material realized gains or losses recognized on the maturity of available-for-sale securities during the year ended December 31, 2017 and, as a result, the Company did not reclassify any amount out of accumulated other comprehensive loss for the same period. In addition, as part of the license and stock purchase agreements entered into with Selecta Biosciences, Inc. (Selecta) (note 14), the Company purchased restricted common shares of Selecta. The investment is classified as available-for-sale and is stated at fair value. (f) Property and equipment Property and equipment consists of computer and laboratory equipment, software, office equipment, furniture and leasehold improvements and is recorded at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of three years for computer equipment and software, five years for laboratory and office equipment and seven years for furniture. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset . The Company reviews long-lived assets, such as property and equipment, for impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated undiscounted future cash flows that the assets are expected to generate. If the carrying amount of an asset exceeds its estimated future cash flows, then an impairment charge is recognized for the amount by which the carrying value of the asset exceeds the fair value of the asset. No impairment charges have been recorded since inception. (g) Acquired in-process research and development and goodwill The acquired in-process research and development (IPR&D) asset was an indefinite-lived intangible asset and was assessed for impairment annually or more frequently if impairment indicators existed. During the year ended December 31, 2017, the Company determined that it would no longer pursue product candidates utilizing the technology acquired from Genable Technologies, Ltd. (Genable) in March 2016 and, accordingly, recorded an impairment charge of $15.7 million within its consolidated statement of operations. Additionally, the Company recognized an income tax benefit of $1.0 million related to the reversal of the deferred tax liability associated with the IPR&D during the year ended December 31, 2017. Goodwill represents the excess of purchase price over the fair value of net assets acquired by the Company. Goodwill is not amortized, but assessed for impairment on an annual basis or more frequently if impairment indicators exist. The Company operates as one reporting unit. The Company has the option to perform a qualitative assessment of goodwill prior to completing the two-step method described below to determine whether or not it is more likely than not that the fair value of its reporting units is less than its carrying amount, including goodwill and other intangible assets. If the Company concludes that this is the case, it must perform the two-step process. The first step compares a reporting unit’s fair value to its carrying amount to identify potential goodwill impairment. If the carrying amount of a reporting unit exceeds the reporting unit’s fair value, the second step of the impairment test must be completed to measure the amount of the reporting unit’s goodwill impairment loss, if any. Step two requires an assignment of the reporting unit’s fair value to the reporting unit’s assets and liabilities to determine the implied fair value of the reporting unit’s goodwill. The implied fair value of the reporting unit’s goodwill is then compared with the carrying amount of the reporting unit’s goodwill to determine the goodwill impairment loss to be recognized, if any. The Company performs its annual goodwill impairment test as of October 1st. The Company performed a qualitative assessment in 2017 and determined that there was no impairment to goodwill for the year ended December 31, 2017 . (h) Research and development and in-process research and development Research and development costs are expensed as incurred. Research and development expenses consist of internal and external expenses. Internal expenses include employee compensation and overhead. External expenses include development, clinical trials, statistical analysis and report writing and regulatory compliance costs incurred with clinical research organizations and other third-party vendors. At the end of the reporting period, the Company compares payments made to third-party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. When the Company is reimbursed by a collaboration partner for work performed, the costs incurred are recorded as research and development expenses and the related reimbursement is recorded as a reduction to research and development expenses. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered. Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use. (i) Income taxes The Company recognizes deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of its assets and liabilities and the expected benefits of net operating loss carryforwards. The impact of changes in tax rates and laws on deferred taxes, if any, is applied during the years in which temporary differences are expected to be settled and net operating losses used and is reflected in the consolidated financial statements in the period of enactment. The measurement of deferred tax assets is reduced, if necessary, if, based on weight of the evidence, it is more likely than not that some, or all, of the deferred tax assets will not be realized. At December 31, 2016 and 2017 , the Company has concluded that a full valuation allowance is necessary for its deferred tax assets. (j) Revenue recognition The Company has generated revenue solely through license and collaborative agreements. The Company recognizes revenue in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Subtopic 605-25, Revenue Recognition: Multiple-Element Arrangements , which addresses the determination of whether an arrangement involving multiple deliverables contains more than one unit of accounting. A delivered item within an arrangement is considered a separate unit of accounting only if both of the following criteria are met: • the delivered item has value to the customer on a stand-alone basis; and • if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in control of the vendor. Under FASB ASC Subtopic 605-25, if both of the criteria above are not met, then separate accounting for the individual deliverables is not appropriate. Revenue recognition for arrangements with multiple deliverables constituting a single unit of accounting is recognized generally over the greater of the term of the arrangement or the expected period of performance, either on a straight-line basis or on a modified proportional performance method. Milestones related to research and development activities are accounted for in accordance with FASB ASC Subtopic 605-28, Revenue Recognition: Milestone Method . FASB ASC Subtopic 605-28 allows for the recognition of consideration, which is contingent on the achievement of a substantive milestone in its entirety, in the period the milestone is achieved. A milestone is considered to be substantive if all of the following criteria are met: • the milestone is commensurate with either: (1) the performance required to achieve the milestone or (2) the enhancement of the value of the delivered items resulting from the performance required to achieve the milestone; • the milestone relates solely to past performance; and • the milestone payment is reasonable relative to all of the deliverables and payment terms within the agreement. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. For licenses with no stand-alone value, revenues are recognized on a straight-line basis over the related performance period. 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. To date, the Company has not generated any revenues from the commercial sale of products. (k) Stock-based compensation and fair value of stock The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation-Stock Compensation . ASC 718 requires all stock-based payments to employees, including grants of employee stock options and restricted stock units and modifications to existing stock options, to be recognized in the consolidated statement of operations based on their fair values. The Company uses the Black-Scholes option pricing model to determine the fair value of options granted. The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees and directors with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to non-employees with service-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period to the extent achievement of the performance condition is probable. The Company expenses restricted stock unit awards to employees based on the fair value of the award on a straight-line basis over the associated service period of the award. Awards of restricted stock units to non-employees are adjusted through stock-based compensation expense at each reporting period end to reflect the current fair value of such awards and expensed over the vesting period. The Company estimates the fair value of its option awards to employees and directors using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of the expected term of the award, (iii) the risk-free interest rate and (iv) expected dividends. Due to the lack of substantial Company-specific historical and implied volatility data of its common stock, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. When selecting these public companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry and with historical share price information sufficient to meet the expected term of the stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. The Company has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid, and does not expect to pay, dividends in the foreseeable future. The Company accounts for forfeitures when they occur. Stock-based compensation expense recognized in the consolidated financial statements is based on actual awards forfeited. Consistent with the guidance in FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees , the fair value of each non-employee stock option and restricted stock award is estimated at the date of grant using the Black-Scholes option pricing model with assumptions generally consistent with those used for employee stock options, with the exception of expected term, which is over the contractual life. (l) 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 December 31, 2015 , 2016 and 2017 as they would be anti-dilutive: December 31, 2015 2016 2017 Unvested restricted common shares 373,655 251,809 798,501 Options issued and outstanding 3,071,372 4,181,993 3,522,874 Amounts in the table above reflect the common stock equivalents of the noted instruments. (m) Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property. Tenant improvement allowances from the lessor are included in the accompanying consolidated balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of December 31, 2016 and 2017 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received. (n) Other comprehensive loss The Company follows the provisions of FASB ASC Topic 220, Comprehensive Income , which establishes standards for the reporting and display of comprehensive income and its components. Comprehensive income is defined to include all changes in equity during a period except those resulting from investments by owners and distributions to owners. The Company recorded gains and losses related to changes in available-for-sale securities and foreign currency translation. The accumulated balances related to each component of other comprehensive loss are summarized as follows: Net unrealized losses on available for sale securities Foreign currency translation adjustments Accumulated other comprehensive loss Balance as of December 31, 2016 $ (801,717 ) $ 7,421 $ (794,296 ) Current period other comprehensive income (loss) (5,162,259 ) 42,960 (5,119,299 ) Balance as of December 31, 2017 $ (5,963,976 ) $ 50,381 $ (5,913,595 ) (o) Recent accounting pronouncements In February 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-02, Leases . ASU 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. ASU 2016-02 is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company is currently evaluating the impact of the updated guidance on the Company's consolidated financial statements. In May 2014, the FASB issued a new standard regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. The new standard provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflect the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. The allowable adoption methods are the full retrospective method, which requires the standard to be applied to each prior period presented, or the modified retrospective method which requires the cumulative effect of adoption to be recognized as an adjustment to opening retained earnings in the period of adoption. The Company will adopt this guidance using the modified retrospective method and the adoption will have no cumulative adjustment to its consolidated financial statements as it relates to the Pfizer Inc. (Pfizer) collaboration agreement discussed in note 14. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities . ASU 2016-01 changes accounting for equity investments, financial liabilities under the fair value option, and presentation and disclosure requirements for financial instruments. ASU 2016-01 does not apply to equity investments in consolidated subsidiaries or those accounted for under the equity method of accounting. In addition, the FASB clarified guidance related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale debt securities. Equity investments with readily determinable fair values will be measured at fair value with changes in fair value recognized in net income. Companies have the option to either measure equity investments without readily determinable fair values at fair value or at cost, adjusted for changes in observable prices, minus impairment. Changes in measurement under either alternative will be recognized in net income. Companies that elect the fair value option for financial liabilities must recognize changes in fair value related to instrument-specific credit risk in other comprehensive income (OCI). Companies must assess valuation allowances for deferred tax assets related to available-for-sale debt securities in combination with their other deferred tax assets. ASU 2016-01 will be effective beginning the first quarter of 2018. The Company currently is evaluating the impact of the updated guidance on the Company's consolidated financial statements.

Marketable securities

Marketable securities	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable securities The following table summarizes the available-for-sale securities held at December 31, 2016 and 2017 : Description Amortized cost Unrealized gains Unrealized losses Fair value December 31, 2016 U.S. government agency $ 133,690,267 $ 10,907 $ (85,714 ) $ 133,615,460 Corporate securities $ 126,253,903 $ — $ (726,579 ) $ 125,527,324 December 31, 2017 U.S. government agency $ 222,179,753 $ — $ (640,341 ) $ 221,539,412 Corporate securities $ 227,237,638 $ — $ (5,322,745 ) $ 221,914,893 No available-for-sale securities held as of December 31, 2017 had remaining maturities greater than two years .

