Sage Therapeutics (SAGE) 10-Q 15 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	SAGE
Entity Registrant Name	Sage Therapeutics, Inc.
Entity Central Index Key	1597553
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		28,366,696

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$113,162	$127,766
Prepaid expenses and other current assets	1,367	1,056
Total current assets	114,529	128,822
Property and equipment, net	269	163
Restricted cash	39	39
Deferred tax assets	641	641
Total assets	115,478	129,665
Current liabilities:
Accounts payable	2,782	2,429
Accrued expenses	5,522	4,687
Deferred tax liabilities	641	641
Total current liabilities	8,945	7,757
Other liabilities	23	23
Total liabilities	8,968	7,780
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.0001 par value; 5,000,000 shares authorized at March 31, 2015 and December 31, 2014, respectively; no shares issued or outstanding at March 31, 2015 and December 31, 2014, respectively
Common stock, $0.0001 par value; 120,000,000 shares authorized at March 31, 2015 and December 31, 2014, respectively; 25,694,560 and 25,621,791 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively	3	3
Additional paid-in capital	190,223	188,727
Accumulated deficit	-83,716	-66,845
Total stockholders' equity	106,510	121,885
Total liabilities and stockholders' equity	$115,478	$129,665

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
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	120,000,000	120,000,000
Common stock, shares issued	25,694,560	25,621,791
Common stock, shares outstanding	25,694,560	25,621,791

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive Loss (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating expenses:
Research and development	$12,900	$4,173
General and administrative	3,997	1,617
Total operating expenses	16,897	5,790
Loss from operations	-16,897	-5,790
Interest income (expense), net	21
Other income (expense), net	5
Net loss and comprehensive loss	-16,871	-5,790
Accretion of redeemable convertible preferred stock to redemption value		-326
Net loss attributable to common stockholders	($16,871)	($6,116)
Net loss per share attributable to common stockholders-basic and diluted	($0.66)	($3.70)
Weighted average number of common shares used in net loss per share attributable to common stockholders-basic and diluted	25,655,883	1,652,726

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities
Net loss	($16,871)	($5,790)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,349	160
Non-cash licensing and consulting fees	424	127
Depreciation and amortization	22	11
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	-134	-52
Accounts payable	343	-471
Accrued expenses and other	261	399
Net cash used in operating activities	-14,606	-5,616
Cash flows from investing activities
Purchase of property and equipment	-110	-3
Net cash used in investing activities	-110	-3
Cash flows from financing activities
Proceeds from the issuance of common stock and restricted stock, net	147	2
Payment of offering costs	-35
Net cash provided by financing activities	112	52,978
Net increase (decrease) in cash and cash equivalents	-14,604	47,359
Cash and cash equivalents at beginning of period	127,766	8,066
Cash and cash equivalents at end of period	113,162	55,425
Supplemental disclosure of non-cash investing and financing activities
Accretion of redeemable convertible preferred stock to redemption value		326
Deferred public offering costs included in accounts payable or accrued expenses	151	842
Series B Preferred Stock [Member]
Cash flows from financing activities
Proceeds from the issuance of preferred stock, net of issuance costs		14,995
Series C Preferred Stock [Member]
Cash flows from financing activities
Proceeds from the issuance of preferred stock, net of issuance costs		$37,981

