Spark Therapeutics (ONCE) 10-Q 10 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	May 02, 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-Q
Trading Symbol	ONCE
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus (Q1,Q2,Q3,FY)	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		31,128,125

Consolidated balance sheets (un

Consolidated balance sheets (unaudited) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 27,495,174	$ 58,923,097
Marketable securities	226,199,646	237,242,655
Other receivables	5,946,802	16,780,917
Prepaid expenses and other current assets	3,713,708	1,647,008
Total current assets	263,355,330	314,593,677
Marketable securities	31,700,984	21,900,129
Property and equipment, net	23,097,630	19,794,306
Acquired in-process research and development	14,950,531	15,490,000
Goodwill	1,119,564	1,160,104
Other assets	853,510	924,579
Total assets	335,077,549	373,862,795
Current liabilities:
Accounts payable	14,021,116	9,928,737
Accrued expenses and other	9,097,776	13,826,920
Current portion of long-term debt	304,473	302,013
Current portion of deferred rent	1,140,735	771,196
Current portion of deferred revenue	5,168,674	5,168,674
Total current liabilities	29,732,774	29,997,540
Long-term debt	1,146,956	1,224,003
Long-term deferred rent	11,086,281	7,498,419
Long-term deferred revenue	2,591,418	3,865,885
Deferred tax liability	965,401	1,000,235
Total liabilities	45,522,830	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 at December 31, 2016; 31,086,809 shares issued and 31,077,603 shares outstanding at March 31, 2017	31,087	30,874
Additional paid-in capital	596,436,232	583,973,682
Accumulated other comprehensive loss	(1,689,721)	(794,296)
Treasury stock, at cost, 9,206 shares at December 31, 2016 and March 31, 2017	(552,636)	(552,636)
Accumulated deficit	(304,670,243)	(252,380,911)
Total stockholders’ equity	289,554,719	330,276,713
Total liabilities and stockholders’ equity	$ 335,077,549	$ 373,862,795

Consolidated balance sheets (u3

Consolidated balance sheets (unaudited) (Parenthetical) - $ / shares	Mar. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock , par value	$ 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	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	31,086,809	30,873,430
Common stock, shares outstanding	31,077,603	30,864,224
Treasury stock, shares	9,206	9,206

Consolidated statements of oper

Consolidated statements of operations and comprehensive (loss) (unaudited) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Revenues	$ 1,274,467	$ 1,288,628
Operating expenses:
Research and development	32,735,033	18,251,900
General and administrative	21,413,818	8,873,861
Total operating expenses	54,148,851	27,125,761
Loss from operations	(52,874,384)	(25,837,133)
Interest income, net	585,052	260,422
Net loss	$ (52,289,332)	$ (25,576,711)
Basic and diluted net loss per common share (in dollars per share)	$ (1.70)	$ (0.95)
Weighted average basic and diluted common shares outstanding (in shares)	30,771,867	26,807,380
Other comprehensive income (loss):
Unrealized loss on available-for-sale securities	$ (300,708)	$ 0
Foreign exchange translation adjustment	(594,717)	0
Total comprehensive loss	$ (53,184,757)	$ (25,576,711)