Fair value of financial instrum

Fair value of financial instruments	12 Months Ended
	Dec. 31, 2017
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: 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) As of December 31, 2016: Assets: Money market funds (included in cash and cash equivalents) $ 53,452,424 — — Corporate securities (included in cash and cash equivalents) $ 5,029,838 — — Marketable securities - U.S. government agencies $ 133,615,460 — — Marketable securities - corporate securities $ 122,144,692 $ 3,382,632 — As of December 31, 2017: Assets: Money market funds (included in cash and cash equivalents) $ 90,347,897 — — Corporate securities (included in cash and cash equivalents) $ 5,548,478 — — Marketable securities - U.S. government agencies $ 221,539,412 — — Marketable securities - corporate securities $ 220,020,150 $ 1,894,743 —

Business acquisition of Genable

Business acquisition of Genable and impairment of acquired in-process research and development	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Business acuisition of genable and impairment of acquired in-process research and development	Business acquisition of Genable and impairment of acquired in-process research and development On March 7, 2016, the Company acquired Genable, an Ireland-based private gene therapy company with which the Company had collaborated since 2014 in the development of Genable's therapeutic program targeting a genetic inherited retinal disease (IRD). With the acquisition, the Company acquired RhoNova™, a potential gene therapy targeting rhodopsin-linked autosomal dominant retinitis pigmentosa (RHO-adRP), an IRD that routinely leads to visual impairment and in the most severe cases to blindness. The consideration paid by the Company to Genable shareholders consisted of $6.1 million in cash and 265,000 shares of the Company's common stock with a fair value of $9.2 million , for total consideration of $15.3 million . In connection with the acquistion, a receivable due from Genable also was settled on the date of acquisition for $0.5 million . The Company incurred acquisition-related costs of approximately $0.3 million , which are included in general and administrative expenses in the consolidated statement of operations for the year ended December 31, 2016. The Company accounted for the acquisition as a business combination under the acquisition method of accounting. The Company allocated the purchase price for the purchase of Genable based upon the estimated fair value of net assets acquired and liabilities assumed at the date of acquisition. Recognition and measurement of assets acquired and liabilities assumed The following table summarizes the fair values of the tangible and intangible assets acquired and liabilities assumed at the acquisition date: Cash acquired $ 196,307 Other current assets 102,506 Acquired in-process research and development 15,490,000 Goodwill 1,160,104 Total assets assumed 16,948,917 Other non-current liabilities 254,753 Deferred tax liability 1,000,235 Total liabilities assumed 1,254,988 Total allocation of purchase price $ 15,693,929 Acquired in-process research and development and related impairment The Company’s allocation of purchase price to acquired IPR&D was $15.5 million . The estimated fair value of the IPR&D was determined using the “income approach,” which is a valuation technique that provides an estimate of the fair value of an asset based on market participant expectations of the cash flows an asset would generate over its remaining useful life. Some of the more significant assumptions inherent in the development of those asset valuations include the estimated net cash flows for each year for the asset or product (including net revenues, cost of sales, research and development costs, selling and marketing costs and working capital/asset contributory asset charges), the appropriate discount rate to select in order to measure the risk inherent in each future cash flow stream, the assessment of the asset’s life cycle, the potential regulatory and commercial success risks, competitive trends impacting the asset cash flow stream as well as other factors. During the year ended December 31, 2017, the Company determined that it would no longer pursue product candidates utilizing the technology acquired from Genable in March 2016 and, accordingly, recorded an impairment charge of $15.7 million within its consolidated statement of operations. Additionally, the Company recognized an income tax benefit of $1.0 million related to the reversal of the deferred tax liability associated with the IPR&D during the year ended December 31, 2017.

Property and equipment, net

Property and equipment, net	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Property and equipment, net	Property and equipment, net Property and equipment consist of the following: December 31, 2016 2017 Laboratory equipment $ 5,833,616 $ 6,380,580 Computer and software 530,663 1,325,403 Furniture and fixtures 470,151 1,770,475 Office equipment 537,756 725,982 Leasehold improvements 16,981,989 23,653,219 Construction in progress 967,945 38,179,619 Property and equipment, gross 25,322,120 72,035,278 Less accumulated depreciation and amortization (5,527,814 ) (10,322,485 ) Property and equipment, net $ 19,794,306 $ 61,712,793 At December 31, 2017, the costs related to the One Drexel Plaza lease (Note 12(a)) approximated $35.0 million within construction in progress. Depreciation and amortization expense was $1.7 million , $3.6 million and $4.9 million for the years ended December 31, 2015 , 2016 and 2017 , respectively.

Accrued expenses

Accrued expenses	12 Months Ended
	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following: December 31, 2016 2017 Compensation and benefits $ 9,613,275 $ 15,011,597 Consulting and professional fees 995,614 4,846,360 Research and development 2,717,777 2,809,043 Other 500,254 2,030,225 $ 13,826,920 $ 24,697,225

Debt

Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt	Debt In August 2016, the Company executed an agreement with the Commonwealth of Pennsylvania to fund machinery and equipment and other assets purchased (MELF Loan) in the amount of $1.6 million . Borrowings under the MELF Loan are secured by equipment, as defined in the loan agreement. Under the terms of the MELF Loan, the Company has a five -year period of monthly payments of approximately $ 29,000 of principal and interest at an annual interest rate of 3.25% . For the years ended December 31, 2016 and 2017 , the Company recorded interest expense of approximately $15,000 and $48,000 , respectively, related to the MELF Loan.

Stockholders' equity

Stockholders' equity	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Stockholders' equity	Stockholders’ equity The Company’s certificate of incorporation and bylaws contain the rights, preferences and privileges of the Company’s stockholders and their respective shares. The Company has authorized 150,000,000 shares of common stock and 5,000,000 shares of preferred stock. In 2013 and 2014, the Company issued restricted stock to various employees, directors and consultants of the Company. The vesting terms of the restricted stock issued varied, but primarily, shares vested 25% on the first anniversary of the vesting commencement date and then quarterly over three years , with accelerated vesting in the event of a change in control, as defined. Any unvested shares are forfeited in the event that the individual ceases to provide services to the Company. Additionally, in 2014, 200,000 restricted shares of common stock were issued to The Trustees of the University of Pennsylvania (Penn) in connection with a license agreement, of which 100,000 shares have vested. The remaining shares are subject to certain milestone-based vesting conditions. No milestone vesting conditions were achieved in 2016 or 2017. For the year ended December 31, 2015 , the Company recorded compensation expense of $0.1 million and $1.8 million in general and administrative expense and research and development expense, respectively, related to restricted shares. For the year ended December 31, 2016 , the Company recorded compensation expense of $0.1 million and $1.3 million in general and administrative expense and research and development expense, respectively, related to restricted shares. For the year ended December 31, 2017 , the Company recorded compensation expense of approximately $34,000 and $4.4 million in general and administrative expense and research and development expense, respectively, related to restricted shares. At December 31, 2017 , there was an immaterial amount of unrecognized compensation expense related to the restricted shares which is expected to be recognized over a weighted-average period of less than a year. The following table summarizes restricted stock activity for the instruments discussed above: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2015 348,555 $ 4.83 Shares canceled (1,213 ) $ 1.15 Shares vested (172,466 ) $ 3.90 Nonvested shares at December 31, 2016 174,876 $ 5.78 Shares canceled (3,107 ) $ 1.15 Shares vested (70,935 ) $ 3.60 Nonvested shares at December 31, 2017 100,834 $ 7.45

Stock incentive plans

Stock incentive plans	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock incentive plans	Stock incentive plans In January 2015, the Company established the 2015 Stock Incentive Plan (the 2015 Plan), which became effective immediately prior to the closing of the IPO. The 2015 Plan replaced the 2014 Stock Incentive Plan (the 2014 Plan) and the 209,500 shares available for future grants under the 2014 Plan were rolled into the 2015 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. Under the 2015 Plan, the number of shares of common stock reserved for issuance is the sum of: (1) 1,830,000 plus; (2) the number of shares (up to 2,543,299 shares) equal to the sum of the number of shares of common stock then available for issuance under the 2014 Plan and the number of shares of common stock subject to outstanding awards under the 2014 Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2025, equal to the lowest of 1,724,000 shares of common stock, 4% of the number of shares of common stock outstanding on the first day of such fiscal year and an amount determined by the board of directors. In January 2017, 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 1,234,641 shares. As of December 31, 2017, 727,229 shares were available for future grants under the 2015 Plan. In January 2015, the Company established the 2015 employee stock purchase plan (the 2015 ESPP), which became effective immediately prior to the closing of the IPO. The 2015 ESPP initially provided participating employees with the opportunity to purchase an aggregate of 220,000 shares of common stock. The number of shares of common stock reserved for issuance under the 2015 ESPP automatically will increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2026, in an amount equal to the lowest of: (1) 440,000 shares of common stock; (2) 1% of the total number of shares of common stock outstanding on the first day of the applicable fiscal year; and (3) an amount determined by the board of directors. For the year ended December 31, 2017 , 11,804 shares were issued under the 2015 ESPP. In January 2017, the number of shares of common stock authorized for issuance under the 2015 ESPP automatically increased, pursuant to the terms of the 2015 ESPP, by 308,660 shares. The 2015 ESPP provides participating employees with the opportunity to purchase an aggregate of 779,672 shares of common stock. Stock-based compensation expense Stock-based compensation expense by award type was as follows: Year ended December 31, 2015 2016 2017 Stock options $ 9,874,709 $ 22,376,794 $ 28,001,032 Restricted stock 1,680,145 632,298 8,452,262 Employee stock purchase plan — 177,767 437,599 $ 11,554,854 $ 23,186,859 $ 36,890,893 Of the $36.9 million of stock-based compensation expense incurred during the year ended December 31, 2017, $15.1 million is classified as research and development expense and $21.8 million is classified as general and administrative expense in the consolidated statement of operations and comprehensive income (loss). Of the $23.2 million of stock-based compensation expense incurred during the year ended December 31, 2016, $9.6 million is classified as research and development expense and $13.6 million is classified as general and administrative expense in the consolidated statement of operations and comprehensive income (loss). Of the $11.6 million of stock-based compensation expense incurred during the year ended December 31, 2015, $5.0 million is classified as research and development expense and $6.6 million is classified as general and administrative expense in the consolidated statement of operations and comprehensive income (loss). Stock options The following table summarizes stock option activity: Number of shares Weighted- average exercise price Aggregate intrinsic value(a) Outstanding at December 31, 2015 3,071,372 $ 22.99 Granted 1,722,950 $ 43.16 Exercised (455,138 ) $ 5.69 Forfeited (157,191 ) $ 21.17 Outstanding at December 31, 2016 4,181,993 $ 33.25 Granted 701,300 $ 56.78 Exercised (953,253 ) $ 21.75 Forfeited (407,166 ) $ 41.30 Outstanding at December 31, 2017 3,522,874 $ 40.11 Vested at December 31, 2017 1,483,330 $ 32.46 $ 34,379,774 Vested at December 31, 2017 and expected to vest 3,170,491 $ 40.11 $ 48,691,490 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock that were in the money at December 31, 2017 . The weighted average remaining contractual term of options outstanding as of December 31, 2017 is 7.9 years. The weighted average remaining contractual term of options exercisable as of December 31, 2017 is 7.3 years. At December 31, 2017 , there was $55.5 million of unrecognized compensation expense related to stock options, which is expected to be recognized over a weighted average period of 2.4 years. The intrinsic value of options exercised during the years ended December 31, 2016 and 2017 was $21.1 million and $45.9 million , respectively. The weighted average grant date fair value of the options granted in 2015 , 2016 and 2017 was $40.51 , $28.56 and $39.27 per share, respectively, using the Black-Scholes option-pricing model with the following weighted-average assumptions: Years Ended December 31, 2015 2016 2017 Expected volatility 76.8 % 75.2 % 78.1 % Risk-free interest rate 1.67 % 1.71 % 2.06 % Expected term (in years) 6.10 5.95 6.10 Expected dividend yield 0.0 % 0.0 % 0.0 % Restricted Stock The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2015 25,100 $ 60.03 Shares granted 59,500 $ 44.35 Shares canceled (1,000 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at December 31, 2016 76,933 $ 49.90 Shares granted 713,400 $ 63.34 Shares canceled (55,457 ) $ 48.93 Shares vested (37,209 ) $ 55.77 Nonvested shares at December 31, 2017 697,667 $ 63.40 At December 31, 2017 , there was $36.9 million of unrecognized compensation expense related to the restricted common stock, which is expected to be recognized over a weighted average period of 3.3 years. ESPP stock activity: For the year ended December 31, 2016 , 8,012 shares were issued under the 2015 ESPP. For the year ended December 31, 2017 , 11,804 shares were issued under the 2015 ESPP. At December 31, 2017 , there was $0.2 million in unrecognized compensation expense related to the 2015 ESPP.