Nature_of_the_Business

Nature of the Business	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Nature of the Business	1	Nature of the Business
	Sage Therapeutics, Inc. (“Sage” or the “Company”) is a biopharmaceutical company committed to developing and commercializing novel medicines to treat life-threatening, rare central nervous system (“CNS”) disorders, where there are inadequate or no approved existing therapies. The Company is targeting CNS indications where patient populations are easily identified, acute treatment is typically initiated in the hospital setting, clinical endpoints are well-defined, and development pathways are feasible. This focus allows the Company to make highly informed decisions when advancing its product candidates through the development process. The Company’s initial product candidates are aimed at treating different stages of status epilepticus, a life-threatening condition in which the brain is in a state of persistent seizure.
	The Company was incorporated under the laws of the state of Delaware on April 16, 2010 and commenced operations on January 19, 2011 as Sterogen Biopharma, Inc. On September 13, 2011, the Company changed its name to Sage Therapeutics, Inc. under its Second Amended and Restated Certificate of Incorporation.
	The Company is subject to risks and uncertainties common to early-stage companies in the biotech industry, including, but not limited to, development by competitors of new technological innovations, dependence on key personnel protection of proprietary technology, compliance with government regulations, and ability to secure additional capital to fund operations.
	We have incurred losses and negative cash flows from operations since our inception. As of March 31, 2015, we had an accumulated deficit of $83.7 million. From our inception through March 31, 2014, we raised aggregate net proceeds of $90.6 million from the issuance of Series A, Series B and Series C redeemable convertible preferred stock. In July 2014, we raised net proceeds of $94.0 million from the sale of common stock in our initial public offering. In April 2015, we raised net proceeds of approximately $129.2 million from the sale of common stock in our underwritten public offering. We believe our cash balance of $113.2 million as of March 31, 2015 will be sufficient to fund our anticipated level of operations for at least the next 12 months.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2	Summary of Significant Accounting Policies
	Basis of Presentation
	The unaudited interim consolidated financial statements of the Company included herein have been prepared, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2014.
	The unaudited interim consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of the Company’s management, the accompanying unaudited interim consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of March 31, 2015, the results of its operations for the three months ended March 31, 2015 and 2014, and its cash flows for the three months ended March 31, 2015 and 2014. Such adjustments are of a normal and recurring nature. The results for the three months ended March 31, 2015 are not indicative of the results for the year ending December 31, 2015, or for any future period.
	On July 23, 2014, the Company completed the sale of 5,750,000 shares of its common stock in its initial public offering (the “IPO”), at a price to the public of $18.00 per share, resulting in net proceeds to the Company of $94.0 million after deducting underwriting discounts and commissions and offering costs paid by the Company. The shares began trading on the Nasdaq Global Market on July 18, 2014.
	In connection with preparing for the IPO, the Company’s board of directors and stockholders approved a 1-for-3.15 reverse stock split of the Company’s common stock effective July 2, 2014. All share and per share amounts in the unaudited consolidated financial statements contained herein and notes thereto have been retroactively adjusted, where necessary, to give effect to this reverse stock split. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock as of July 23, 2014, resulting in the issuance by the Company of an additional 18,007,575 shares of common stock. The significant increase in common stock outstanding in July 2014 will impact the year-over-year comparability of the Company’s net loss per share calculations over the next year.
	On April 20, 2015, the Company completed the sale of 2,628,571 shares of common stock in its underwritten public offering of its common stock at a price to the public of $52.50 per share, resulting in net proceeds to the Company of approximately $129.2 million after deducting underwriting discounts and commissions and offering costs paid by the Company.
	Principles of Consolidation
	The consolidated financial statements include the accounts of the Company and its wholly owned subsidiary as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying its Annual Report on Form 10-K. Intercompany accounts and transactions have been eliminated.
	Recently Issued Accounting Pronouncements
	In May 2014, the Financial Accounting Standards Board (“FASB”), issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606), (“ASU 2014-09”). ASU 2014-09 outlines a new, single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. This new revenue recognition model provides a five-step analysis in determining when and how revenue is recognized. The new model will require revenue recognition to depict the transfer of promised goods or services to customers in an amount that reflects the consideration a company expects to receive in exchange for those goods or services. In April 2015, the FASB proposed a one year delay in the effective date of ASU 2014-09, which was originally effective for public entities for annual reporting periods beginning after December 15, 2016 and interim periods within those periods. Early adoption is not permitted. Companies may use either a full retrospective or a modified retrospective approach to adopt ASU 2014-09. The Company is currently assessing the method of adoption and the impact of this new accounting guidance will have on its consolidated financial statements and footnote disclosures.
	In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements — Going Concern (Subtopic 205-40). The new guidance addresses management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern and to provide related footnote disclosures. Management’s evaluation should be based on relevant conditions and events that are known and reasonably knowable at the date that the financial statements are issued. The standard will be effective for the first interim period within annual reporting periods beginning after December 15, 2016. Early adoption is permitted. The Company is evaluating the effect that this guidance will have on its consolidated financial statements.
	In April 2015, the FASB issued guidance simplifying the presentation of debt issuance costs. To simplify presentation of debt issue costs, the amendments require that debt issue costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The guidance becomes effective for the Company in its year ending December 31, 2016, and early adoption is permitted. The Company is currently assessing the impact that this standard will have on its consolidated financial statements.
	Use of Estimates
	The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the 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 those estimates.
	Fair Value Measurements
	Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three categories:
	Level 1		-	Quoted market prices in active markets for identical assets or liabilities. At March 31, 2015 and December 31, 2014, the Company’s Level 1 assets consisted of money market funds totaling $113,162 and $127,766 respectively.
	Level 2		-	Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. At March 31, 2015 and December 31, 2014, the Company had no Level 2 assets or liabilities.
	Level 3		-	Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. At March 31, 2015 and December 31, 2014, the Company had no Level 3 assets or liabilities.
	The Company’s financial instruments generally consist of cash equivalents, accounts payable and accrued expenses. The carrying amounts for the applicable financial instruments reported in the balance sheets approximate their fair values at March 31, 2015 (unaudited) and December 31, 2014.
	Deferred Offering Costs
	The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as other assets until such financings are consummated. After consummation of the equity financing in July 2014, $2,285 of these costs were recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the initial public offering. As of March 31, 2015 and December 31, 2014, the Company had recorded $186 and $10, respectively, of deferred offering costs, which are included in prepaid expenses and other current assets in the accompanying consolidated balance sheet. These costs were in contemplation of the Company’s public offering of common stock which closed in April 2015.
	Segment Data
	The company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on advancing medicines to treat central nervous system disorders, where there are inadequate or no approved existing therapies, including status epilepticus. All tangible assets are held within the U.S.