Consolidated statements of cash

Consolidated statements of cash flows (unaudited) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net loss	$ (52,289,332)	$ (25,576,711)
Adjustments to reconcile net loss to net cash used in operating activities:
Noncash rent expense	303,499	(180,992)
Depreciation expense	1,031,404	744,058
Acquired in-process research and development	386,784	0
Stock-based compensation expense	9,028,829	4,865,169
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,969,377)	(860,829)
Other receivables	10,700,763	15,726,138
Accounts payable and accrued expenses	(2,690,064)	(1,146,536)
Deferred rent	3,653,902	0
Deferred revenue	(1,274,467)	(1,288,628)
Net cash used in operating activities	(33,118,059)	(7,718,331)
Cash flows from investing activities:
Payment for acquisition, net of cash acquired	0	(5,907,850)
Purchases of marketable securities	(24,225,338)	0
Proceeds from maturities of marketable securities	24,780,000	0
Purchases of property and equipment	(2,230,192)	(4,473,966)
Net cash used in investing activities	(1,675,530)	(10,381,816)
Cash flows from financing activities:
Proceeds from exercise of options	3,433,933	196,180
Proceeds from public offerings of common stock, net	0	(657,331)
Payments on long-term debt	(74,587)	0
Net cash provided by (used in) financing activities	3,359,346	(461,151)
Effect of exchange rate changes on cash and cash equivalents	6,320	0
Net decrease in cash and cash equivalents	(31,427,923)	(18,561,298)
Cash and cash equivalents, beginning of period	58,923,097	293,530,590
Cash and cash equivalents, end of period	27,495,174	274,969,292
Supplemental disclosure of cash flow information:
Property and equipment purchases included in accounts payable and accrued expenses	$ 2,786,326	$ 348,108

The Company

The Company	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
The Company	The Company Spark Therapeutics, Inc. was formed on March 13, 2013, in the state of Delaware as AAVenue Therapeutics, LLC and amended its Certificate of Formation in October 2013 to change its name to Spark Therapeutics LLC. In May 2014, it converted from a limited liability company (LLC) to a C corporation, Spark Therapeutics, Inc. (the Company). The Company is a gene therapy company, seeking to transform the lives of patients suffering from debilitating genetic diseases by developing potentially one-time, life-altering treatments. The Company operates in one segment and has its principal offices in Philadelphia, Pennsylvania.

Development-stage risks

Development-stage risks	3 Months Ended
	Mar. 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 $304.7 million as of March 31, 2017 . The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of its product candidates in development. The Company will need additional financing to fund its operations and to commercially develop its 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 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	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies (a) Basis of presentation The accompanying unaudited interim consolidated financial statements of the Company and its wholly-owned subsidiaries have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying consolidated financial statements include all normal and recurring adjustments (which consist primarily of accruals, estimates and assumptions that impact the financial statements) considered necessary to present fairly the Company’s financial position as of March 31, 2017 , its results of operations for the three months ended March 31, 2016 and 2017 and cash flows for the three months ended March 31, 2016 and 2017 . Operating results for the three months ended March 31, 2017 are not necessarily indicative of the results that may be expected for the year ending December 31, 2017 or any other period. The interim consolidated financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual consolidated financial statements. The accompanying unaudited interim consolidated financial statements should be read in conjunction with the annual audited consolidated financial statements and related notes as of and for the year ended December 31, 2016 , included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2016 . (b) Use of estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates. (c) Fair value of financial instruments Management believes that the carrying amounts of the Company’s consolidated 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) 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. (e) 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 March 31, 2016 and 2017 as they would be anti-dilutive: March 31, 2016 2017 Unvested restricted common shares 330,600 574,341 Stock options issued and outstanding 4,029,103 4,401,360 Amounts in the table above reflect the common stock equivalents of the noted instruments. (f) 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 March 31, 2017 represents the net excess of rent expense over the actual cash paid for rent and tenant improvement allowances received. (g) 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. Comprehensive loss includes gains and losses related to changes in the fair value of 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 loss (300,708 ) (594,717 ) (895,425 ) Balance as of March 31, 2017 $ (1,102,425 ) $ (587,296 ) $ (1,689,721 ) (h) Recent accounting pronouncements In February 2016, the FASB issued Accounting Standards Update (ASU) 2016-02, “Leases.” ASU 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. The updated guidance is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company has not evaluated the impact of the updated guidance on the Company's consolidated financial statements. In May 2014, the FASB issued updated guidance regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. The update 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 Company currently is evaluating the effect that this guidance may have on its consolidated financial statements as it relates to the Pfizer Inc. (Pfizer) collaboration agreement discussed in note 11.

