Spark Therapeutics (ONCE) 10-Q 11 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	1-May-15
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	ONCE
Entity Registrant Name	Spark Therapeutics, Inc.
Entity Central Index Key	1609351
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		24,543,923

Balance_sheets

Balance sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$234,412,118	$74,566,963
Other receivables	669,399	244,393
Prepaid expenses and deferred financing costs	776,113	2,551,912
Total current assets	235,857,630	77,363,268
Property and equipment, net	13,399,394	12,674,372
Other assets	408,211	408,211
Total assets	249,665,235	90,445,851
Current liabilities:
Accounts payable	3,135,451	2,676,697
Accrued expenses and other	2,448,366	3,163,154
Current portion of deferred revenue	5,578,558	10,014,377
Total current liabilities	11,162,375	15,854,228
Deferred rent	8,651,297	8,618,489
Long-term deferred revenue	12,928,766	10,767,414
Total liabilities	32,742,438	35,240,131
Stockholders' equity:
Common stock, $0.001 par value. Authorized, 150,000,000 shares; issued and outstanding, 6,290,317 and 24,543,923 shares at December 31, 2014 and March 31, 2015, respectively.	24,544	6,290
Additional paid-in capital	308,234,366	54,364,833
Accumulated deficit	-91,336,113	-81,602,606
Total stockholders' equity	216,922,797	55,205,720
Total liabilities and stockholders' equity	249,665,235	90,445,851
Series A Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value		10,000,000
Series B Convertible Preferred Stock [Member]
Stockholders' equity:
Preferred stock, value		$72,437,203

Balance_sheets_Parenthetical

Balance sheets (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Preferred stock , par value	$0.00	$0.00
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	$0.00	$0.00
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	24,543,923	6,290,317
Common stock, shares outstanding	24,543,923	6,290,317
Series A Convertible Preferred Stock [Member]
Preferred stock , par value	$0.00	$0.00
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued		5,000,000
Preferred stock, shares outstanding		5,000,000
Series B Convertible Preferred Stock [Member]
Preferred stock , par value	$0.00	$0.00
Preferred stock, shares authorized	45,186,334	45,186,334
Preferred stock, shares issued		45,186,334
Preferred stock, shares outstanding		45,186,334

Statements_of_operations

Statements of operations (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Revenues	$2,274,467	$20,000
Operating expenses:
Research and development	8,334,108	3,387,733
General and administrative	3,684,880	1,016,123
Total operating expenses	12,018,988	4,403,856
Loss from operations	-9,744,521	-4,383,856
Interest income	11,014
Net loss	-9,733,507	-4,383,856
Preferred stock dividends	-634,794
Net loss applicable to common stockholders	($10,368,301)	($4,383,856)
Basic and diluted net loss per common share	($0.58)	($0.86)
Weighted average basic and diluted common shares outstanding	17,925,296	5,100,885

Statement_of_stockholders_equi

Statement of stockholders' equity (USD $)	Total	Series A Convertible Preferred Stock [Member]	Series A Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Series B Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
			Series A Preferred Stock [Member]		Series B Preferred Stock [Member]		Series A Preferred Stock [Member]	Series B Preferred Stock [Member]		Series A Preferred Stock [Member]	Series B Preferred Stock [Member]
			Initial Public Offering [Member]		Initial Public Offering [Member]		Initial Public Offering [Member]	Initial Public Offering [Member]		Initial Public Offering [Member]	Initial Public Offering [Member]
Beginning Balance at Dec. 31, 2014	$55,205,720	$10,000,000		$72,437,203		$6,290			$54,364,833			($81,602,606)
Beginning Balance, Shares at Dec. 31, 2014		5,000,000		45,186,334		6,290,317
Conversion of preferred stock and dividends to common stock upon initial public offering			-10,000,000		-72,437,203		1,016	9,184		9,998,984	72,428,019
Conversion of preferred stock and dividends to common stock upon initial public offering, Shares			-5,000,000		-45,186,334		1,016,219	9,183,831
Issuance of common stock, net of issuance costs	168,892,493					8,050			168,884,443
Issuance of common stock, net of issuance costs, Shares						8,050,000
Issuance of common stock for services	193,909					4			193,905
Issuance of common stock for services, Shares						3,556
Stock-based compensation expense	2,364,182								2,364,182
Net loss	-9,733,507											-9,733,507
Ending Balance at Mar. 31, 2015	$216,922,797					$24,544			$308,234,366			($91,336,113)
Ending Balance, Shares at Mar. 31, 2015						24,543,923