Commitments and contingencies

Commitments and contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and contingencies (a) Leases In March 2014, the Company entered into an operating lease for laboratory and office space at its corporate headquarters in Philadelphia, Pennsylvania, through October 2025. Under this lease, the Company received $8.0 million of tenant improvement allowances during 2014. In November 2015, the Company entered into a sublease agreement for approximately 14,000 square feet of additional office space at its corporate headquarters. The sublease will terminate on November 30, 2018. In February 2016, the Company leased 6,500 square feet of additional office space in Philadelphia under a lease that expires in June 2021. In addition, the Company leases approximately 5,400 square feet of office space in Waltham, Massachusetts under a lease that expires in March 2022. In November 2016, the Company entered into a lease agreement for approximately 49,000 square feet of office space in Philadelphia that will terminate on March 31, 2027. Under this lease, the Company received $1.6 million of tenant improvement allowances during 2017. In January 2017, the Company amended its lease for office space in Philadelphia to lease an additional 24,800 square feet that commenced on January 1, 2018, and terminate on December 31, 2028. In November 2017, the Company amended this lease to accelerate the termination date of approximately 50,000 square feet of office space, with such termination to occur, at the latest, in December 2022. The Company recorded an expense of $6.9 million associated with the change in termination date in 2017. As of December 31, 2017 , $5.3 million is recorded as long-term other liabilities and $1.5 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: Balance as of November 10, 2017 Recognized during the quarter Balance as of December 31, 2017 Contract termination liability $6,880,161 $53,328 $6,826,833 In November 2017, the Company entered into a lease for the new research facility at One Drexel Plaza in Philadelphia, Pennsylvania for approximately 108,000 square feet through June 2033. Based on the terms of the lease agreement for One Drexel Plaza, the Company has construction period risks during the construction period and the Company is deemed the owner of the building (for accounting purposes only) during the construction period. Accordingly, the Company recorded an asset of $35.0 million at December 31, 2017, representing the Company’s leased portion of the building, and recorded a corresponding liability. Upon completion of leasehold improvement construction, the Company may not meet the sale-leaseback criteria for de-recognition of the building asset and liability. Therefore, the lease may be accounted for as a financing obligation. The asset will be depreciated over the expected duration of the lease of 15.5 years, and rental payments will be treated as principal and interest payments on the lease financing obligation liability. The underlying accounting for this transaction has no impact on cash flows associated with the underlying lease or construction in process. At December 31, 2017, the lease financing obligation balance was $35.0 million and was recorded as a long-term liability within other liabilities on the consolidated balance sheets. The remaining future minimum payments under the lease financing obligation as of December 31, 2017 are included in the table below. Rent expense under these leases was $1.9 million and $10.5 million for the years ended December 31, 2016 and 2017 , respectively. Future minimum lease payments under these leases are as follows: Year Ending December 31, 2018 $ 5,520,167 2019 7,156,064 2020 7,819,394 2021 8,071,918 2022 8,126,544 2023 and thereafter 58,897,069 Total $ 95,591,156 (b) License agreements See note 13 for a discussion of the Children's Hospital of Philadelphia (CHOP) license agreement. In October 2013, the Company entered into a patent license agreement with Penn, as amended, for certain intellectual property licenses to be provided by Penn to the Company in the fields of research, development, manufacture and commercialization. The license agreement requires the Company to reimburse Penn for the patent costs related to the underlying licensed rights. The Company is obligated to make payments to Penn upon the occurrence of first commercial sale for certain licensed products in both the United States and Europe. The Company must pay a low-single-digit royalty based on net sales of licensed products by territory, which royalties will be reduced if the Company is required to license patents or intellectual property from third parties. In December 2014, the Company entered into a license agreement with Penn for certain intellectual property licenses. The Company issued to Penn 200,000 shares of restricted common stock (note 10), which are subject to performance-based vesting conditions, in connection with the agreement and is obligated to make milestone payments upon the achievement of certain regulatory milestones up to $5.5 million in the aggregate. Additionally, the Company is obligated to pay Penn single-digit-royalties based on its net sales of licensed products by territory. In October 2013, the Company entered into a license agreement with the University of Iowa Research Foundation (UIRF) for certain intellectual property licenses. The license agreement requires the Company to reimburse UIRF for the patent costs related to the underlying licensed rights. The Company is obligated to make payments to UIRF upon the occurrence of various development and commercialization milestones. The Company must pay a low-single-digit royalty to UIRF based on net sales of licensed products by territory.

Related-party transactions

Related-party transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions As of December 31, 2016 , CHOP was considered a significant equity holder. In October 2013, the Company entered into technology and license agreements with CHOP for certain commercialization licenses to be provided to the Company in order to develop services, methods and marketable products for commercialization. The license agreement requires the Company to reimburse CHOP for the patent costs related to the underlying licensed rights incurred after the effective date. For the years ended December 31, 2015 , 2016 and 2017 , the Company recorded $0.7 million , $0.9 million and $0.8 million , respectively, of 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 years ended December 31, 2015 , 2016 and 2017 , the Company recorded $5.2 million , $7.4 million and $6.1 million , respectively, as research and development expense. As of December 31, 2016 , $1.1 million and $0.9 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP. As of December 31, 2017 , $0.3 million and $1.4 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP.

Collaboration and license agree

Collaboration and license agreements	12 Months Ended
	Dec. 31, 2017
Collaboration and License Agreements [Abstract]
Collaboration and license agreements	Collaboration and license agreements (a) Pfizer In December 2014, the Company entered into a global collaboration agreement with Pfizer for the development and commercialization of SPK-FIX product candidates for the treatment of hemophilia B. Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024. The Company will be 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 is no stand-alone value for the license, the Company is recognizing revenue (through the estimated completion date of Phase 1/2 clinical trials). In December of both 2015 and 2016, the Company earned a $15.0 million milestone payment. The $15.0 million payment earned in 2016 is included in other receivables on the accompanying consolidated balance sheet as of December 31, 2016. In November 2017, the Company amended its global collaboration agreement with Pfizer. Under the terms of this amendment, the Company received a $10.0 million upfront payment and is eligible to receive an additional $15.0 million in payments upon completion of certain transition activities. The $25.0 million of consideration is being recognized as revenue over the remaining estimated performance period associated with the global collaboration agreement. During the years ended December 31, 2015 , 2016 and 2017 , the Company recognized $20.2 million , $20.2 million and $12.1 million of revenue, respectively. As of December 31, 2017 , there is $12.0 million of current deferred revenue on the consolidated balance sheet. During the years ended December 31, 2015 , 2016 and 2017 , the Company recorded $1.3 million , $2.4 million and $2.9 million , respectively, as a reduction to research and development expenses for the reimbursement of costs from Pfizer. The Company is eligible to receive up to an additional $230.0 million in aggregate milestone payments, $110.0 million of which relate to potential development, regulatory and commercial milestones for the first product candidate to achieve each milestone and $120.0 million of which relate to potential regulatory milestones for additional product candidates. In addition, the Company is entitled to receive royalties calculated as a low-teen percentage of net sales of licensed products. The royalties may be subject to certain reductions, including for a specified portion of royalty payments that Pfizer may become required to pay under any third-party license agreements, subject to a minimum royalty. Under the agreement, the Company remains solely responsible for the payment of license payments payable by the Company under specified license agreements. The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product; or (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements. In February 2018, the Company entered into a supply agreement with Pfizer to begin production in 2018 for one batch of drug substance expected to be used for Phase 3 development. The Company will receive $7.0 million upfront and will receive up to $7.0 million upon delivery. (b) Selecta In December 2016, the Company entered into a License and Option Agreement (the 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 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 License Agreement. In connection with the execution of the 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 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 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 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 License Agreement on a target-by-target basis for material breach with respect to such target. In connection with the License Agreement, the Company entered into a Stock Purchase Agreement (SPA) with Selecta pursuant to which the Company purchased 197,238 unregistered shares of Selecta’s common stock for $5.0 million in December 2016. An additional 324,362 unregistered shares of Selecta's common stock were purchased for $5.0 million in June 2017, and 205,254 unregistered shares of Selecta's common stock were purchased for $5.0 million in October 2017. The fair value of these shares is classified as available-for-sale securities at December 31, 2017 . In December 2016, the Company accounted for payments under the License Agreement and SPA as a basket transaction and allocated the $15.0 million in cash payments to the shares of Selecta’s common stock and the License Agreement in the amounts of $3.5 million and $11.5 million , respectively. The Company calculated the $3.5 million allocated to the Selecta shares acquired based on the closing market price on the date of purchase. The Company allocated the remaining $11.5 million to the License Agreement, which was expensed as acquired in-process research and development as the Company determined there was no alternative future use. In June 2017, the Company accounted for the payments under the License Agreement, Letter Agreement and SPA as a basket transaction and allocated the $7.5 million in cash payments to the shares of Selecta’s common stock and the License Agreement in the amounts of $4.4 million and $3.1 million , respectively. The Company calculated the $4.4 million allocated to the Selecta shares acquired based on the closing market price on the date of purchase. The Company allocated the remaining $3.1 million to the License Agreement, which was expensed as acquired in-process research and development as the Company determined there was no alternative future use. In October 2017, the Company accounted for the payments under the License Agreement, Letter Agreement and SPA as a basket transaction and allocated the $7.5 million in cash payments to the shares of Selecta’s common stock and the License Agreement in the amounts of $4.1 million and $3.4 million , respectively. The Company calculated the $4.1 million allocated to the Selecta shares acquired based on the closing market price on the date of purchase. The Company allocated the remaining $3.4 million to the License Agreement, which was expensed as acquired in-process research and development as the Company determined there was no alternative future use.