Marketable securities

Marketable securities	3 Months Ended
	Mar. 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 March 31, 2017 : Description 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 March 31, 2017 U.S. government agency $ 136,701,100 $ — $ (117,043 ) $ 136,584,057 Corporate securities $ 122,301,624 $ 5,183 $ (990,234 ) $ 121,316,573 No available-for-sale securities held as of March 31, 2017 had remaining maturities greater than two years .

Fair value of financial instrum

Fair value of financial instruments	3 Months Ended
	Mar. 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) At 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 — At March 31, 2017: Assets: Money market funds (included in cash and cash equivalents) $ 26,648,005 — — Marketable securities - U.S. government agencies $ 136,584,057 — — Marketable securities - corporate securities $ 118,672,889 $ 2,643,684 — (a) Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of March 31, 2017 consisted primarily of money market funds. (b) Marketable securities The Company classifies its marketable security investments as available-for-sale securities and the securities are stated at fair value. There were no realized gains or losses recognized on the maturity of available-for-sale securities during the quarter ended March 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 11), the Company purchased restricted common shares of Selecta. The investment is classified as available-for-sale and is stated at fair value.

Accrued expenses

Accrued expenses	3 Months Ended
	Mar. 31, 2017
Payables and Accruals [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following: December 31, March 31, Compensation and benefits $ 9,613,275 $ 3,893,575 Consulting and professional fees 995,614 1,759,313 Research and development 2,717,777 2,279,076 Other 500,254 1,165,812 $ 13,826,920 $ 9,097,776

Stockholders' equity

Stockholders' equity	3 Months Ended
	Mar. 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. For the three months ended March 31, 2016 , the Company recorded compensation expense of $15,769 in general and administrative expense and a reduction in compensation expense of $0.1 million in research and development expense related to the restricted shares. For the three months ended March 31, 2017 , the Company recorded compensation expense of $10,246 in general and administrative expense and expense of $0.4 million in research and development expense related to the restricted shares. 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 vested. The remaining shares are subject to certain milestone-based vesting conditions. No milestone vesting conditions were achieved in 2016 or during the three months ended March 31, 2017. The following table summarizes restricted stock activity: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2016 174,876 $ 5.78 Shares canceled (3,107 ) $ 1.15 Shares vested (42,811 ) $ 3.92 Nonvested shares at March 31, 2017 128,958 $ 6.52

Stock incentive plans

Stock incentive plans	3 Months Ended
	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock incentive plans	Stock incentive plans The Company's 2015 Stock Incentive Plan (the 2015 Plan) provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to employees, officers, directors, consultants and advisors. In January 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 March 31, 2017 , 867,094 shares were available for future grants under the 2015 Plan. In January 2017, the number of shares of common stock authorized for issuance under the 2015 Employee Stock Purchase Plan (the 2015 ESPP) automatically increased, pursuant to the terms of the 2015 ESPP, by 308,660 shares. The 2015 ESPP provides participating employees with the opportunity to purchase an aggregate of 791,476 shares of common stock. As of March 31, 2017, no shares were issued under the 2015 ESPP. Stock-based compensation expense Stock-based compensation expense by award type was as follows: Three months ended March 31, 2016 2017 Stock options $ 4,626,039 $ 6,716,344 Restricted stock 330,342 1,879,305 Employee stock purchase plan — 45,176 $ 4,956,381 $ 8,640,825 Of the $8.6 million of stock-based compensation expense incurred during the first quarter of 2017, $3.8 million is classified as research and development expense and $4.8 million is classified as general and administrative expense in the consolidated statements of operations and comprehensive loss. Of the $5.0 million of stock-based compensation expense incurred during the first quarter of 2016, $2.4 million is classified as research and development expense and $2.6 million is classified as general and administrative expense in the consolidated statements of operations and comprehensive loss. Stock options The following table summarizes stock option activity: Number of options Weighted- average exercise price Outstanding at December 31, 2016 4,181,993 $ 33.25 Granted 547,700 $ 56.43 Exercised (213,379 ) $ 16.09 Forfeited (114,954 ) $ 33.70 Outstanding at March 31, 2017 4,401,360 $ 36.95 Vested at March 31, 2017 1,351,681 $ 28.08 Restricted stock The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2016 76,933 $ 49.90 Shares granted 374,500 $ 57.64 Shares canceled (6,050 ) $ 55.97 Nonvested shares at March 31, 2017 445,383 $ 56.31 Included in the above table are non-vested restricted stock grants to certain employees and consultants totaling 42,600 shares for which the vesting provisions are based on achievement of certain Company milestones.