Statements_of_cash_flows

Statements of cash flows (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities:
Net loss	($9,733,507)	($4,383,856)
Adjustments to reconcile net loss to net cash used in operating activities:
Noncash rent expense	208,027
Depreciation expense	370,570
Stock-based compensation expense	2,558,091	389,253
Changes in operating assets and liabilities:
Prepaid expenses and other assets	-939,021	-156,070
Accounts payable and accrued expenses	870,391	722,601
Deferred revenue	-2,274,467
Net cash used in operating activities	-8,939,916	-3,428,072
Cash flows from investing activities:
Purchases of property and equipment	-1,685,771	-37,060
Net cash used in investing activities	-1,685,771	-37,060
Cash flows from financing activities:
Proceeds from issuance of Series A convertible preferred units		4,861,285
Proceeds from issuance of common stock, net of financing costs	170,470,842
Net cash provided by financing activities	170,470,842	4,861,285
Net increase in cash and cash equivalents	159,845,155	1,396,153
Cash and cash equivalents, beginning of period	74,566,963
Cash and cash equivalents, end of period	234,412,118	1,396,153
Supplemental disclosure of cash flow information:
Deferred financing costs included in accounts payable and accrued expenses	157,407	10,519
Property and equipment purchases included in accounts payable and accrued expenses	$557,021	$59,160

Background

Background	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Background	(1) 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.
	(a) Initial Public Offering (IPO)
	On February 4, 2015, the Company completed its 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 expenses. 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.

Developmentstage_risks

Development-stage risks	3 Months Ended
	Mar. 31, 2015
Development Stage Enterprises [Abstract]
Development-stage risks	(2) Development-stage risks
	The Company has incurred losses and negative cash flows from operations since inception and had an accumulated deficit of $91.3 million at March 31, 2015. The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of its product candidates in development. Additional financing will be needed by the Company 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_account

Summary of significant accounting policies	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of significant accounting policies	(3) Summary of significant accounting policies
	(a) Basis of presentation
	The accompanying unaudited interim financial statements of the Company have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying 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, 2015 and its results of operations and cash flows for the three months ended March 31, 2014 and 2015. Operating results for the three months ended March 31, 2015 are not necessarily indicative of the results that may be expected for the year ending December 31, 2015. The interim financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual financial statements.
	The accompanying unaudited interim financial statements should be read in conjunction with the annual audited financial statements and related notes as of and for the year ended December 31, 2014 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2014.
	(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 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 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.
	(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 March 31, 2015 consisted primarily of money market funds.
	(e) 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.
	(f) 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.
	(g) 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) Topic 605-25, Revenue Recognition for Arrangements with Multiple Elements, 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 Topic 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 Topic 605-28, milestone method of revenue recognition. FASB ASC Topic 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 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.
	(h) Recapitalization
	On January 16, 2015, the Company effected a reverse stock split of the Company’s common stock at a ratio of one share for every five shares previously held. All common stock share and common stock per share data included in these financial statements reflect the reverse stock split.
	(i) 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, the outstanding shares of Series A convertible preferred stock (Series A Stock), 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, 2014 and 2015 as they would be anti-dilutive:
			March 31,		March 31,
			2014		2015
	Convertible preferred shares			1,000,000		—
	Unvested restricted common shares			923,283		507,673
	Options issued and outstanding			—		3,027,947
	Amounts in the table above reflect the common stock equivalents of the noted instruments.
	(j) 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, which was May 1, 2014 for the Company’s corporate headquarters. Tenant improvement allowances from the lessor are included in the accompanying 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, 2015 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received/receivable.