Income taxes Income taxes

Income taxes Income taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income taxes	Income taxes On December 22, 2017, the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (the Tax Act). The Tax Act makes broad and complex changes to the U.S. tax code, including, but not limited to, reducing the U.S. federal corporate tax rate from 35 percent to 21 percent; eliminating the corporate alternative minimum tax (AMT) and changing how existing AMT credits can be realized; creating a new limitation on deductible interest expense; changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017; limitations on the deductibility of certain executive compensation; and changes to the calculation of the orphan drug credit. In December 2017, the SEC issued Staff Accounting Bulletin No. 118 (SAB 118), which addresses situations where the accounting is incomplete for the income tax effects of the Tax Act. SAB 118 directs taxpayers to consider the impact of the Tax Act as “provisional” when the Company does not have the necessary information available, prepared or analyzed (including computations) to finalize the accounting for the change in tax law. Companies are provided a measurement period of up to one year to obtain, prepare, and analyze information necessary to finalize the accounting for provisional amounts or amounts that cannot be estimated as of December 31, 2017. With regards to the Tax Act impact on the tax provision as it relates to the Company for the year ended December 31, 2017, the Company has recognized the provisional impact of tax reform related to the revaluation of deferred tax assets and liabilities from 35% to 21% of $44.9 million of tax expense, which is offset by a reduction in the valuation allowance. As a result of changes made by the Tax Act, starting with executive compensation paid in 2018, Section 162(m) of the Internal Revenue Code will limit the Company from deducting compensation, including performance-based compensation, in excess of $1 million paid to certain executives. The only exception to this rule is for compensation that is paid pursuant to a binding contract in effect on November 2, 2017 that would have otherwise been deductible under the prior Section 162(m) rules. The Company has not yet completed an analysis of the binding contract requirement on the various compensation plans to determine the impact of the law change that may affect its deferred tax asset for stock compensation. With regards to the one-time transition tax, the Company did not record any tax liability as it believes that the accumulated earnings and profits of foreign subsidiaries will be in a deficit position. Because of the complexity of the new international tax provisions included in the Tax Act that are not applicable to the Company until 2018, and the Company's recent international expansion, the Company is continuing to evaluate these provisions of the Tax Act and the application of ASC 740, Income Taxes . The Company will continue to refine its calculations as additional analysis is completed related to the Tax Act. Additional information that may affect its provisional amounts would include further clarification and guidance on how the IRS will implement tax reform, including guidance with respect to the above, further clarification and guidance on how state taxing authorities will implement tax reform and the related effect on its state income tax returns, completion of its 2017 tax return filings, and the potential for additional guidance from the SEC or the FASB related to tax reform. The accounting is expected to be completed when the Company's 2017 U.S. corporate income tax return is filed in 2018. Loss before income taxes attributable to domestic and international operations, consists of the following: Year Ended December 31, 2015 2016 2017 Domestic $ (47,125,778 ) $ (122,809,629 ) $ (239,300,526 ) Foreign — (842,898 ) (15,144,487 ) Loss before income taxes $ (47,125,778 ) $ (123,652,527 ) $ (254,445,013 ) Income tax expense (benefit) consists of the following: Year Ended December 31, 2015 2016 2017 Current tax: Domestic $ — $ — $ — Foreign — — 110,305 Deferred tax: Domestic — — — Foreign — — (1,073,346 ) Total income tax benefit $ — $ — $ (963,041 ) A reconciliation of income tax benefit computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows: Year Ended December 31, 2015 2016 2017 Federal income tax benefit at statutory rate 34.0 % 34.0 % 34.0 % State and local tax, net of federal benefit 8.0 10.4 10.3 Permanent differences (2.7 ) 3.1 (0.3 ) Tax credits 13.1 9.1 9.2 Tax Cuts and Jobs Act impact — — (17.6 ) Foreign rate differential — (0.1 ) (1.3 ) Change in valuation allowance (52.4 ) (56.5 ) (33.9 ) Effective income tax rate — % — % 0.4 % Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes and for net operating loss and tax credit carryforwards. The significant components of the Company’s deferred tax assets are comprised of the following: December 31, 2016 2017 Deferred tax assets (liabilities): Net operating loss carryforwards $ 55,710,846 $ 114,152,143 Tax credit carryforwards 28,327,477 51,585,618 Stock-based compensation 10,342,547 14,643,667 Deferred rent 3,356,430 4,697,221 Deferred revenue 4,057,998 956,071 Accruals and other 9,262,530 8,997,857 Total deferred tax assets 111,057,828 195,032,577 Less valuation allowance (107,616,937 ) (193,795,798 ) Net deferred tax assets $ 3,440,891 $ 1,236,779 Intangible assets $ (1,936,250 ) $ — Fixed assets (2,504,876 ) (1,236,779 ) Total deferred tax liabilities $ (4,441,126 ) $ (1,236,779 ) Net deferred tax liability $ (1,000,235 ) $ — As of December 31, 2017 , the Company had U.S. federal net operating loss carryforwards of $331.0 million , which may be available to offset future income tax liabilities and will expire beginning in 2034. As of December 31, 2017 , the Company also had U.S. state net operating loss carryforwards of $342.8 million , which may be available to offset future income tax liabilities and will expire beginning in 2034 . The Company has recorded a full valuation allowance against its net deferred tax assets as of December 31, 2017 and 2016 , respectively, because the Company has determined that is it more likely than not that these assets will not be fully realized due to historic net operating losses incurred. The Company experienced a net change in valuation allowance of $73.8 million and $86.2 million in the years ended December 31, 2016 and 2017 , respectively. As of December 31, 2017 , the Company had federal research and development and orphan drug tax credit carryforwards of $46.9 million and $4.6 million , respectively, available to reduce future tax liabilities, which expire beginning in 2034 . Under the provisions of the Internal Revenue Code, the net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three -year period in excess of 50% , as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed financing since its inception which may have resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code, or could result in a change in control in the future. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal, state, and foreign jurisdictions, where applicable. The company’s tax years are still open under status from 2014 to present. All open years maybe be examined to the extent that tax credit or net operating loss carryforward are used in future periods. The Company will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2017 , the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s consolidated statements of operations.

Subsequent events

Subsequent events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent events	Subsequent events In January 2018, the Company entered into a license and commercialization agreement with Novartis Pharma AG (Novartis) for the development and commercialization of investigational voretigene neparvovec outside the United States. Novartis paid the Company a non-refundable, non-creditable, one-time cash payment of $105.0 million . The Company is eligible to receive an additional $25.0 million cash payment if investigational voretigene neparvovec is approved by the European Medicines Agency, as well as an aggregate $40.0 million in cash based upon the achievement of certain aggregate net sales levels in certain markets outside the United States. In addition, the Company is entitled to royalty payments at a flat mid-twenties percentage of net sales on a royalty-region by royalty-region basis, subject to reduction and extension in certain circumstances. The Company will manufacture and supply all of the requirements of Novartis for voretigene neparvovec.

Selected quarterly financial da

Selected quarterly financial data (unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected quarterly financial data (unaudited)	Selected quarterly financial data (unaudited) The following table contains quarterly financial information for 2016 and 2017 . The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2016 (1) First quarter Second quarter Third quarter Fourth quarter Total Revenues $ 1,288,628 $ 1,288,629 $ 1,302,789 $ 16,302,789 $ 20,182,835 Research and development 18,251,900 19,621,536 22,384,109 26,121,860 86,379,405 Acquired in-process research and development — — — 11,132,146 11,132,146 General and administrative 8,873,861 10,676,752 12,049,954 16,469,750 48,070,317 Total operating expenses 27,125,761 30,298,288 34,434,063 53,723,756 145,581,868 Loss from operations (25,837,133 ) (29,009,659 ) (33,131,274 ) (37,420,967 ) (125,399,033 ) Net loss $ (25,576,711 ) $ (28,676,115 ) $ (32,562,407 ) $ (36,837,294 ) $ (123,652,527 ) Basic and diluted net loss per common share $ (0.95 ) $ (1.04 ) $ (1.07 ) $ (1.21 ) $ (4.29 ) 2017 (1) First Second quarter Third quarter Fourth quarter Total Revenues $ 1,274,467 $ 1,483,233 $ 1,899,575 $ 7,408,369 $ 12,065,644 Research and development 32,348,249 32,989,267 39,341,386 30,481,145 135,160,047 Acquired in-process research and development 386,784 3,070,358 1,750,000 3,397,116 8,604,258 Impairment of acquired in-process research and development — 15,696,017 — — 15,696,017 General and administrative 21,413,818 26,728,606 26,640,443 36,340,380 111,123,247 Total operating expenses 54,148,851 78,484,248 67,731,829 70,218,641 270,583,569 Loss from operations (52,874,384 ) (77,001,015 ) (65,832,254 ) (62,810,272 ) (258,517,925 ) Net loss $ (52,289,332 ) $ (74,359,781 ) $ (65,011,911 ) $ (61,820,948 ) $ (253,481,972 ) Basic and diluted net loss per common share $ (1.70 ) $ (2.40 ) $ (1.90 ) $ (1.68 ) $ (7.63 ) (1) The sum of the quarterly per share amounts may not equal per share amounts reported for the year due to rounding.