Related-party transactions

Related-party transactions	3 Months Ended
	Mar. 31, 2017
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions As of December 31, 2016 and March 31, 2017 , 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 three months ended March 31, 2016 and 2017 , the Company recorded $0.3 million and $0.1 million , respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying consolidated statements of operations and comprehensive 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 three months ended March 31, 2016 and 2017 , the Company recorded $1.9 million and $1.8 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 March 31, 2017 , $1.3 million and $2.0 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP.

Commitments and contingencies

Commitments and contingencies	3 Months Ended
	Mar. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Commitments and contingencies In January 2017, the Company amended its lease for office space in Philadelphia to lease approximately an additional 24,800 square feet that will commence on January 1, 2018 , and terminate on December 31, 2028 . The additional leased space will increase its future minimum lease payments by $11.5 million over the life of the lease.

Collaboration and license agree

Collaboration and license agreements	3 Months Ended
	Mar. 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 is primarily responsible for conducting all research and development activities through completion of Phase 1/2 clinical trials of hemophilia B product candidates. Pfizer and the Company will share development costs incurred under an agreed product development plan for each product candidate with the Company’s share of development costs under the agreement limited to $10.6 million . Following the completion of Phase 1/2 clinical trials, Pfizer will be primarily responsible for development, manufacture, regulatory approval and commercialization, including all costs associated therewith. In connection with this agreement, the Company received a $20.0 million upfront payment for the license in December 2014. As there is no stand-alone value for the license, the Company is recognizing revenue through the estimated completion date of Phase 1/2 clinical trials. During each of the three month periods ended March 31, 2016 and 2017 , the Company recognized $1.3 million of revenue related to the upfront payment. As of March 31, 2017 , there is $5.2 million and $2.6 million of current and long-term deferred revenue, respectively, related to this payment. During each of the three month periods ended March 31, 2016 and 2017 , the Company recorded $0.3 million and $1.0 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. (b) Selecta In December 2016, the Company entered into a License and Option Agreement (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. Each party is responsible for its own costs and expenses incurred in connection with its respective activities under the License Agreement, except that 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 . Additional payments in the aggregate of $5.0 million are due to Selecta within 12 months of execution of the agreement. On a target-by-target basis, the Company will be responsible to pay up to an aggregate of $430.0 million in milestone payments for each target, with up to $65.0 million being based on the Company’s achievement of specified development and regulatory milestones and up to $365.0 million for commercial milestones, as well as tiered royalties on global net sales at percentages ranging from mid-single to low-double digits. For a period of 3 years , the Company has the right to fund up to 50% of any development or regulatory milestone payable to Selecta by issuing to Selecta shares of the Company’s common stock having a fair market value equal to the percentage of such development or regulatory milestone, as applicable. The 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 . The fair value of these shares is classified as available for sale securities as of March 31, 2017. Under the terms of the SPA the Company will purchase additional shares of Selecta’s common stock for an aggregate of $10.0 million in 2017, unless the License Agreement is terminated. The Company accounted for 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 for the Selecta shares acquired based on the closing market price on the date of purchase. The Company allocated the remaining $11.5 million to the License Agreement, which was expensed as acquired in-process research and development as the Company determined there was no alternative future use.