Fair_value_of_financial_instru

Fair value of financial instruments	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair value of financial instruments	(4) 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		Significant		Significant
			in active		other		unobservable
			markets for		observable		inputs
			identical		inputs		(Level 3)
			assets		(Level 2)
			(Level 1)
	At December 31, 2014:
	Assets:
	Money market funds (included in cash and cash equivalents)		$	74,025,841		—		—
	At March 31, 2015:
	Assets:
	Money market funds (included in cash and cash equivalents)		$	233,976,601		—		—

Accrued_expenses

Accrued expenses	3 Months Ended
	Mar. 31, 2015
Payables and Accruals [Abstract]
Accrued expenses	(5) Accrued expenses
	Accrued expenses consist of the following:
		December 31,		March 31,
		2014		2015
	Compensation and benefits	$	1,385,013	$	1,235,473
	Consulting and professional fees		1,327,942		188,363
	Research and development		247,448		702,466
	Other		202,751		146,845
		$	3,163,154	$	2,273,147

Stockholders_equity

Stockholders' equity	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Stockholders' equity	(6) 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.
	(a) Convertible preferred
	October 2013 Series A financing
	In October 2013, the Company entered into an agreement with The Children’s Hospital of Philadelphia (CHOP) to sell 5,000,000 Series A Units at $2.00 per unit for proceeds of $10.0 million. Each Series A Unit was convertible into one Series 1 Unit (subject to certain antidilution adjustments) at any time at the option of the holder. The Series A Units were mandatorily convertible into common stock in the event of an IPO, as defined.
	May 2014 conversion to C corporation
	Upon conversion of the Company into a C corporation in May 2014, each outstanding Series A Unit converted into one share of Series A Stock.
	May 2014 Series B financing
	In May 2014, the Company issued 45,186,334 shares of Series B convertible preferred stock (Series B Stock) for $72.4 million, net of transaction costs. In conjunction with the issuance of Series B Stock, certain Series A Stock terms were amended. Every five shares of Series A Stock and Series B Stock were to automatically convert into one share of common stock at a qualified IPO, as defined, or upon approval by at least 87.5% of the Series B Stock holders, subject to certain customary antidilution adjustments contained in the Company’s certificate of incorporation. The Series A Stock and Series B Stock were entitled to receive cumulative dividends at 8% per annum, which accrued from day to day beginning November 23, 2014 and were payable upon conversion, an event of liquidation or a qualified IPO, in each case, in shares of Series A Stock and Series B Stock, as applicable. As of February 4, 2015, dividends of $1.3 million had accumulated, and in connection with the IPO, were declared and converted along with all outstanding shares of Series A Stock and Series B Stock into an aggregate of 10,200,050 shares of common stock.
	(b) Common
	Through May 1, 2014, the Board designated Series 1 Units, Series 2 Units and Series 3 common units (Series 3 Units). Capital distributions were to be made to and among the holders, in the following order of priority: Series A Units, Series 1 Units, Series 2 Units and Series 3 Units. Upon conversion of the Company into a C corporation in May 2014, each outstanding Series 1 Unit converted into 0.2 shares (post-split) of common stock, each outstanding Series 2 Unit converted into 0.2 shares (post-split) of common stock and each outstanding Series 3 Unit converted into 0.03883773 shares (post-split) of common stock. In 2013 and 2014, the Company issued Series 2 Units and Series 3 Units to various employees, directors and consultants of the Company. The Series 2 Units and Series 3 Units vesting terms vary, but primarily, shares vest 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. Upon conversion, the vesting terms of the previously issued equity remained consistent.
	For the three months ended March 31, 2014, the Company recorded compensation expense of $0.2 million and $0.2 million in general and administrative expense and research and development expense, respectively, related to the restricted units. For the three months ended March 31, 2015, the Company recorded compensation expense of $15,847 and $1.2 million in general and administrative expense and research and development expense, respectively, related to the restricted shares. At March 31, 2015, there was $3.7 million of unrecognized compensation expense related to restricted common shares which is expected to be recognized over a weighted-average period of 2.3 years.
	The following table summarizes restricted stock activity:
		Number			Weighted-
		of shares			average
					grant date
					fair value
	Nonvested shares at December 31, 2014		578,994		$	4.46
	Shares vested		(71,321	)		5.61
	Nonvested shares at March 31, 2015		507,673			4.56