Summary of significant accoun24

Summary of significant accounting policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Use of estimates	Use of estimates The preparation of consolidated financial statements in conformity with U.S. generally accepted accounting principles (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.
Principles of consolidation	Principles of consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries: Spark Therapeutics, Inc., Spark Therapeutics Ireland Limited, Spark Therapeutics UK Limited, Spark Therapeutics Argentina Limited, Spark Therapeutics Switzerland Limited, Spark Therapeutics Germany Limited and Spark Therapeutics France Limited. All intercompany balances and transactions have been eliminated in consolidation.
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, other receivables and 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.
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 December 31, 2016 and 2017 consisted primarily of money market funds.
Marketable securities	Marketable securities The Company classifies its marketable security investments as available-for-sale securities and the securities are stated at fair value. At December 31, 2017 , the balance in the Company’s accumulated other comprehensive loss included activity related to the Company’s available-for-sale marketable securities. There were no material realized gains or losses recognized on the maturity of available-for-sale securities during the year ended December 31, 2017 and, as a result, the Company did not reclassify any amount out of accumulated other comprehensive loss for the same period. In addition, as part of the license and stock purchase agreements entered into with Selecta Biosciences, Inc. (Selecta) (note 14), the Company purchased restricted common shares of Selecta. The investment is classified as available-for-sale and is stated at fair value.
Property and equipment	Property and equipment Property and equipment consists of computer and laboratory equipment, software, office equipment, furniture and leasehold improvements and is recorded at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of three years for computer equipment and software, five years for laboratory and office equipment and seven years for furniture. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset . The Company reviews long-lived assets, such as property and equipment, for impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated undiscounted future cash flows that the assets are expected to generate. If the carrying amount of an asset exceeds its estimated future cash flows, then an impairment charge is recognized for the amount by which the carrying value of the asset exceeds the fair value of the asset. No impairment charges have been recorded since inception.
Acquired in-process research and development and goodwill	Acquired in-process research and development and goodwill The acquired in-process research and development (IPR&D) asset was an indefinite-lived intangible asset and was assessed for impairment annually or more frequently if impairment indicators existed. During the year ended December 31, 2017, the Company determined that it would no longer pursue product candidates utilizing the technology acquired from Genable Technologies, Ltd. (Genable) in March 2016 and, accordingly, recorded an impairment charge of $15.7 million within its consolidated statement of operations. Additionally, the Company recognized an income tax benefit of $1.0 million related to the reversal of the deferred tax liability associated with the IPR&D during the year ended December 31, 2017. Goodwill represents the excess of purchase price over the fair value of net assets acquired by the Company. Goodwill is not amortized, but assessed for impairment on an annual basis or more frequently if impairment indicators exist. The Company operates as one reporting unit. The Company has the option to perform a qualitative assessment of goodwill prior to completing the two-step method described below to determine whether or not it is more likely than not that the fair value of its reporting units is less than its carrying amount, including goodwill and other intangible assets. If the Company concludes that this is the case, it must perform the two-step process. The first step compares a reporting unit’s fair value to its carrying amount to identify potential goodwill impairment. If the carrying amount of a reporting unit exceeds the reporting unit’s fair value, the second step of the impairment test must be completed to measure the amount of the reporting unit’s goodwill impairment loss, if any. Step two requires an assignment of the reporting unit’s fair value to the reporting unit’s assets and liabilities to determine the implied fair value of the reporting unit’s goodwill. The implied fair value of the reporting unit’s goodwill is then compared with the carrying amount of the reporting unit’s goodwill to determine the goodwill impairment loss to be recognized, if any. The Company performs its annual goodwill impairment test as of October 1st. The Company performed a qualitative assessment in 2017 and determined that there was no impairment to goodwill for the year ended December 31, 2017 .
Research and development and in-process research and development	Research and development and in-process research and development Research and development costs are expensed as incurred. Research and development expenses consist of internal and external expenses. Internal expenses include employee compensation and overhead. External expenses include development, clinical trials, statistical analysis and report writing and regulatory compliance costs incurred with clinical research organizations and other third-party vendors. At the end of the reporting period, the Company compares payments made to third-party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. When the Company is reimbursed by a collaboration partner for work performed, the costs incurred are recorded as research and development expenses and the related reimbursement is recorded as a reduction to research and development expenses. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered. Costs incurred in obtaining technology licenses are charged to research and development expense as acquired in-process research and development if the technology licensed has not reached technological feasibility and has no alternative future use.
Income tax	Income taxes The Company recognizes deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of its assets and liabilities and the expected benefits of net operating loss carryforwards. The impact of changes in tax rates and laws on deferred taxes, if any, is applied during the years in which temporary differences are expected to be settled and net operating losses used and is reflected in the consolidated financial statements in the period of enactment. The measurement of deferred tax assets is reduced, if necessary, if, based on weight of the evidence, it is more likely than not that some, or all, of the deferred tax assets will not be realized. At December 31, 2016 and 2017 , the Company has concluded that a full valuation allowance is necessary for its deferred tax assets.
Revenue recognition	Revenue recognition The Company has generated revenue solely through license and collaborative agreements. The Company recognizes revenue in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Subtopic 605-25, Revenue Recognition: Multiple-Element Arrangements , which addresses the determination of whether an arrangement involving multiple deliverables contains more than one unit of accounting. A delivered item within an arrangement is considered a separate unit of accounting only if both of the following criteria are met: • the delivered item has value to the customer on a stand-alone basis; and • if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in control of the vendor. Under FASB ASC Subtopic 605-25, if both of the criteria above are not met, then separate accounting for the individual deliverables is not appropriate. Revenue recognition for arrangements with multiple deliverables constituting a single unit of accounting is recognized generally over the greater of the term of the arrangement or the expected period of performance, either on a straight-line basis or on a modified proportional performance method. Milestones related to research and development activities are accounted for in accordance with FASB ASC Subtopic 605-28, Revenue Recognition: Milestone Method . FASB ASC Subtopic 605-28 allows for the recognition of consideration, which is contingent on the achievement of a substantive milestone in its entirety, in the period the milestone is achieved. A milestone is considered to be substantive if all of the following criteria are met: • the milestone is commensurate with either: (1) the performance required to achieve the milestone or (2) the enhancement of the value of the delivered items resulting from the performance required to achieve the milestone; • the milestone relates solely to past performance; and • the milestone payment is reasonable relative to all of the deliverables and payment terms within the agreement. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. For licenses with no stand-alone value, revenues are recognized on a straight-line basis over the related performance period. 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. To date, the Company has not generated any revenues from the commercial sale of products.
Share-based compensation and fair value of stock	Stock-based compensation and fair value of stock The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation-Stock Compensation . ASC 718 requires all stock-based payments to employees, including grants of employee stock options and restricted stock units and modifications to existing stock options, to be recognized in the consolidated statement of operations based on their fair values. The Company uses the Black-Scholes option pricing model to determine the fair value of options granted. The Company’s stock-based awards are subject to either service or performance-based vesting conditions. Compensation expense related to awards to employees and directors with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to non-employees with service-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period to the extent achievement of the performance condition is probable. The Company expenses restricted stock unit awards to employees based on the fair value of the award on a straight-line basis over the associated service period of the award. Awards of restricted stock units to non-employees are adjusted through stock-based compensation expense at each reporting period end to reflect the current fair value of such awards and expensed over the vesting period. The Company estimates the fair value of its option awards to employees and directors using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including (i) the expected stock price volatility, (ii) the calculation of the expected term of the award, (iii) the risk-free interest rate and (iv) expected dividends. Due to the lack of substantial Company-specific historical and implied volatility data of its common stock, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. When selecting these public companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry and with historical share price information sufficient to meet the expected term of the stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. The Company has estimated the expected term of its employee stock options using the “simplified” method, whereby, the expected term equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid, and does not expect to pay, dividends in the foreseeable future. The Company accounts for forfeitures when they occur. Stock-based compensation expense recognized in the consolidated financial statements is based on actual awards forfeited. Consistent with the guidance in FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees , the fair value of each non-employee stock option and restricted stock award is estimated at the date of grant using the Black-Scholes option pricing model with assumptions generally consistent with those used for employee stock options, with the exception of expected term, which is over the contractual life.
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.
Deferred rent	Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property. Tenant improvement allowances from the lessor are included in the accompanying consolidated balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of December 31, 2016 and 2017 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received
Other comprehensive loss	Other comprehensive loss The Company follows the provisions of FASB ASC Topic 220, Comprehensive Income , which establishes standards for the reporting and display of comprehensive income and its components. Comprehensive income is defined to include all changes in equity during a period except those resulting from investments by owners and distributions to owners. The Company recorded gains and losses related to changes in available-for-sale securities and foreign currency translation.
Recent accounting pronouncements	Recent accounting pronouncements In February 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-02, Leases . ASU 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. ASU 2016-02 is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company is currently evaluating the impact of the updated guidance on the Company's consolidated financial statements. In May 2014, the FASB issued a new standard regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. The new standard provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflect the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. The allowable adoption methods are the full retrospective method, which requires the standard to be applied to each prior period presented, or the modified retrospective method which requires the cumulative effect of adoption to be recognized as an adjustment to opening retained earnings in the period of adoption. The Company will adopt this guidance using the modified retrospective method and the adoption will have no cumulative adjustment to its consolidated financial statements as it relates to the Pfizer Inc. (Pfizer) collaboration agreement discussed in note 14. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities . ASU 2016-01 changes accounting for equity investments, financial liabilities under the fair value option, and presentation and disclosure requirements for financial instruments. ASU 2016-01 does not apply to equity investments in consolidated subsidiaries or those accounted for under the equity method of accounting. In addition, the FASB clarified guidance related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale debt securities. Equity investments with readily determinable fair values will be measured at fair value with changes in fair value recognized in net income. Companies have the option to either measure equity investments without readily determinable fair values at fair value or at cost, adjusted for changes in observable prices, minus impairment. Changes in measurement under either alternative will be recognized in net income. Companies that elect the fair value option for financial liabilities must recognize changes in fair value related to instrument-specific credit risk in other comprehensive income (OCI). Companies must assess valuation allowances for deferred tax assets related to available-for-sale debt securities in combination with their other deferred tax assets. ASU 2016-01 will be effective beginning the first quarter of 2018. The Company currently is evaluating the impact of the updated guidance on the Company's consolidated financial statements.

Summary of significant accoun25

Summary of significant accounting policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
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 December 31, 2015 , 2016 and 2017 as they would be anti-dilutive: December 31, 2015 2016 2017 Unvested restricted common shares 373,655 251,809 798,501 Options issued and outstanding 3,071,372 4,181,993 3,522,874
Schedule of Accumulated Other Comprehensive Income (Loss)	The accumulated balances related to each component of other comprehensive loss are summarized as follows: Net unrealized losses on available for sale securities Foreign currency translation adjustments Accumulated other comprehensive loss Balance as of December 31, 2016 $ (801,717 ) $ 7,421 $ (794,296 ) Current period other comprehensive income (loss) (5,162,259 ) 42,960 (5,119,299 ) Balance as of December 31, 2017 $ (5,963,976 ) $ 50,381 $ (5,913,595 )

Marketable securities (Tables)

Marketable securities (Tables)	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	The following table summarizes the available-for-sale securities held at December 31, 2016 and 2017 : Description Amortized cost Unrealized gains Unrealized losses Fair value December 31, 2016 U.S. government agency $ 133,690,267 $ 10,907 $ (85,714 ) $ 133,615,460 Corporate securities $ 126,253,903 $ — $ (726,579 ) $ 125,527,324 December 31, 2017 U.S. government agency $ 222,179,753 $ — $ (640,341 ) $ 221,539,412 Corporate securities $ 227,237,638 $ — $ (5,322,745 ) $ 221,914,893

Fair value of financial instr27

Fair value of financial instruments (Tables)	12 Months Ended
	Dec. 31, 2017
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: 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) As of December 31, 2016: Assets: Money market funds (included in cash and cash equivalents) $ 53,452,424 — — Corporate securities (included in cash and cash equivalents) $ 5,029,838 — — Marketable securities - U.S. government agencies $ 133,615,460 — — Marketable securities - corporate securities $ 122,144,692 $ 3,382,632 — As of December 31, 2017: Assets: Money market funds (included in cash and cash equivalents) $ 90,347,897 — — Corporate securities (included in cash and cash equivalents) $ 5,548,478 — — Marketable securities - U.S. government agencies $ 221,539,412 — — Marketable securities - corporate securities $ 220,020,150 $ 1,894,743 —

Business acquisition of Genab28

Business acquisition of Genable and impairment of acquired in-process research and development (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Summarized Fair Value of Tanigle and Intangible Assets Acquired and Liabilities Assumed	The following table summarizes the fair values of the tangible and intangible assets acquired and liabilities assumed at the acquisition date: Cash acquired $ 196,307 Other current assets 102,506 Acquired in-process research and development 15,490,000 Goodwill 1,160,104 Total assets assumed 16,948,917 Other non-current liabilities 254,753 Deferred tax liability 1,000,235 Total liabilities assumed 1,254,988 Total allocation of purchase price $ 15,693,929

Property and equipment, net (Ta

Property and equipment, net (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	Property and equipment consist of the following: December 31, 2016 2017 Laboratory equipment $ 5,833,616 $ 6,380,580 Computer and software 530,663 1,325,403 Furniture and fixtures 470,151 1,770,475 Office equipment 537,756 725,982 Leasehold improvements 16,981,989 23,653,219 Construction in progress 967,945 38,179,619 Property and equipment, gross 25,322,120 72,035,278 Less accumulated depreciation and amortization (5,527,814 ) (10,322,485 ) Property and equipment, net $ 19,794,306 $ 61,712,793

Accrued expenses (Tables)

Accrued expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following: December 31, 2016 2017 Compensation and benefits $ 9,613,275 $ 15,011,597 Consulting and professional fees 995,614 4,846,360 Research and development 2,717,777 2,809,043 Other 500,254 2,030,225 $ 13,826,920 $ 24,697,225

Stockholders' equity (Tables)

Stockholders' equity (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Summary of Restricted Common Stock Activity	The following table summarizes restricted stock activity for the instruments discussed above: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2015 348,555 $ 4.83 Shares canceled (1,213 ) $ 1.15 Shares vested (172,466 ) $ 3.90 Nonvested shares at December 31, 2016 174,876 $ 5.78 Shares canceled (3,107 ) $ 1.15 Shares vested (70,935 ) $ 3.60 Nonvested shares at December 31, 2017 100,834 $ 7.45 The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2015 25,100 $ 60.03 Shares granted 59,500 $ 44.35 Shares canceled (1,000 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at December 31, 2016 76,933 $ 49.90 Shares granted 713,400 $ 63.34 Shares canceled (55,457 ) $ 48.93 Shares vested (37,209 ) $ 55.77 Nonvested shares at December 31, 2017 697,667 $ 63.40