Accrued_Expenses

Accrued Expenses	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Accrued Expenses	3	Accrued Expenses
	Accrued expenses consist of the following:
			March 31, 2015		December 31, 2014
	Employee-related expenses		$	619	$	1,279
	Development costs			4,290		2,788
	Professional services			432		574
	Other accrued expenses			181		46
			$	5,522	$	4,687

Commitments_and_contingencies

Commitments and contingencies	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	4	Commitments and contingencies
	CyDex License Agreement
	In October 2011, the Company entered into a research and development license with CyDex Pharmaceuticals, Inc. (“CyDex”) for the development of drug product using licensed technology for a period of one year. Under the terms of the license agreement, the Company paid an initial licensing fee of $200 and an additional fee of $100 for CyDex to perform research and development services to evaluate the licensed technology for formulation with the Company’s developmental product.
	The $200 payment was recorded as research and development expense as the acquired technology was in-process research and development, and the $100 payment was recorded to research and development expense in 2011 and 2012 as services were performed.
	In December 2012, the Company exercised its option to enter into a commercial license and supply agreement for CyDex’s proprietary technology and paid $100 for the perpetual license, which was recorded as research and development expense.
	In August 2013, the Company entered into a commercial license agreement as a result of which the December 2012 license was terminated and the December 2012 supply agreement was amended. Specifically, CyDex granted the Company an exclusive license to the CyDex technology for use in the fields of status epilepticus and traumatic brain injury. In exchange, the Company is required to pay upfront, milestone and royalty-based compensation. In addition, CyDex granted the Company a research license to Captisol for allopregnanolone for use in proof of concept studies. The August 2013 agreement will continue in effect unless and until terminated. In consideration for the amended license rights, the Company paid $300. The Company is obligated to make milestone payments based on achievement of clinical development and regulatory milestones of $900 and $3,750, respectively. Also under this agreement, the Company is required to pay royalties in the low single digits based on levels of net sales.
	Under the amended supply agreement with CyDex, the Company is required to purchase all of its supply of Captisol from CyDex and CyDex is required to supply the Company with Captisol, subject to certain limitations.
	In April 2014, the Company amended its commercial license and supply agreements with CyDex to expand the fields of use to include the treatment, prevention or diagnosis of any disease or symptom in humans or animals. In consideration for the amended terms, the Company paid $200 upfront and is obligated to make milestone payments, once per field, based on the achievement of clinical development and regulatory milestones for the development of SAGE-547 in the fields of status epilepticus and traumatic brain injury of $750 and $3,750, respectively. For the development in two additional fields, the Company is obligated to make milestone payments, once per field, based on the achievement of clinical development and regulatory milestones of $1,250 and $8,500, respectively.
	In March 2015, a clinical development milestone was met for one of the programs. Accordingly, the Company recorded research and development expense for the three months ended March 31, 2015 of $250.
	In April 2015, an additional clinical development milestone was met for one of the programs. Accordingly, the Company will record research and development expense for the three months ended June 30, 2015 of $500.
	University of California License Agreement
	In October 2013, the Company entered into a non-exclusive license agreement with The Regents of the University of California whereby the Company was granted a non-exclusive license to certain clinical data and clinical material for use in the development and commercialization of biopharmaceutical products in the licensed field, including status epilepticus and post-partum depression. In May 2014, the license agreement was amended to add the treatment of essential tremor to the licensed field of use, materials and milestone fee provisions of the agreement.
	The Company will be required to pay clinical development milestones of up to $100 and pay royalties of less than 1% on net sales for a period of fifteen years following the sale of the first commercial product.
	The license will terminate on the earlier to occur of (i) 27 years after the effective date or (ii) 15 years after the last-derived product is first commercially sold.
	In March 2015, a clinical development milestone was met. Accordingly, the Company recorded research and development expenses for the three months ended March 31, 2015 totaling $75.
	Consulting Agreement
	In January 2014, the Company entered into a consulting agreement with a nonemployee advisor whereby the Company is obligated to make cash payments of up to $2,000 and to issue up to 126,984 shares of common stock upon attainment of certain clinical development and regulatory milestones.
	In January and March 2014, the first clinical development milestones for each of two programs included in the consulting agreement were met. Accordingly, the Company recorded research and development expense for the year ended December 31, 2014 of $177, comprised of $50 in cash and $127 related to the issuance of 15,872 shares of the Company’s common stock.
	In March 2015, the second clinical development milestone for one of the programs included in the consulting agreement was met. Accordingly, the Company recorded research and development expense for the three months ended March 31, 2015 of $574, comprised of $150 in cash and $424 related to the issuance of 7,936 shares of the Company’s common stock.
	In April 2015, the third clinical development milestone for one of the programs included in the consulting agreement was met. Accordingly, the Company will record research and development expense for the three months ended June 30, 2015 of $1,087, comprised of $300 in cash and $787 related to the issuance of 15,873 shares of the Company’s common stock.