Stock_incentive_plans

Stock incentive plans	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock incentive plans	(7) Stock incentive plans
	In May 2014, the Company established the 2014 Stock Incentive Plan (the 2014 Plan), which allows for the granting of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards and other stock awards of the Company’s common stock to employees, officers, directors, consultants and advisors. In January 2015, upon the IPO, the plan was terminated and the 209,500 shares available for future grants under the 2014 Plan were rolled into the 2015 Stock Incentive Plan (the 2015 Plan).
	In January 2015, the Company established the 2015 Plan, which became effective immediately prior to the closing of the IPO. 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, cancelled, 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. As of March 31, 2015, 1,276,050 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 will provide 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. No shares were issued under the 2015 ESPP plan as of March 31, 2015.
	The following table summarizes stock option activity:
			Number			Weighted-		Aggregate
			of shares			average		intrinsic
						exercise		value(a)
						price
	Outstanding at December 31, 2014			2,264,497		$	4.52
	Granted			763,450			60.15
	Outstanding at March 31, 2015			3,027,947			18.55
	Vested at March 31, 2015			80,100			3.45	$	5,931,405
	Vested at March 31, 2015 and expected to vest			3,027,947			18.55	$	178,499,471
	(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.
	The weighted average remaining contractual term of options outstanding as of March 31, 2015 is 9.5 years. The weighted average remaining contractual term of options exercisable as of March 31, 2015 is 9.2 years.
	During the three months ended March 31, 2015, the Company recorded compensation expense of $0.5 million and $0.7 million in research and development expense and general and administrative expense, respectively, related to stock options. At March 31, 2015, there was $36.7 million of unrecognized compensation expense related to stock options, which is expected to be recognized over a weighted average period of 3.8 years.
	The weighted average grant date fair value of the options granted during the three months ended March 31, 2015 was estimated at $40.27 per share using the Black-Scholes option-pricing model with the following weighted-average assumptions:
	Expected volatility			75.3	%
	Risk-free interest rate			1.66	%
	Expected term (in years)			6.1
	Expected dividend yield			0	%

Relatedparty_transactions

Related-party transactions	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Related-party transactions	(8) Related-party transactions
	As of December 31, 2014 and March 31, 2015, 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, 2014 and 2015, the Company recorded $23,848 and $0.2 million, respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying statement of operations.
	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, 2014, the Company recorded $2.4 million as research and development expense and $10,266 as general and administrative expense related to these agreements. For the three months ended March 31, 2015, the Company recorded $0.9 million as research and development expense related to these agreements.
	As of December 31, 2014, $0.1 million and $0.9 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP. As of March 31, 2015, $0.1 million and $0.7 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP.

Collaboration_and_license_agre

Collaboration and license agreements	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Collaboration and license agreements	(9) Collaboration and license agreements
	In March 2014, the Company entered into an agreement with Genable Technologies Limited (Genable) in which the Company will be the exclusive manufacturer and provide development advice and expertise in the ongoing development of Genable’s lead therapeutic product. Under a license agreement, the Company also granted certain rights to manufacturing patent applications. The Company is eligible to earn development milestone payments and mid-single-digit royalties on future product sales. During the three months ended March 31, 2014, the Company received $20,000 for the license and recognized it as revenue. As the final product being manufactured is subject to final testing, the Company will recognize manufacturing revenues when product is released. As of March 31, 2015, $0.4 million remains in current deferred revenue related to the Genable agreement.
	In April 2014, the Company began discussions with a biopharmaceutical company concerning a potential manufacturing technology agreement. The Company received a one-time, nonrefundable payment of $1.0 million to engage in due diligence. In March 2015, the Company concluded discussions on a potential arrangement with the biopharmaceutical company and, as a result, the Company recognized the nonrefundable payment of $1.0 million as revenue during the three months ended March 31, 2015.
	In December 2014, the Company entered into a global collaboration agreement with Pfizer Inc. (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. During the three months ended March 31, 2015, the Company recognized $1.3 million of revenue. As of March 31, 2015, there is $5.2 million and $12.9 million of current and long term deferred revenue for this payment, respectively. During the three months ended March 31, 2015, the Company recorded $0.4 million as a reduction to research and development expenses for the reimbursement of costs from Pfizer.
	The Company also is eligible to receive up to $260.0 million in aggregate milestone payments, $140.0 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 April 2015, the Company entered into a research, license and option agreement with Clearside Biomedical, Inc. (Clearside) under which the Company acquired exclusive rights to license Clearside’s microinjector technology and the option to further develop and commercialize gene therapy products delivered using the Clearside technology. Under the agreement, the companies will explore the feasibility of using Clearside’s microinjector technology to deliver viral vectors to the choroid and the retina through the suprachoroidal space.