Summary of significant accoun17

Summary of significant accounting policies (Policies)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying unaudited interim consolidated financial statements of the Company and its wholly-owned subsidiaries have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying consolidated financial statements include all normal and recurring adjustments (which consist primarily of accruals, estimates and assumptions that impact the financial statements) considered necessary to present fairly the Company’s financial position as of March 31, 2017 , its results of operations for the three months ended March 31, 2016 and 2017 and cash flows for the three months ended March 31, 2016 and 2017 . Operating results for the three months ended March 31, 2017 are not necessarily indicative of the results that may be expected for the year ending December 31, 2017 or any other period. The interim consolidated financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual consolidated financial statements. The accompanying unaudited interim consolidated financial statements should be read in conjunction with the annual audited consolidated financial statements and related notes as of and for the year ended December 31, 2016 , included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2016 .
Use of estimates	Use of estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates.
Fair value of financial instruments	Fair value of financial instruments Management believes that the carrying amounts of the Company’s consolidated 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.
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.
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 March 31, 2017 represents the net excess of rent expense over the actual cash paid for rent and 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. Comprehensive loss includes gains and losses related to changes in the fair value of available for sale securities and foreign currency translation.
Recent accounting pronouncements	Recent accounting pronouncements In February 2016, the FASB issued Accounting Standards Update (ASU) 2016-02, “Leases.” ASU 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. The updated guidance is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company has not evaluated the impact of the updated guidance on the Company's consolidated financial statements. In May 2014, the FASB issued updated guidance regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. The update 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 Company currently is evaluating the effect that this guidance may have on its consolidated financial statements as it relates to the Pfizer Inc. (Pfizer) collaboration agreement discussed in note 11.
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 March 31, 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. There were no realized gains or losses recognized on the maturity of available-for-sale securities during the quarter ended March 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 11), the Company purchased restricted common shares of Selecta. The investment is classified as available-for-sale and is stated at fair value.

Summary of significant accoun18

Summary of significant accounting policies (Tables)	3 Months Ended
	Mar. 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 March 31, 2016 and 2017 as they would be anti-dilutive: March 31, 2016 2017 Unvested restricted common shares 330,600 574,341 Stock options issued and outstanding 4,029,103 4,401,360
Schedule of Components of Accumulated Other Comprehensive 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 loss (300,708 ) (594,717 ) (895,425 ) Balance as of March 31, 2017 $ (1,102,425 ) $ (587,296 ) $ (1,689,721 )

Marketable securities (Tables)

Marketable securities (Tables)	3 Months Ended
	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Summary of Marketable Securities	The following table summarizes the available-for-sale securities held at December 31, 2016 and March 31, 2017 : Description 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 March 31, 2017 U.S. government agency $ 136,701,100 $ — $ (117,043 ) $ 136,584,057 Corporate securities $ 122,301,624 $ 5,183 $ (990,234 ) $ 121,316,573

Fair value of financial instr20

Fair value of financial instruments (Tables)	3 Months Ended
	Mar. 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) At 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 — At March 31, 2017: Assets: Money market funds (included in cash and cash equivalents) $ 26,648,005 — — Marketable securities - U.S. government agencies $ 136,584,057 — — Marketable securities - corporate securities $ 118,672,889 $ 2,643,684 —

Accrued expenses (Tables)

Accrued expenses (Tables)	3 Months Ended
	Mar. 31, 2017
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following: December 31, March 31, Compensation and benefits $ 9,613,275 $ 3,893,575 Consulting and professional fees 995,614 1,759,313 Research and development 2,717,777 2,279,076 Other 500,254 1,165,812 $ 13,826,920 $ 9,097,776

Stockholders' equity (Tables)

Stockholders' equity (Tables)	3 Months Ended
	Mar. 31, 2017
Equity [Abstract]
Summary of Restricted Stock Activity	The following table summarizes restricted stock activity: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2016 174,876 $ 5.78 Shares canceled (3,107 ) $ 1.15 Shares vested (42,811 ) $ 3.92 Nonvested shares at March 31, 2017 128,958 $ 6.52 The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2016 76,933 $ 49.90 Shares granted 374,500 $ 57.64 Shares canceled (6,050 ) $ 55.97 Nonvested shares at March 31, 2017 445,383 $ 56.31