Stock incentive plans (Tables)

Stock incentive plans (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-based Compensation Expense	Stock-based compensation expense by award type was as follows: Year ended December 31, 2015 2016 2017 Stock options $ 9,874,709 $ 22,376,794 $ 28,001,032 Restricted stock 1,680,145 632,298 8,452,262 Employee stock purchase plan — 177,767 437,599 $ 11,554,854 $ 23,186,859 $ 36,890,893
Summary of Stock Option Activity	The following table summarizes stock option activity: Number of shares Weighted- average exercise price Aggregate intrinsic value(a) Outstanding at December 31, 2015 3,071,372 $ 22.99 Granted 1,722,950 $ 43.16 Exercised (455,138 ) $ 5.69 Forfeited (157,191 ) $ 21.17 Outstanding at December 31, 2016 4,181,993 $ 33.25 Granted 701,300 $ 56.78 Exercised (953,253 ) $ 21.75 Forfeited (407,166 ) $ 41.30 Outstanding at December 31, 2017 3,522,874 $ 40.11 Vested at December 31, 2017 1,483,330 $ 32.46 $ 34,379,774 Vested at December 31, 2017 and expected to vest 3,170,491 $ 40.11 $ 48,691,490 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock that were in the money at December 31, 2017 .
Summary of Weighted-Average Assumptions	The weighted average grant date fair value of the options granted in 2015 , 2016 and 2017 was $40.51 , $28.56 and $39.27 per share, respectively, using the Black-Scholes option-pricing model with the following weighted-average assumptions: Years Ended December 31, 2015 2016 2017 Expected volatility 76.8 % 75.2 % 78.1 % Risk-free interest rate 1.67 % 1.71 % 2.06 % Expected term (in years) 6.10 5.95 6.10 Expected dividend yield 0.0 % 0.0 % 0.0 %
Summary of Restricted Common Stock Activity	The following table summarizes restricted stock activity for the instruments discussed above: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2015 348,555 $ 4.83 Shares canceled (1,213 ) $ 1.15 Shares vested (172,466 ) $ 3.90 Nonvested shares at December 31, 2016 174,876 $ 5.78 Shares canceled (3,107 ) $ 1.15 Shares vested (70,935 ) $ 3.60 Nonvested shares at December 31, 2017 100,834 $ 7.45 The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2015 25,100 $ 60.03 Shares granted 59,500 $ 44.35 Shares canceled (1,000 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at December 31, 2016 76,933 $ 49.90 Shares granted 713,400 $ 63.34 Shares canceled (55,457 ) $ 48.93 Shares vested (37,209 ) $ 55.77 Nonvested shares at December 31, 2017 697,667 $ 63.40

Commitments and contingencies (

Commitments and contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Reconciliation of Lease Termination Costs	The following table reconciles the termination cost discussed above: Balance as of November 10, 2017 Recognized during the quarter Balance as of December 31, 2017 Contract termination liability $6,880,161 $53,328 $6,826,833
Schedule of Future Minimum Rental Payments for Operating Leases	Future minimum lease payments under these leases are as follows: Year Ending December 31, 2018 $ 5,520,167 2019 7,156,064 2020 7,819,394 2021 8,071,918 2022 8,126,544 2023 and thereafter 58,897,069 Total $ 95,591,156

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Loss Before Income Taxes Attributable to Domestic and International Operations	Loss before income taxes attributable to domestic and international operations, consists of the following: Year Ended December 31, 2015 2016 2017 Domestic $ (47,125,778 ) $ (122,809,629 ) $ (239,300,526 ) Foreign — (842,898 ) (15,144,487 ) Loss before income taxes $ (47,125,778 ) $ (123,652,527 ) $ (254,445,013 )
Schedule of Components of Income Tax Expense (Benefit)	Income tax expense (benefit) consists of the following: Year Ended December 31, 2015 2016 2017 Current tax: Domestic $ — $ — $ — Foreign — — 110,305 Deferred tax: Domestic — — — Foreign — — (1,073,346 ) Total income tax benefit $ — $ — $ (963,041 )
Schedule of Effective Income Tax Rate Reconciliation	A reconciliation of income tax benefit computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows: Year Ended December 31, 2015 2016 2017 Federal income tax benefit at statutory rate 34.0 % 34.0 % 34.0 % State and local tax, net of federal benefit 8.0 10.4 10.3 Permanent differences (2.7 ) 3.1 (0.3 ) Tax credits 13.1 9.1 9.2 Tax Cuts and Jobs Act impact — — (17.6 ) Foreign rate differential — (0.1 ) (1.3 ) Change in valuation allowance (52.4 ) (56.5 ) (33.9 ) Effective income tax rate — % — % 0.4 %
Schedule of Deferred Tax Assets	The significant components of the Company’s deferred tax assets are comprised of the following: December 31, 2016 2017 Deferred tax assets (liabilities): Net operating loss carryforwards $ 55,710,846 $ 114,152,143 Tax credit carryforwards 28,327,477 51,585,618 Stock-based compensation 10,342,547 14,643,667 Deferred rent 3,356,430 4,697,221 Deferred revenue 4,057,998 956,071 Accruals and other 9,262,530 8,997,857 Total deferred tax assets 111,057,828 195,032,577 Less valuation allowance (107,616,937 ) (193,795,798 ) Net deferred tax assets $ 3,440,891 $ 1,236,779 Intangible assets $ (1,936,250 ) $ — Fixed assets (2,504,876 ) (1,236,779 ) Total deferred tax liabilities $ (4,441,126 ) $ (1,236,779 ) Net deferred tax liability $ (1,000,235 ) $ —

Selected quarterly financial 35

Selected quarterly financial data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of quarterly financial information	The following table contains quarterly financial information for 2016 and 2017 . The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period. 2016 (1) First quarter Second quarter Third quarter Fourth quarter Total Revenues $ 1,288,628 $ 1,288,629 $ 1,302,789 $ 16,302,789 $ 20,182,835 Research and development 18,251,900 19,621,536 22,384,109 26,121,860 86,379,405 Acquired in-process research and development — — — 11,132,146 11,132,146 General and administrative 8,873,861 10,676,752 12,049,954 16,469,750 48,070,317 Total operating expenses 27,125,761 30,298,288 34,434,063 53,723,756 145,581,868 Loss from operations (25,837,133 ) (29,009,659 ) (33,131,274 ) (37,420,967 ) (125,399,033 ) Net loss $ (25,576,711 ) $ (28,676,115 ) $ (32,562,407 ) $ (36,837,294 ) $ (123,652,527 ) Basic and diluted net loss per common share $ (0.95 ) $ (1.04 ) $ (1.07 ) $ (1.21 ) $ (4.29 ) 2017 (1) First Second quarter Third quarter Fourth quarter Total Revenues $ 1,274,467 $ 1,483,233 $ 1,899,575 $ 7,408,369 $ 12,065,644 Research and development 32,348,249 32,989,267 39,341,386 30,481,145 135,160,047 Acquired in-process research and development 386,784 3,070,358 1,750,000 3,397,116 8,604,258 Impairment of acquired in-process research and development — 15,696,017 — — 15,696,017 General and administrative 21,413,818 26,728,606 26,640,443 36,340,380 111,123,247 Total operating expenses 54,148,851 78,484,248 67,731,829 70,218,641 270,583,569 Loss from operations (52,874,384 ) (77,001,015 ) (65,832,254 ) (62,810,272 ) (258,517,925 ) Net loss $ (52,289,332 ) $ (74,359,781 ) $ (65,011,911 ) $ (61,820,948 ) $ (253,481,972 ) Basic and diluted net loss per common share $ (1.70 ) $ (2.40 ) $ (1.90 ) $ (1.68 ) $ (7.63 ) (1) The sum of the quarterly per share amounts may not equal per share amounts reported for the year due to rounding.

Background - Additional informa

Background - Additional information (Details) $ / shares in Units, $ in Millions	Aug. 09, 2017USD ($)$ / sharesshares	Jun. 20, 2016USD ($)$ / sharesshares	Dec. 28, 2015USD ($)$ / sharesshares	Feb. 04, 2015USD ($)$ / sharesshares	Dec. 31, 2017segmentshares	Dec. 31, 2016shares	Dec. 31, 2015shares
Class of Stock [Line Items]
Number of operating segments | segment					1
Common stock [Member]
Class of Stock [Line Items]
Common stock, shares issued during IPO (in shares) | shares					5,296,053	3,025,000	10,316,995
Preferred stock, shares issued upon conversion (in shares) | shares				10,200,050
Initial Public Offering [Member]
Class of Stock [Line Items]
IPO completion date					Feb. 4, 2015
Initial Public Offering [Member] | Common stock [Member]
Class of Stock [Line Items]
Common stock, shares issued during IPO (in shares) | shares				8,050,000
Stock issuance price per share (in usd per share) | $ / shares				$ 23
Aggregate gross proceeds from IPO | $				$ 185.2
Net proceeds from sale of stock | $				$ 168.9
Public stock offering [Member] | Common stock [Member]
Class of Stock [Line Items]
Net proceeds from sale of stock | $	$ 379.9	$ 127.6	$ 99.4
Number of shares issued (in shares) | shares	5,296,053	3,025,000	2,266,995
Shares issuance price per share (in dollars per share) | $ / shares	$ 76	$ 45	$ 47
Proceeds from follow-on public offering | $	$ 402.5	$ 136.1	$ 106.5

Development-stage risks - Addit

Development-stage risks - Additional information (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Development-stage Risk [Abstract]
Accumulated deficit	$ 505,862,883	$ 252,380,911

Summary of significant accoun38

Summary of significant accounting policies - Additional information (Details)	12 Months Ended			58 Months Ended
	Dec. 31, 2017USD ($)reporting_unit	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2017USD ($)
Property, Plant and Equipment [Line Items]
Impairment charges of long-lived assets				$ 0
Impairment of acquired in-process research and development	$ 15,696,017	$ 0	$ 0
Non-cash income tax benefit	$ 1,000,000
Number of reporting units | reporting_unit	1
Impairment of goodwill	$ 0
Computers [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	5 years
Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	7 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset

Summary of significant accoun39

Summary of significant accounting policies - Schedule of antidilutive securities excluded from computation of diluted weighted average shares outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unvested restricted common shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	798,501	251,809	373,655
Options issued and outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	3,522,874	4,181,993	3,071,372

Summary of significant accoun40

Summary of significant accounting policies - Accumulated other comprehensive loss (Details)	12 Months Ended
	Dec. 31, 2017USD ($)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 330,276,713
Ending balance	513,624,212
Net unrealized losses on available for sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(801,717)
Current period other comprehensive income (loss)	(5,162,259)
Ending balance	(5,963,976)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	7,421
Current period other comprehensive income (loss)	42,960
Ending balance	50,381
Accumulated other comprehensive loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(794,296)
Current period other comprehensive income (loss)	(5,119,299)
Ending balance	$ (5,913,595)

Marketable securities - Schedul

Marketable securities - Schedule of available-for-sale securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Remaining maturity period	2 years
U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 222,179,753	$ 133,690,267
Unrealized gains	0	10,907
Unrealized losses	(640,341)	(85,714)
Fair value	221,539,412	133,615,460
Corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	227,237,638	126,253,903
Unrealized gains	0	0
Unrealized losses	(5,322,745)	(726,579)
Fair value	$ 221,914,893	$ 125,527,324