Summary_of_significant_account1

Summary of significant accounting policies (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of presentation	(a) Basis of presentation
	The accompanying unaudited interim financial statements of the Company have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying 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, 2015 and its results of operations and cash flows for the three months ended March 31, 2014 and 2015. Operating results for the three months ended March 31, 2015 are not necessarily indicative of the results that may be expected for the year ending December 31, 2015. The interim financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual financial statements.
	The accompanying unaudited interim financial statements should be read in conjunction with the annual audited financial statements and related notes as of and for the year ended December 31, 2014 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2014.
Use of estimates	(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 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	(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.
Cash and cash equivalents	(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 March 31, 2015 consisted primarily of money market funds.
Property and equipment	(e) 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.
Research and development and in-process research and development	(f) 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.
Revenue recognition	(g) 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) Topic 605-25, Revenue Recognition for Arrangements with Multiple Elements, 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 Topic 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 Topic 605-28, milestone method of revenue recognition. FASB ASC Topic 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 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.
Recapitalization	(h) Recapitalization
	On January 16, 2015, the Company effected a reverse stock split of the Company’s common stock at a ratio of one share for every five shares previously held. All common stock share and common stock per share data included in these financial statements reflect the reverse stock split.
Net loss per common share	(i) 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, the outstanding shares of Series A convertible preferred stock (Series A Stock), 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, 2014 and 2015 as they would be anti-dilutive:
			March 31,		March 31,
			2014		2015
	Convertible preferred shares			1,000,000		—
	Unvested restricted common shares			923,283		507,673
	Options issued and outstanding			—		3,027,947
	Amounts in the table above reflect the common stock equivalents of the noted instruments.
Deferred rent	(j) 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, which was May 1, 2014 for the Company’s corporate headquarters. Tenant improvement allowances from the lessor are included in the accompanying 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, 2015 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received/receivable.

Summary_of_significant_account2

Summary of significant accounting policies (Tables)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Schedule of Antidilutive 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, 2014 and 2015 as they would be anti-dilutive:
		March 31,		March 31,
		2014		2015
	Convertible preferred shares		1,000,000		—
	Unvested restricted common shares		923,283		507,673
	Options issued and outstanding		—		3,027,947

Fair_value_of_financial_instru1

Fair value of financial instruments (Tables)	3 Months Ended
	Mar. 31, 2015
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		Significant		Significant
		in active		other		unobservable
		markets for		observable		inputs
		identical		inputs		(Level 3)
		assets		(Level 2)
		(Level 1)
	At December 31, 2014:
	Assets:
	Money market funds (included in cash and cash equivalents)	$	74,025,841		—		—
	At March 31, 2015:
	Assets:
	Money market funds (included in cash and cash equivalents)	$	233,976,601		—		—

Accrued_expenses_Tables

Accrued expenses (Tables)	3 Months Ended
	Mar. 31, 2015
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following:
		December 31,		March 31,
		2014		2015
	Compensation and benefits	$	1,385,013	$	1,235,473
	Consulting and professional fees		1,327,942		188,363
	Research and development		247,448		702,466
	Other		202,751		146,845
		$	3,163,154	$	2,273,147

Stockholders_equity_Tables

Stockholders' equity (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Summary of Restricted Stock Activity	The following table summarizes restricted stock activity:
		Number			Weighted-
		of shares			average
					grant date
					fair value
	Nonvested shares at December 31, 2014		578,994		$	4.46
	Shares vested		(71,321	)		5.61
	Nonvested shares at March 31, 2015		507,673			4.56

Stock_incentive_plans_Tables

Stock incentive plans (Tables)	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Stock Option Activity	The following table summarizes stock option activity:
			Number			Weighted-		Aggregate
			of shares			average		intrinsic
						exercise		value(a)
						price
	Outstanding at December 31, 2014			2,264,497		$	4.52
	Granted			763,450			60.15
	Outstanding at March 31, 2015			3,027,947			18.55
	Vested at March 31, 2015			80,100			3.45	$	5,931,405
	Vested at March 31, 2015 and expected to vest			3,027,947			18.55	$	178,499,471
	(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.
Summary of Weighted-Average Assumptions	The weighted average grant date fair value of the options granted during the three months ended March 31, 2015 was estimated at $40.27 per share using the Black-Scholes option-pricing model with the following weighted-average assumptions:
	Expected volatility			75.3	%
	Risk-free interest rate			1.66	%
	Expected term (in years)			6.1
	Expected dividend yield			0	%