StockBased_Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	5	Stock-Based Compensation
	2014 Stock Option Plan
	On July 2, 2014, the Company’s stockholders approved the 2014 Stock Option and Incentive Plan (the “2014 Stock Option Plan”), which became effective upon the completion of the IPO. The 2014 Stock Option Plan provides for the grant of restricted stock awards, incentive stock options, non-statutory stock options, among others. The 2014 Stock Option Plan replaced the Company’s 2011 Stock Option and Grant Plan (the “2011 Stock Option Plan”). The Company will grant no further stock options or other awards under the 2011 Stock Option Plan. Any options or awards outstanding under the 2011 Stock Option Plan remained outstanding and effective. As of March 31, 2015, the total number of shares reserved under all equity plans is 4,240,053 and the Company had 1,459,354 shares available for future issuance under such plans.
	The 2014 Stock Option Plan provides for an annual increase, to be added on the first day of each fiscal year, by up to 4% of the Company’s issued and outstanding shares of common stock on the immediately preceding December 31. On January 1, 2015, 773,779 shares of common stock, representing 3% of the Company’s issued and outstanding shares of common stock as of December 31, 2014, were added to the 2014 Stock Option Plan. Such shares are included in the equity plan totals specified in the paragraph above.
	2014 Employee Stock Purchase Plan
	On July 2, 2014, the Company’s stockholders approved the 2014 Employee Stock Purchase Plan. A total of 282,000 shares of common stock were initially authorized for issuance under this plan. The 2014 Employee Stock Purchase Plan became effective upon the completion of the IPO. As of March 31, 2015, no shares have been issued under this plan.
	Stock-Based Compensation
	Terms of restricted stock awards and stock option agreements, including vesting requirements, are determined by the Compensation Committee of the Board of Directors or the Board of Directors, subject to the provisions of the applicable stock option plan. Options and restricted stock awards granted by the Company generally vest based on the grantee’s continued service with the Company during a specified period following grant. Awards generally vest ratably over four years, with a 25% cliff vesting at the one year anniversary for new employee awards. During 2013, the Company also granted a pool of option awards which vest ratably over one year. During the three months ended March 31, 2015, the Company granted options to purchase 497,100 shares of common stock to employees, which contain performance-based vesting criteria. Performance-based vesting criteria for these options primarily relate to milestone events specific to the Company’s corporate goals, including but not limited to certain clinical and regulatory development milestones related to the Company’s product candidates. Recognition of stock-based compensation expense associated with these performance-based stock options commences when the performance condition is considered probable of achievement using management’s best estimates. As of March 31, 2015, no performance-based milestones had been achieved, achievement of the milestones was not deemed probable and no expense has been recognized related to these awards.
	All awards are exercisable from the date of grant for a period of ten years.
	The stock-based compensation expense recognized during the three months ended March 31, 2015 and 2014 was as follows:
			Three Months Ended March 31,
			2015			2014
	Stock compensation expense:
	Research and development		$	523		$	106
	General and administrative			826			54
			$	1,349		$	160
	For stock option awards, the fair value of the options is estimated at the grant date using the Black-Scholes option-pricing model, taking into account the terms and conditions upon which options are granted. The fair value of the options is amortized on a straight-line basis over the requisite service period of the awards. The weighted average grant date exercise price per share relating to outstanding stock options granted under the Company’s stock option plans during the three months ended March 31, 2015 and 2014 was $38.28 and $4.64, respectively. The weighted average Black-Scholes value per share relating to outstanding stock options granted under the Company’s stock option plans during the three months ended March 31, 2015 was $28.76.