Fair value of financial instr42

Fair value of financial instruments - Summary of assets and liabilities measured at fair value on recurring basis (Details) - Fair Value Measurements Recurring [Member] - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Money market funds [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds/Corporate securities (included in cash and cash equivalents)	$ 90,347,897	$ 53,452,424
Money market funds [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds/Corporate securities (included in cash and cash equivalents)	0	0
Money market funds [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds/Corporate securities (included in cash and cash equivalents)	0	0
Corporate securities [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds/Corporate securities (included in cash and cash equivalents)	5,548,478	5,029,838
Marketable securities	220,020,150	122,144,692
Corporate securities [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds/Corporate securities (included in cash and cash equivalents)	0	0
Marketable securities	1,894,743	3,382,632
Corporate securities [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds/Corporate securities (included in cash and cash equivalents)	0	0
Marketable securities	0	0
U.S. government agencies [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Marketable securities	221,539,412	133,615,460
U.S. government agencies [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Marketable securities	0	0
U.S. government agencies [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Marketable securities	$ 0	$ 0

Business acquisition of Genab43

Business acquisition of Genable and impairment of acquired in-process research and development - Additional information (Details) - USD ($)	Mar. 07, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Impairment of acquired in-process research and development		$ 15,696,017	$ 0	$ 0
Non-cash income tax benefit		$ 1,000,000
Genable [Member]
Business Acquisition [Line Items]
Consideration paid	$ 6,100,000
Fair value of shares issued for consideration	9,200,000
Consideration transferred	15,300,000
Acquisition related costs	$ 300,000
Common stock [Member] | Genable [Member]
Business Acquisition [Line Items]
Shares issued for consideration (in shares)	265,000
Genable [Member] | Genable [Member]
Business Acquisition [Line Items]
Receivables due from acquisition	$ 500,000
In-process research and development [Member] | Genable [Member]
Business Acquisition [Line Items]
Acquired in-process research and development	$ 15,500,000

Business acquisition of Genab44

Business acquisition of Genable and impairment of acquired in-process research and development - Allocation of purchase price (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Mar. 07, 2016
Business Acquisition [Line Items]
Goodwill	$ 1,254,005	$ 1,160,104
Genable [Member]
Business Acquisition [Line Items]
Cash acquired			$ 196,307
Other current assets			102,506
Acquired in-process research and development			15,490,000
Goodwill			1,160,104
Total assets assumed			16,948,917
Other non-current liabilities			254,753
Deferred tax liability			1,000,235
Total liabilities assumed			1,254,988
Total allocation of purchase price			$ 15,693,929

Property and equipment, net - S

Property and equipment, net - Schedule of property and equipment, net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 72,035,278	$ 25,322,120
Less accumulated depreciation and amortization	(10,322,485)	(5,527,814)
Property and equipment, net	61,712,793	19,794,306
Depreciation and amortization expense	4,860,428	3,634,349	$ 1,732,983
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	6,380,580	5,833,616
Computer and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,325,403	530,663
Furniture & Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,770,475	470,151
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	725,982	537,756
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	23,653,219	16,981,989
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	38,179,619	$ 967,945
One Drexel Plaza Research Facility [Member]
Property, Plant and Equipment [Line Items]
Construction in progress	$ 35,000,000

Accrued expenses - Schedule of

Accrued expenses - Schedule of accrued expenses (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Compensation and benefits	$ 15,011,597	$ 9,613,275
Consulting and professional fees	4,846,360	995,614
Research and development	2,809,043	2,717,777
Other	2,030,225	500,254
Accrued expenses	$ 24,697,225	$ 13,826,920

Debt - Additional Information (

Debt - Additional Information (Details) - Commonwealth of Pennsylvania [Member] - Machinery and equipment and other assets purchased (MELF Loan) [Member] - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Loan amount	$ 1,600
Debt term	5 years
Principle and interest payment	$ 29
Stated percentage rate	3.25%
Debt interest expense		$ 48	$ 15

Stockholders' equity - Addition

Stockholders' equity - Additional information (Details) - USD ($)	1 Months Ended	12 Months Ended				36 Months Ended
	Dec. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, shares authorized (in shares)		150,000,000	150,000,000			150,000,000
Preferred stock, shares authorized (in shares)		5,000,000	5,000,000			5,000,000
Compensation expense		$ 36,890,893	$ 23,186,859	$ 11,554,854
Unrecognized compensation expense related to restricted common shares		0				$ 0
General and administrative expense [Member]
Class of Stock [Line Items]
Compensation expense		21,800,000	13,600,000	6,600,000
Research and development expense [Member]
Class of Stock [Line Items]
Compensation expense		$ 15,100,000	$ 9,600,000	5,000,000
Common stock [Member] | Tranche One [Member]
Class of Stock [Line Items]
Percentage of vesting of units on first anniversary		25.00%
Common stock [Member] | Tranche Two [Member]
Class of Stock [Line Items]
Units, vesting period		3 years
License agreement [Member] | University of Pennsylvania [Member]
Class of Stock [Line Items]
Issuance of restricted stock in connection with license agreement (in shares)	200,000				200,000
Shares vested (in shares)						100,000
Restricted shares [Member]
Class of Stock [Line Items]
Shares vested (in shares)		70,935	172,466
Restricted shares [Member] | General and administrative expense [Member]
Class of Stock [Line Items]
Compensation expense		$ 34,000	$ 100,000	100,000
Restricted shares [Member] | Research and development expense [Member]
Class of Stock [Line Items]
Compensation expense		$ 4,400,000	$ 1,300,000	$ 1,800,000

Stockholders' equity - Summary

Stockholders' equity - Summary of restricted stock activity (Details) - Restricted shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Number of shares (in shares):
Nonvested beginning balance (in shares)	174,876	348,555
Shares canceled (in shares)	(3,107)	(1,213)
Shares vested (in shares)	(70,935)	(172,466)
Nonvested ending balance (in shares)	100,834	174,876
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in usd per share)	$ 5.78	$ 4.83
Shares canceled (in dollars per share)	1.15	1.15
Shares vested (in usd per share)	3.60	3.90
Nonvested shares ending balance (in usd per share)	$ 7.45	$ 5.78

Stock incentive plans - Additio

Stock incentive plans - Additional information (Details) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2017	Jan. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			$ 36,890,893	$ 23,186,859	$ 11,554,854
Weighted average remaining contractual term of options outstanding			7 years 10 months 24 days
Weighted average remaining contractual term of options exercisable			7 years 3 months 18 days
Weighted average grant date fair value of options (in usd per share)			$ 39.27	$ 28.56	$ 40.51
Unrecognized compensation expense related to the performance-based restricted common stock			$ 0
Research and development expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			15,100,000	$ 9,600,000	$ 5,000,000
General and administrative expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			21,800,000	13,600,000	6,600,000
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			437,599	$ 177,767	0
Unrecognized compensation expense related to the performance-based restricted common stock			$ 200,000
Grants in period (in shares)			11,804	8,012
Stock options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense			$ 28,001,032	$ 22,376,794	9,874,709
Unrecognized compensation expense related to stock options			$ 55,500,000
Weighted average period recognition for unrecognized compensation expense			2 years 4 months 24 days
Intrinsic value of options exercised			$ 45,900,000	$ 21,100,000
Restricted stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)			713,400	59,500
Compensation expense			$ 8,452,262	$ 632,298	$ 1,680,145
Weighted average period recognition for unrecognized compensation expense			3 years 3 months 18 days
Unrecognized compensation expense related to the performance-based restricted common stock			$ 36,900,000
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants (in shares)		209,500
Common stock reserved for issuance (in shares)		1,830,000	727,229
Maximum shares authorized (in shares)		2,543,299
Shares of common stock (in shares)		1,724,000
Percentage of common stock outstanding		4.00%
Shares for ESPP automatically increased (in shares)	1,234,641
Stock incentive plans description			Under the 2015 Plan, the number of shares of common stock reserved for issuance is the sum of: (1) 1,830,000 plus; (2) the number of shares (up to 2,543,299 shares) equal to the sum of the number of shares of common stock then available for issuance under the 2014 Plan and the number of shares of common stock subject to outstanding awards under the 2014 Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right; plus (3) an annual increase, to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2025, equal to the lowest of 1,724,000 shares of common stock, 4% of the number of shares of common stock outstanding on the first day of such fiscal year and an amount determined by the board of directors.
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock incentive plans description			The number of shares of common stock reserved for issuance under the 2015 ESPP automatically will increase on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2016 and continuing until, and including, the fiscal year ending December 31, 2026, in an amount equal to the lowest of: (1) 440,000 shares of common stock; (2) 1% of the total number of shares of common stock outstanding on the first day of the applicable fiscal year; and (3) an amount determined by the board of directors. For the year ended December 31, 2017, 11,804 shares were issued under the 2015 ESPP.
2015 Employee Stock Purchase Plan [Member] | ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for issuance (in shares)		220,000	779,672
Shares of common stock (in shares)		440,000
Percentage of common stock outstanding		1.00%
Shares for ESPP automatically increased (in shares)	308,660
Shares granted (in shares)			11,804

Stock incentive plans - Compens

Stock incentive plans - Compensation expense (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 36,890,893	$ 23,186,859	$ 11,554,854
Stock options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	28,001,032	22,376,794	9,874,709
Restricted stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	8,452,262	632,298	1,680,145
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 437,599	$ 177,767	$ 0

Stock incentive plans - Summary

Stock incentive plans - Summary of stock option activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Number of shares
Outstanding beginning balance (in shares)	4,181,993	3,071,372
Granted (in shares)	701,300	1,722,950
Exercised (in shares)	(953,253)	(455,138)
Forfeited (in shares)	(407,166)	(157,191)
Outstanding ending balance (in shares)	3,522,874	4,181,993
Vested, outstanding (in shares)	1,483,330
Vested and expected to vest, outstanding (in shares)	3,170,491
Weighted-average exercise price
Outstanding beginning balance (in usd per share)	$ 33.25	$ 22.99
Granted (in usd per share)	56.78	43.16
Exercised (in usd per share)	21.75	5.69
Forfeited (in usd per share)	41.30	21.17
Outstanding ending balance (in usd per share)	40.11	$ 33.25
Vested, outstanding, weighted-average exercise price (in usd per share)	32.46
Vested and expected to vest, outstanding, weighted-average exercise price (in usd per share)	$ 40.11
Vested, outstanding, aggregate intrinsic value	$ 34,379,774
Vested and expected to vest, outstanding, aggregate intrinsic value	$ 48,691,490

Stock incentive plans - Summa53

Stock incentive plans - Summary of weighted-average assumptions (Details) - Stock options [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	78.10%	75.20%	76.80%
Risk-free interest rate	2.06%	1.71%	1.67%
Expected term	6 years 1 month 6 days	5 years 11 months 12 days	6 years 1 month 6 days
Expected dividend yield	0.00%	0.00%	0.00%