Stock incentive plans (Tables)

Stock incentive plans (Tables)	3 Months Ended
	Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Compensation Expense by Award Type	Stock-based compensation expense by award type was as follows: Three months ended March 31, 2016 2017 Stock options $ 4,626,039 $ 6,716,344 Restricted stock 330,342 1,879,305 Employee stock purchase plan — 45,176 $ 4,956,381 $ 8,640,825
Summary of Stock Option Activity	The following table summarizes stock option activity: Number of options Weighted- average exercise price Outstanding at December 31, 2016 4,181,993 $ 33.25 Granted 547,700 $ 56.43 Exercised (213,379 ) $ 16.09 Forfeited (114,954 ) $ 33.70 Outstanding at March 31, 2017 4,401,360 $ 36.95 Vested at March 31, 2017 1,351,681 $ 28.08
Summary of Restricted Common Stock Activity	The following table summarizes restricted stock activity: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2016 174,876 $ 5.78 Shares canceled (3,107 ) $ 1.15 Shares vested (42,811 ) $ 3.92 Nonvested shares at March 31, 2017 128,958 $ 6.52 The following table summarizes restricted common stock activity: Number Weighted- Nonvested shares at December 31, 2016 76,933 $ 49.90 Shares granted 374,500 $ 57.64 Shares canceled (6,050 ) $ 55.97 Nonvested shares at March 31, 2017 445,383 $ 56.31

The Company - Additional Inform

The Company - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2017segment
Accounting Policies [Abstract]
Number of operating segment	1

Development-stage risks - Addit

Development-stage risks - Additional Information (Detail) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Development-stage Risk [Abstract]
Retained earnings (accumulated deficit)	$ 304,670,243	$ 252,380,911

Summary of significant accoun26

Summary of significant accounting policies - Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted Average Shares Outstanding (Detail) - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Unvested restricted common shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding (in shares)	574,341	330,600
Stock options issued and outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding (in shares)	4,401,360	4,029,103

Summary of significant accoun27

Summary of significant accounting policies - Other Comprehensive Income (Details)	3 Months Ended
	Mar. 31, 2017USD ($)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2016	$ 330,276,713
Balance as of March 31, 2017	289,554,719
Net Unrealized Losses on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2016	(801,717)
Current period other comprehensive loss	(300,708)
Balance as of March 31, 2017	(1,102,425)
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2016	7,421
Current period other comprehensive loss	(594,717)
Balance as of March 31, 2017	(587,296)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of December 31, 2016	(794,296)
Current period other comprehensive loss	(895,425)
Balance as of March 31, 2017	$ (1,689,721)

Marketable securities - Schedul

Marketable securities - Schedule of Available for Sale Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Mar. 31, 2017
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Remaining maturity period	2 years
U.S. government agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 133,690,267	$ 136,701,100
Unrealized gains	10,907	0
Unrealized losses	(85,714)	(117,043)
Fair value	133,615,460	136,584,057
Corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	126,253,903	122,301,624
Unrealized gains	0	5,183
Unrealized losses	(726,579)	(990,234)
Fair value	$ 125,527,324	$ 121,316,573

Fair value of financial instr29

Fair value of financial instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Fair Value Measurements Recurring [Member] | Money Market Funds [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	$ 26,648,005	$ 53,452,424
Fair Value Measurements Recurring [Member] | Money Market Funds [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Fair Value Measurements Recurring [Member] | Money Market Funds [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Fair Value Measurements Recurring [Member] | Corporate securities [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds (included in cash and cash equivalents)		5,029,838
U.S. government agency [Member]
Assets:
Fair value of marketable securities	136,584,057	133,615,460
U.S. government agency [Member] | Fair Value Measurements Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Fair value of marketable securities	136,584,057	133,615,460
U.S. government agency [Member] | Fair Value Measurements Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Fair value of marketable securities		0
Corporate securities [Member]
Assets:
Fair value of marketable securities	121,316,573	125,527,324
Corporate securities [Member] | Fair Value Measurements Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Fair value of marketable securities	118,672,889	122,144,692
Corporate securities [Member] | Fair Value Measurements Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Fair value of marketable securities	$ 2,643,684	$ 3,382,632