Background_Additional_Informat

Background - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended
In Millions, except Share data, unless otherwise specified	Feb. 04, 2015	Mar. 31, 2015	Feb. 04, 2015
		Segment
Class of Stock [Line Items]
Number of operating segment		1
Common stock, shares issued during IPO	8,050,000
Aggregate gross proceeds from IPO	$185.20
Net proceeds from IPO	$168.90
Preferred stock, shares issued upon conversion	10,200,050		10,200,050
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued during IPO		8,050,000
Initial Public Offering [Member]
Class of Stock [Line Items]
IPO Completion date		4-Feb-15
Initial Public Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Stock issuance price per share	$23		23

DevelopmentStage_Risks_Additio

Development-Stage Risks - Additional Information (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Refinery Through Puts [Abstract]
Accumulated deficit	($91,336,113)	($81,602,606)

Recovered_Sheet1

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended	25 Months Ended
	Jan. 16, 2015	Mar. 31, 2015	Mar. 31, 2015
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Impairment charges of long-lived assets			$0
Reverse stock split, description		One share for every five shares previously held.
Conversion ratio of common stock	0.2
Computers & Software [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful life		3 years
Laboratory and Office Equipment [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful life		5 years
Furniture & Fixtures [Member]
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful life		7 years
Leasehold Improvements [Member]
Schedule Of Summary Of Significant Accounting Policies [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

Recovered_Sheet2

Summary of Significant Accounting Policies - Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted Average Shares Outstanding (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
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	507,673	923,283
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	3,027,947
Convertible Preferred Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding		1,000,000

Recovered_Sheet3

Fair Value of Financial Instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Fair Value Measurements Recurring [Member], Money Market Funds [Member], Quoted Prices in Active Markets for Identical Assets (Level 1) [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
Fair Value Measurements Recurring [Member] | Money Market Funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets:
Assets fair value disclosure	$233,976,601	$74,025,841

Accrued_Expenses_Schedule_of_A

Accrued Expenses - Schedule of Accrued Expenses (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Payables and Accruals [Abstract]
Compensation and benefits	$1,235,473	$1,385,013
Consulting and professional fees	188,363	1,327,942
Research and development	702,466	247,448
Other	146,845	202,751
Accrued expenses	$2,273,147	$3,163,154

Stockholders_Equity_Additional

Stockholders' Equity - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended	1 Months Ended	3 Months Ended	1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 04, 2015	31-May-14	31-May-15	Oct. 31, 2013	Dec. 31, 2014
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
Preferred stock, shares issued	0						0
Preferred stock, shares issued upon conversion			10,200,050
Accumulated dividends			1,300,000
Unrecognized compensation expense related to restricted common shares	3,700,000
Unrecognized compensation expense related to restricted common shares expected to be recognized, weighted-average period	2 years 3 months 18 days
General and Administrative Expense [Member]
Class of Stock [Line Items]
Compensation expense	15,847	200,000
Research and Development Expense [Member]
Class of Stock [Line Items]
Compensation expense	1,200,000	200,000
Common Stock [Member]
Class of Stock [Line Items]
Preferred shares converted into number of common stock			10,200,050
Percentage of vesting of units on first anniversary	25.00%
Units, vesting period	3 years
Units, vesting description	The Series 2 Units and Series 3 Units vesting terms vary, but primarily, shares vest 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.
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock terms of conversion	Upon conversion of the Company into a C corporation in May 2014, each outstanding Series A Unit converted into one share of Series A Stock.
Preferred stock, shares issued upon conversion	1
Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized	45,186,334						45,186,334
Preferred stock, shares issued				45,186,334			45,186,334
Convertible preferred stock terms of conversion	Every five shares of Series A Stock and Series B Stock were to automatically convert into one share of common stock at a qualified IPO, as defined, or upon approval by at least 87.5% of the Series B Stock holders
Preferred stock, shares issued upon conversion				0.2
Preferred stock shares, Value				72,400,000			72,437,203
Minimum percentage of approval by outstanding preferred share holders for preferred stock conversion to common stock				87.50%
Series A and B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock dividend rate				8.00%
Series 1 Unit [Member]
Class of Stock [Line Items]
Convertible preferred stock terms of conversion					Each Series A Unit was convertible into one Series 1 Unit (subject to certain antidilution adjustments) at any time at the option of the holder.
Preferred stock, shares issued upon conversion						1
Series 1 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion				0.2
Series 2 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion				0.2
Series 3 Unit [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion				0.03883773
CHOP [Member] | Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued						5,000,000
Stock issuance price per share						$2
Gross proceeds of common stock						$10,000,000