Stock incentive plans - Summa54

Stock incentive plans - Summary of restricted common stock activity (Details) - Restricted stock [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Number of shares (in shares):
Nonvested beginning balance (in shares)	76,933	25,100
Shares granted (in shares)	713,400	59,500
Shares canceled (in shares)	(55,457)	(1,000)
Shares vested (in shares)	(37,209)	(6,667)
Nonvested ending balance (in shares)	697,667	76,933
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in usd per share)	$ 49.90	$ 60.03
Shares granted (in usd per share)	63.34	44.35
Shares canceled (in dollars per share)	48.93	60.03
Shares vested (in usd per share)	55.77	37.06
Nonvested shares ending balance (in usd per share)	$ 63.40	$ 49.90

Commitments and contingencies -

Commitments and contingencies - Additional information (Details)	1 Months Ended			2 Months Ended	12 Months Ended
	Nov. 30, 2017USD ($)ft²	Nov. 30, 2016USD ($)ft²	Dec. 31, 2014shares	Dec. 31, 2017USD ($)ft²	Dec. 31, 2017USD ($)ft²	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014shares	Nov. 20, 2017ft²	Nov. 10, 2017USD ($)	Jan. 31, 2017ft²	Feb. 29, 2016ft²	Nov. 30, 2015ft²
Property Subject to or Available for Operating Lease [Line Items]
Long-term other liabilities				$ 40,255,605	$ 40,255,605	$ 0
Current other liabilities				1,557,062	1,557,062	0
Contractual obligation				35,000,000	35,000,000
Rent expense					10,500,000	$ 1,900,000
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018				5,520,167	5,520,167
2,019				7,156,064	7,156,064
2,020				7,819,394	7,819,394
2,021				8,071,918	8,071,918
2,022				8,126,544	8,126,544
2023 and thereafter				58,897,069	58,897,069
Total				95,591,156	95,591,156
University of Pennsylvania [Member] | License agreement [Member]
Property Subject to or Available for Operating Lease [Line Items]
Issuance of restricted stock in connection with license agreement (in shares) | shares			200,000					200,000
Lease of Laboratory and Office Space [Member]
Property Subject to or Available for Operating Lease [Line Items]
Allowance for leasehold improvements							$ 8,000,000
Lease of office space [Member]
Property Subject to or Available for Operating Lease [Line Items]
Loss on contract termination	$ 6,900,000
Long-term other liabilities				5,300,000	5,300,000
Current other liabilities				1,500,000	1,500,000
Accrued expenses				6,826,833	$ 6,826,833					$ 6,880,161
Contract termination recognized				$ 53,328
Lease of office space [Member] | Corporate HQ [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of real estate property (in sq ft) | ft²		49,000									24,800	6,500
Terminated lease property (in sq ft) | ft²	50,000
Lease of office space [Member] | Office space in Waltham, Mass [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of real estate property (in sq ft) | ft²				5,400	5,400
Sublease of office space [Member] | Corporate HQ [Member]
Property Subject to or Available for Operating Lease [Line Items]
Allowance for leasehold improvements		$ 1,600,000
Area of real estate property (in sq ft) | ft²													14,000
One Drexel Plaza Research Facility [Member]
Property Subject to or Available for Operating Lease [Line Items]
Area of real estate property (in sq ft) | ft²									108,000
Construction in progress				$ 35,000,000	$ 35,000,000
Leased asset under construction, useful life					15 years 6 months
Maximum [Member] | Milestone payments [Member] | University of Pennsylvania [Member] | License agreement [Member]
Property Subject to or Available for Operating Lease [Line Items]
Agreement obligation, milestone payments							$ 5,500,000

Related-party transactions - Ad

Related-party transactions - Additional information (Details) - CHOP [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Accrued expenses due to related party	$ 0.3	$ 1.1
Accounts payable due to related party	1.4	0.9
Master Research Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense related to agreements	6.1	7.4	$ 5.2
Technology and License Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement of patent costs	$ 0.8	$ 0.9	$ 0.7

Collaboration and license agr57

Collaboration and license agreements - Additional information (Details)	1 Months Ended						5 Months Ended	12 Months Ended
	Feb. 28, 2018USD ($)	Nov. 30, 2017USD ($)	Oct. 31, 2017USD ($)shares	Jun. 30, 2017USD ($)shares	Dec. 31, 2016USD ($)targetshares	Dec. 31, 2014USD ($)	Oct. 31, 2017USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Current portion of deferred revenue					$ 5,168,674			$ 11,968,915	$ 5,168,674
Purchase of acquired in-process research and development								$ 8,217,474	11,132,146	$ 0
Pfizer collaboration and license agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement rights and obligations description								Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024.
Research and development expense related to agreements						$ 10,600,000
Upfront payment received for license		$ 10,000,000				$ 20,000,000
Milestone payment earned									15,000,000	15,000,000
Other receivables, milestone payment earned					15,000,000				15,000,000
Revenue recognition from aggregate milestone payments		$ 15,000,000
Deferred revenue								$ 25,000,000
Licenses revenue								12,100,000	$ 20,200,000	20,200,000
Current portion of deferred revenue								12,000,000
Additional revenue potential to be earned								$ 230,000,000
Collaboration agreement expiration, period								15 years
Collaboration and license agreement expiration description								The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product or; (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements.
First product candidate [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Additional revenue potential to be earned								$ 110,000,000
Additional product candidates [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Additional revenue potential to be earned								120,000,000
Selecta license agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Prepaid expense, current					$ 10,000,000		$ 5,000,000
Shares purchased (in shares) | shares			205,254	324,362	197,238		205,254		197,238
Investment shares purchased, amount			$ 5,000,000	$ 5,000,000	$ 5,000,000
Payments to acquire project			7,500,000	7,500,000	15,000,000
Purchase of acquired in-process research and development			3,400,000	3,100,000	$ 11,500,000
Development and regulatory milestone [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaboration agreement expiration, period					90 days
Milestone payments					$ 65,000,000
Milestone payment, option period					3 years
Commercial milestone [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments					$ 365,000,000
Research and development expense [Member] | Pfizer collaboration and license agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Reimbursement revenue								$ 2,900,000	$ 2,400,000	$ 1,300,000
Minimum [Member] | Selecta license agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of additional licensing targets | target					4
Option exercise fees					$ 1,400,000
Maximum [Member] | Selecta license agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option exercise fees					2,000,000
Milestone payments					$ 430,000,000
Milestone payment, rights to fund percentage					50.00%
Common stock [Member] | Selecta license agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments to acquire project			4,100,000	4,400,000	$ 3,500,000
Licensing agreements [Member] | Selecta license agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments to acquire project			$ 3,400,000	$ 3,100,000	$ 11,500,000
Subsequent event [Member] | Pfizer collaboration and license agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront payment received for license	$ 7,000,000
Additional revenue potential to be earned	$ 7,000,000

Income taxes - Narrative (Detai

Income taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Tax Act, provisional impact of tax reform from revaluation of deferred taxes	$ 44,900,000
Valuation allowance changes	86,200,000	$ 73,800,000
Accrued interest and income tax penalties for uncertain tax positions	0
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	331,000,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	342,800,000
Research Tax Credit Carryforward [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, amount	46,900,000
Orphan Drug Tax Credit Carryforward [Member] | Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, amount	$ 4,600,000

Income taxes - Loss from domest

Income taxes - Loss from domestic and foreign operations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Domestic	$ (239,300,526)	$ (122,809,629)	$ (47,125,778)
Foreign	(15,144,487)	(842,898)	0
Loss before income taxes	$ (254,445,013)	$ (123,652,527)	$ (47,125,778)

Income taxes - Components of in

Income taxes - Components of income tax expense (benefit) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax:
Domestic	$ 0	$ 0	$ 0
Foreign	110,305	0	0
Deferred tax:
Domestic	0	0	0
Foreign	(1,073,346)	0	0
Total income tax benefit	$ (963,041)	$ 0	$ 0

Income taxes - Effective rate r

Income taxes - Effective rate reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax benefit at statutory rate	34.00%	34.00%	34.00%
State and local tax, net of federal benefit	10.30%	10.40%	8.00%
Permanent differences	(0.30%)	3.10%	(2.70%)
Tax credits	9.20%	9.10%	13.10%
Tax Cuts and Jobs Act impact	(17.60%)	0.00%	0.00%
Foreign rate differential	(1.30%)	(0.10%)	0.00%
Change in valuation allowance	(33.90%)	(56.50%)	(52.40%)
Effective income tax rate	0.40%	0.00%	0.00%

Income taxes - Deferred tax ass

Income taxes - Deferred tax assets (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 114,152,143	$ 55,710,846
Tax credit carryforwards	51,585,618	28,327,477
Stock-based compensation	14,643,667	10,342,547
Deferred rent	4,697,221	3,356,430
Deferred revenue	956,071	4,057,998
Accruals and other	8,997,857	9,262,530
Total deferred tax assets	195,032,577	111,057,828
Less valuation allowance	(193,795,798)	(107,616,937)
Net deferred tax assets	1,236,779	3,440,891
Intangible assets	0	(1,936,250)
Fixed assets	(1,236,779)	(2,504,876)
Total deferred tax liabilities	(1,236,779)	(4,441,126)
Net deferred tax liability	$ 0	$ (1,000,235)

Subsequent events - Additional

Subsequent events - Additional information (Details) - Subsequent event [Member] $ in Millions	1 Months Ended
	Jan. 31, 2018USD ($)
Novartis collaboration, license and supply agreement [Member]
Subsequent Event [Line Items]
Upfront payment received for license	$ 105
Novartis collaboration, license and supply agreement, drug approval [Member]
Subsequent Event [Line Items]
Additional revenue potential to be earned	25
Novartis collaboration, license and supply agreement, net sales achievements [Member]
Subsequent Event [Line Items]
Additional revenue potential to be earned	$ 40

Selected quarterly financial 64

Selected quarterly financial data (unaudited) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 7,408,369	$ 1,899,575	$ 1,483,233	$ 1,274,467	$ 16,302,789	$ 1,302,789	$ 1,288,629	$ 1,288,628	$ 12,065,644	$ 20,182,835	$ 22,063,674
Research and development	30,481,145	39,341,386	32,989,267	32,348,249	26,121,860	22,384,109	19,621,536	18,251,900	135,160,047	86,379,405	46,029,314
Acquired in-process research and development	3,397,116	1,750,000	3,070,358	386,784	11,132,146	0	0	0	8,604,258	11,132,146	0
Impairment of acquired in-process research and development	0	0	15,696,017	0					15,696,017	0	0
General and administrative	36,340,380	26,640,443	26,728,606	21,413,818	16,469,750	12,049,954	10,676,752	8,873,861	111,123,247	48,070,317	23,352,171
Total operating expenses	70,218,641	67,731,829	78,484,248	54,148,851	53,723,756	34,434,063	30,298,288	27,125,761	270,583,569	145,581,868	69,381,485
Loss from operations	(62,810,272)	(65,832,254)	(77,001,015)	(52,874,384)	(37,420,967)	(33,131,274)	(29,009,659)	(25,837,133)	(258,517,925)	(125,399,033)	(47,317,811)
Net loss	$ (61,820,948)	$ (65,011,911)	$ (74,359,781)	$ (52,289,332)	$ (36,837,294)	$ (32,562,407)	$ (28,676,115)	$ (25,576,711)	$ (253,481,972)	$ (123,652,527)	$ (47,125,778)
Basic and diluted net loss per common share (in usd per share)	$ (1.68)	$ (1.90)	$ (2.40)	$ (1.70)	$ (1.21)	$ (1.07)	$ (1.04)	$ (0.95)	$ (7.63)	$ (4.29)	$ (2.10)