Accrued expenses - Schedule of

Accrued expenses - Schedule of Accrued Expenses (Detail) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Compensation and benefits	$ 3,893,575	$ 9,613,275
Consulting and professional fees	1,759,313	995,614
Research and development	2,279,076	2,717,777
Other	1,165,812	500,254
Accrued expenses	$ 9,097,776	$ 13,826,920

Stockholders' equity - Addition

Stockholders' equity - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended
	Dec. 31, 2014	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2016
Class of Stock [Line Items]
Common stock, shares authorized		150,000,000			150,000,000
Preferred stock, shares authorized		5,000,000			5,000,000
Compensation expense (reduction of expense)		$ 8,600,000	$ 8,640,825	$ 4,956,381
General and Administrative Expense [Member]
Class of Stock [Line Items]
Compensation expense (reduction of expense)		4,800,000		2,600,000
Research and Development Expense [Member]
Class of Stock [Line Items]
Compensation expense (reduction of expense)		$ 3,800,000		2,400,000
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]
Shares, vesting period		3 years
Restricted stock [Member]
Class of Stock [Line Items]
Compensation expense (reduction of expense)			1,879,305	$ 330,342
Shares vested (in shares)		42,811
Restricted stock [Member] | General and Administrative Expense [Member]
Class of Stock [Line Items]
Compensation expense (reduction of expense)		$ 10,246	15,769
Restricted stock [Member] | Research and Development Expense [Member]
Class of Stock [Line Items]
Compensation expense (reduction of expense)		$ 400,000	$ (100,000)
University of Pennsylvania [Member] | License Agreement [Member] | Restricted stock [Member]
Class of Stock [Line Items]
Issuance of restricted stock in connection with license agreement (in shares)	200,000
Shares vested (in shares)	100,000

Stockholders' equity - Summary

Stockholders' equity - Summary of Restricted Stock Activity (Detail) - Restricted stock [Member]	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Number of shares (in shares):
Nonvested beginning balance (in shares) | shares	174,876
Shares canceled (in shares) | shares	(3,107)
Shares vested (in shares) | shares	(42,811)
Nonvested ending balance (in shares) | shares	128,958
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in dollars per share) | $ / shares	$ 5.78
Shares canceled (in dollars per share) | $ / shares	1.15
Shares vested (in dollars per share) | $ / shares	3.92
Nonvested shares ending balance (in dollars per share) | $ / shares	$ 6.52

Stock incentive plans - Additio

Stock incentive plans - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended
	Jan. 31, 2017	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 8,600,000	$ 8,640,825	$ 4,956,381
Employee stock purchase plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 45,176	0
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock grants issued (in shares)		374,500
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized	1,234,641
Shares available for future grants		867,094
2015 Employee Stock Purchase Plan [Member] | Employee stock purchase plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized	308,660
Common stock reserved for issuance	791,476
Shares issued (in shares)		0
Certain employees and consultants [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock grants issued (in shares)		42,600
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 3,800,000		2,400,000
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 4,800,000		$ 2,600,000

Stock incentive plans - Share-b

Stock incentive plans - Share-based Compensation Expense (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 8,600,000	$ 8,640,825	$ 4,956,381
Stock option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense		6,716,344	4,626,039
Restricted stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense		1,879,305	330,342
Employee stock purchase plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense		$ 45,176	$ 0