	The fair value of each option granted to employees and directors during the three months ended March 31, 2015 and 2014 under the Company’s stock option plans has been calculated on the date of grant using the following weighted average assumptions:
	Black-Scholes Assumptions:
			Three Months Ended			Three Months Ended
			31-Mar-15			31-Mar-14
	Expected dividend yield			0	%		0	%
	Expected volatility			93	%		99.85	%
	Risk free interest rate			1.46	%		1.98	%
	Expected term			5.89 years			5.98 years
	Expected dividend yield: The Company has not paid and does not anticipate paying any dividends in the foreseeable future.
	Risk-free interest rate: The Company determined the risk-free interest rate by using a weighted average equivalent to the expected term based on the U.S. Treasury yield curve in effect as of the date of grant.
	Expected volatility: As the Company has only been a public company since July 2014, there is not sufficient historical volatility for the expected term of the options. Therefore, the Company used an average historical share price volatility based on an analysis of reported data for a peer group of comparable companies.
	Expected term (in years): Expected term represents the period that the Company’s share option grants are expected to be outstanding. As the Company has only been a public company since July 2014, there is not sufficient historical term data to calculate the expected term of the options. Therefore, the Company elected to utilize the “simplified” method to estimate the expected term of option grants issued to employees. Under this approach, the weighted average expected life is presumed to be the average of the vesting term and the contractual term of the option.
	Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates. The Company estimates forfeitures based on historical termination behavior. For the three months ended March 31, 2015 and 2014, a forfeiture rate of 10% was applied.
	For options granted to nonemployees, the expected life of the option used is ten years, which is the contractual term of each such option. All other assumptions used to calculate the grant date fair value are generally consistent with the assumptions used for options granted to employees.
	The table below summarizes activity related to stock options:
			Shares			Weighted-Average			Weighted-Average		Aggregate Intrinsic
						Exercise Price			Remaining Life		Value
									(in years)
	Outstanding as of December 31, 2014			1,996,615		$	7.01			8.98	$	59,362
	Granted			904,475			38.28
	Exercised			(39,594	)		3.72					1,756
	Forfeited			(80,797	)		26.01					1,595
	Outstanding as of March 31, 2015			2,780,699		$	16.67			9.07	$	93,152
	Vested or expected to vest as of March 31, 2015			2,462,001		$	15.89			9.04	$	84,546
	Exercisable as of March 31, 2015			458,113		$	1.06			8.44	$	22,370
	As of March 31, 2015, the Company had unrecognized stock-based compensation expense related to its unvested stock option awards of $17,223, which is expected to be recognized over the remaining weighted average vesting period of 3.01 years. The total fair value of shares vested for the three months ended March 31, 2015 and 2014 was $380 and 81, respectively. During the three months ended March 31, 2015 and 2014, stock option exercises resulted in proceeds of $147 and $2, respectively. The intrinsic value of stock options exercised during the three months ended March 31, 2015 and 2014 was $1,756 and $30, respectively.
	Restricted Stock Awards
	The Company granted restricted stock awards to certain officers, employees, directors, and consultants of the Company. During the three months ended March 31, 2015 and 2014, the Company recorded $82 and $22, respectively, of stock-based compensation expense related to its restricted stock.
	The table below summarizes activity relating to restricted stock:
			Shares			Weighted Average
						Grant Date Fair
						Value Per Share
	Outstanding as of December 31, 2014			170,832		$	—
	Issued			—			—
	Vested			(33,175	)		—
	Forfeited			—			—
	Repurchased			—			—
	Outstanding as of March 31, 2015			137,657			—
	As of March 31, 2015 and 2014, the Company had unrecognized stock-based compensation expense related to its unvested restricted stock awards of $266 and $123, respectively, which is expected to be recognized over the remaining weighted average vesting period of 0.91 years and 1.9 years, respectively.
	Unvested shares are subject to repurchase by the Company, at the issuance price, upon the employee’s termination at the Company’s sole discretion. No shares of restricted stock were repurchased in the three months ended March 31, 2015.