Stockholders_Equity_Summary_of

Stockholders' Equity - Summary of Restricted Stock Activity (Detail) (Restricted Stock [Member], USD $)	3 Months Ended
	Mar. 31, 2015
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested beginning balance, Number of shares	578,994
Shares vested, Number of shares	-71,321
Nonvested ending balance, Number of shares	507,673
Nonvested shares Beginning balance, Weighted Average Grant Date Fair Value	$4.46
Shares vested, Weighted Average Grant Date Fair Value	$5.61
Nonvested shares Ending balance, Weighted Average Grant Date Fair Value	$4.56

Stock_Incentive_Plans_Addition

Stock Incentive Plans - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Jan. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining contractual term of options exercisable	9 years 6 months
Weighted average remaining contractual term of options outstanding	9 years 2 months 12 days
Unrecognized compensation expense related to stock options	$36,700,000
Weighted average period recognition for unrecognized compensation expense	2 years 3 months 18 days
Weighted average grant date fair value of options	$40.27
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	15,847	200,000
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	1,200,000	200,000
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period recognition for unrecognized compensation expense	3 years 9 months 18 days
Employee Stock Option [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	700,000
Employee Stock Option [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$500,000
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants	1,276,050		209,500
Common stock reserved for issuance			1,830,000
Shares of common stock			1,724,000
Percentage of common stock outstanding			4.00%
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, cancelled, 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]
Common stock reserved for issuance			220,000
Shares of common stock			440,000
Percentage of common stock outstanding			1.00%
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. No shares were issued under the 2015 ESPP plan as of March 31, 2015.
Number of shares issued	0

Stock_Incentive_Plans_Summary_

Stock Incentive Plans - Summary of Stock Option Activity (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of shares, Outstanding	2,264,497
Number of shares, Granted	763,450
Number of shares, Outstanding	3,027,947
Number of shares, Vested	80,100
Number of shares, Vested and expected to vest	3,027,947
Weighted-average exercise price, Outstanding	$4.52
Weighted-average exercise price, Granted	$60.15
Weighted-average exercise price, Outstanding	$18.55
Weighted-average exercise price, Vested	$3.45
Weighted-average exercise price, Vested and expected to vest	$18.55
Aggregate intrinsic value, Vested	$5,931,405
Aggregate intrinsic value, Vested and expected to vest	$178,499,471

Stock_Incentive_Plans_Summary_1

Stock Incentive Plans - Summary of Weighted-Average Assumptions (Detail)	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Expected volatility	75.30%
Risk-free interest rate	1.66%
Expected term (in years)	6 years 1 month 6 days
Expected dividend yield	0.00%

RelatedParty_Transactions_Addi

Related-Party Transactions - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Related Party Transaction [Line Items]
Research and development expense related to agreements	$8,334,108	$3,387,733
General and administrative expense related to agreements	3,684,880	1,016,123
CHOP [Member]
Related Party Transaction [Line Items]
Accrued expenses due to related party	100,000		100,000
Accounts payable due to related party	700,000		900,000
CHOP [Member] | Master Research Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense related to agreements	900,000	2,400,000
General and administrative expense related to agreements		10,266
CHOP [Member] | General and Administrative Expense [Member] | Technology and License Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement of patent costs	$200,000	$23,848

Recovered_Sheet4

Collaboration and License Agreements - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2014	Apr. 30, 2014
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Current portion of deferred revenue	$5,578,558		$10,014,377	$10,014,377
Nonrefundable payment received					1,000,000
Development costs	8,334,108	3,387,733
Long term deferred revenue	12,928,766		10,767,414	10,767,414
Revenue recognition from aggregate milestone payments	260,000,000
First Product Candidate [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition from aggregate milestone payments	140,000,000
Additional Product Candidates [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognition from aggregate milestone payments	120,000,000
Genable License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Licenses revenue		20,000
Current portion of deferred revenue	400,000
Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Licenses revenue	1,300,000
Current portion of deferred revenue	5,200,000
Development costs				10,600,000
Upfront payment received for license			20,000,000
Long term deferred revenue	12,900,000
Reimbursement revenue	$100,000
Collaboration agreement rights, 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.
Collaboration 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.