Stock incentive plans - Summary

Stock incentive plans - Summary of Stock Option Activity (Detail)	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Number of shares (in shares):
Outstanding at beginning of period (in shares) | shares	4,181,993
Granted (in shares) | shares	547,700
Exercised (in shares) | shares	(213,379)
Forfeited (in shares) | shares	(114,954)
Outstanding at end of period (in shares) | shares	4,401,360
Vested at end of period (in shares) | shares	1,351,681
Weighted-average exercise price (in dollars per share):
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 33.25
Granted (in dollars per share) | $ / shares	56.43
Exercised (in dollars per share) | $ / shares	16.09
Forfeited (in dollars per share) | $ / shares	33.70
Outstanding at end of period (in dollars per share) | $ / shares	36.95
Vested at end of period (in dollars per share) | $ / shares	$ 28.08

Stock incentive plans - Summa36

Stock incentive plans - Summary of Restricted Common Stock Activity (Details) - Performance Shares [Member]	3 Months Ended
	Mar. 31, 2017$ / sharesshares
Number of shares (in shares):
Nonvested beginning balance (in shares) | shares	76,933
Shares granted (in shares) | shares	374,500
Shares canceled (in shares) | shares	(6,050)
Nonvested ending balance (in shares) | shares	445,383
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in dollars per share) | $ / shares	$ 49.90
Shares granted (in dollars per share) | $ / shares	57.64
Shares canceled (in dollars per share) | $ / shares	55.97
Nonvested shares ending balance (in dollars per share) | $ / shares	$ 56.31

Related-party transactions - Ad

Related-party transactions - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Related Party Transaction [Line Items]
Research and development expense related to agreements	$ 32,735,033	$ 18,251,900
General and administrative due to related party	21,413,818	8,873,861
CHOP [Member]
Related Party Transaction [Line Items]
Accrued expenses due to related party	1,300,000		$ 1,100,000
Accounts payable due to related party	2,000,000		$ 900,000
CHOP [Member] | Master Research Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense related to agreements	1,800,000	1,900,000
CHOP [Member] | General and Administrative Expense [Member] | Technology and License Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement of patent costs	$ 100,000	$ 300,000

Commitments and contingencies -

Commitments and contingencies - Additional Information (Details) - Sublease of Office Space [Member] - Corporate Headquarters in Philadelphia [Member] $ in Millions	Jan. 31, 2017USD ($)ft²
Operating Leased Assets [Line Items]
Area of leased property (in sq ft) | ft²	24,800
Increase future minimum payments due | $	$ 11.5

Collaboration and license agr39

Collaboration and license agreements - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		8 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2014	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2014
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development costs			$ 32,735,033	$ 18,251,900
Current portion of deferred revenue	$ 5,168,674		5,168,674
Long-term deferred revenue	3,865,885		2,591,418
Milestone payment, additional revenue potential to be earned			$ 230,000,000
Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement rights and obligations			Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024.
Development costs							$ 10,600,000
Upfront payment received for license		$ 20,000,000
Licenses revenue			$ 1,300,000	1,300,000
Current portion of deferred revenue			5,200,000
Long-term deferred revenue			$ 2,600,000
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]
Milestone payment, additional revenue potential to be earned			$ 110,000,000
Additional Product Candidates [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment, additional revenue potential to be earned			120,000,000
Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Prepaid expense, current	$ 10,000,000
Shares purchased (in shares)	197,238
Investment shares purchased, amount	$ 5,000,000
Payments to acquire project			15,000,000
Payments to acquire in process research and development			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]
Reimbursed costs			1,000,000	$ 300,000
Minimum [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option exercise fee	1,400,000
Maximum [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Option exercise fee	2,000,000
Milestone payments	$ 430,000,000
Milestone payment, rights to fund percentage	50.00%
Scenario, Forecast [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Prepaid expense, current						$ 5,000,000
Investment shares purchased, amount					$ 10,000,000
Common Stock [Member] | Selecta License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments to acquire project			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			$ 11,500,000