Net_Loss_Per_Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Net Loss Per Share	6	Net Loss Per Share
	Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the three months ended March 31, 2015 and 2014:
			Three Months Ended March 31,
			2015			2014
	Basic net loss per share attributable to common stockholders:
	Numerator:
	Net loss		$	(16,871	)	$	(6,116	)
	Denominator:
	Weighted average common shares outstanding—basic			25,655,883			1,652,726
	Dilutive effect of common share equivalents resulting from common share options and preferred common shares (as converted)			—			—
	Weighted average common shares outstanding—diluted			25,655,883			1,652,726
	Net loss per share attributable to common stockholders—basic and diluted		$	(0.66	)	$	(3.70	)
	The following common stock equivalents outstanding as of March 31, 2015 and 2014 were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been anti-dilutive:
			Three Months Ended March 31,
			2015			2014
	Options to purchase common stock			1,736,871			1,269,418
	Restricted stock			137,184			268,960
	Redeemable convertible preferred stock (presented on a weighted average basis)			—			15,121,600
				1,874,055			16,659,978

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	7	Income Taxes
	The Company did not record a federal or state income tax benefit for the Company’s losses for the three months ended March 31, 2015 and 2014 due to the Company’s conclusion that a valuation allowance is required.

Related_Party_Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Related Party Transactions	8	Related Party Transactions
	Since inception, the Company has received consulting and management services from Third Rock Ventures LLC, which through its affiliates, owns 37.8% of the Company’s common stock at March 31, 2015. The Company incurred $8 and $99 for these services during the three months ended March 31, 2015 and 2014, respectively. At March 31, 2015, the Company owed Third Rock Ventures LLC $4, which is included in accounts payable. At December 31, 2014, the Company owed Third Rock Ventures LLC $5, which is included in accrued expenses.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation
	The unaudited interim consolidated financial statements of the Company included herein have been prepared, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these consolidated financial statements should be read in conjunction with the consolidated financial statements as of and for the year ended December 31, 2014.
	The unaudited interim consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of the Company’s management, the accompanying unaudited interim consolidated financial statements contain all adjustments which are necessary to present fairly the Company’s financial position as of March 31, 2015, the results of its operations for the three months ended March 31, 2015 and 2014, and its cash flows for the three months ended March 31, 2015 and 2014. Such adjustments are of a normal and recurring nature. The results for the three months ended March 31, 2015 are not indicative of the results for the year ending December 31, 2015, or for any future period.
	On July 23, 2014, the Company completed the sale of 5,750,000 shares of its common stock in its initial public offering (the “IPO”), at a price to the public of $18.00 per share, resulting in net proceeds to the Company of $94.0 million after deducting underwriting discounts and commissions and offering costs paid by the Company. The shares began trading on the Nasdaq Global Market on July 18, 2014.
	In connection with preparing for the IPO, the Company’s board of directors and stockholders approved a 1-for-3.15 reverse stock split of the Company’s common stock effective July 2, 2014. All share and per share amounts in the unaudited consolidated financial statements contained herein and notes thereto have been retroactively adjusted, where necessary, to give effect to this reverse stock split. In connection with the closing of the IPO, all of the Company’s outstanding redeemable convertible preferred stock automatically converted into shares of common stock as of July 23, 2014, resulting in the issuance by the Company of an additional 18,007,575 shares of common stock. The significant increase in common stock outstanding in July 2014 will impact the year-over-year comparability of the Company’s net loss per share calculations over the next year.
	On April 20, 2015, the Company completed the sale of 2,628,571 shares of common stock in its underwritten public offering of its common stock at a price to the public of $52.50 per share, resulting in net proceeds to the Company of approximately $129.2 million after deducting underwriting discounts and commissions and offering costs paid by the Company.
Principles of Consolidation	Principles of Consolidation
	The consolidated financial statements include the accounts of the Company and its wholly owned subsidiary as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying its Annual Report on Form 10-K. Intercompany accounts and transactions have been eliminated.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements
	In May 2014, the Financial Accounting Standards Board (“FASB”), issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606), (“ASU 2014-09”). ASU 2014-09 outlines a new, single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. This new revenue recognition model provides a five-step analysis in determining when and how revenue is recognized. The new model will require revenue recognition to depict the transfer of promised goods or services to customers in an amount that reflects the consideration a company expects to receive in exchange for those goods or services. In April 2015, the FASB proposed a one year delay in the effective date of ASU 2014-09, which was originally effective for public entities for annual reporting periods beginning after December 15, 2016 and interim periods within those periods. Early adoption is not permitted. Companies may use either a full retrospective or a modified retrospective approach to adopt ASU 2014-09. The Company is currently assessing the method of adoption and the impact of this new accounting guidance will have on its consolidated financial statements and footnote disclosures.
	In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements — Going Concern (Subtopic 205-40). The new guidance addresses management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern and to provide related footnote disclosures. Management’s evaluation should be based on relevant conditions and events that are known and reasonably knowable at the date that the financial statements are issued. The standard will be effective for the first interim period within annual reporting periods beginning after December 15, 2016. Early adoption is permitted. The Company is evaluating the effect that this guidance will have on its consolidated financial statements.
	In April 2015, the FASB issued guidance simplifying the presentation of debt issuance costs. To simplify presentation of debt issue costs, the amendments require that debt issue costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The guidance becomes effective for the Company in its year ending December 31, 2016, and early adoption is permitted. The Company is currently assessing the impact that this standard will have on its consolidated financial statements.
Use of Estimates	Use of Estimates
	The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the 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 those estimates.
Fair Value Measurements	Fair Value Measurements
	Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three categories:
	Level 1	-	Quoted market prices in active markets for identical assets or liabilities. At March 31, 2015 and December 31, 2014, the Company’s Level 1 assets consisted of money market funds totaling $113,162 and $127,766 respectively.
	Level 2	-	Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. At March 31, 2015 and December 31, 2014, the Company had no Level 2 assets or liabilities.
	Level 3	-	Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. At March 31, 2015 and December 31, 2014, the Company had no Level 3 assets or liabilities.
	The Company’s financial instruments generally consist of cash equivalents, accounts payable and accrued expenses. The carrying amounts for the applicable financial instruments reported in the balance sheets approximate their fair values at March 31, 2015 (unaudited) and December 31, 2014.
Deferred Offering Costs	Deferred Offering Costs
	The Company capitalizes certain legal, accounting and other third-party fees that are directly associated with in-process equity financings as other assets until such financings are consummated. After consummation of the equity financing in July 2014, $2,285 of these costs were recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the initial public offering. As of March 31, 2015 and December 31, 2014, the Company had recorded $186 and $10, respectively, of deferred offering costs, which are included in prepaid expenses and other current assets in the accompanying consolidated balance sheet. These costs were in contemplation of the Company’s public offering of common stock which closed in April 2015.
Segment Data	Segment Data
	The company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on advancing medicines to treat central nervous system disorders, where there are inadequate or no approved existing therapies, including status epilepticus. All tangible assets are held within the U.S.

Accrued_Expenses_Tables

Accrued Expenses (Tables)	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Summary of Accrued Expenses	Accrued expenses consist of the following:
		March 31, 2015		December 31, 2014
	Employee-related expenses	$	619	$	1,279
	Development costs		4,290		2,788
	Professional services		432		574
	Other accrued expenses		181		46
		$	5,522	$	4,687

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Recognized Stock-Based Compensation Expense	The stock-based compensation expense recognized during the three months ended March 31, 2015 and 2014 was as follows:
		Three Months Ended March 31,
		2015			2014
	Stock compensation expense:
	Research and development	$	523		$	106
	General and administrative		826			54
		$	1,349		$	160
Summary of Weighted Average Assumptions Used to Compute Fair Value of Option Granted	The fair value of each option granted to employees and directors during the three months ended March 31, 2015 and 2014 under the Company’s stock option plans has been calculated on the date of grant using the following weighted average assumptions:
	Black-Scholes Assumptions:
		Three Months Ended			Three Months Ended
		31-Mar-15			31-Mar-14
	Expected dividend yield		0	%		0	%
	Expected volatility		93	%		99.85	%
	Risk free interest rate		1.46	%		1.98	%
	Expected term		5.89 years			5.98 years
Summary of Activity Relating to Stock Options	The table below summarizes activity related to stock options:
		Shares			Weighted-Average			Weighted-Average		Aggregate Intrinsic
					Exercise Price			Remaining Life		Value
								(in years)
	Outstanding as of December 31, 2014		1,996,615		$	7.01			8.98	$	59,362
	Granted		904,475			38.28
	Exercised		(39,594	)		3.72					1,756
	Forfeited		(80,797	)		26.01					1,595
	Outstanding as of March 31, 2015		2,780,699		$	16.67			9.07	$	93,152
	Vested or expected to vest as of March 31, 2015		2,462,001		$	15.89			9.04	$	84,546
	Exercisable as of March 31, 2015		458,113		$	1.06			8.44	$	22,370
Summary of Activity Relating to Restricted Stock	The table below summarizes activity relating to restricted stock:
		Shares			Weighted Average
					Grant Date Fair
					Value Per Share
	Outstanding as of December 31, 2014		170,832		$	—
	Issued		—			—
	Vested		(33,175	)		—
	Forfeited		—			—
	Repurchased		—			—
	Outstanding as of March 31, 2015		137,657			—

Net_Loss_Per_Share_Tables

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Summary of Basic and Diluted Net Income (Loss) Per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the three months ended March 31, 2015 and 2014:
		Three Months Ended March 31,
		2015			2014
	Basic net loss per share attributable to common stockholders:
	Numerator:
	Net loss	$	(16,871	)	$	(6,116	)
	Denominator:
	Weighted average common shares outstanding—basic		25,655,883			1,652,726
	Dilutive effect of common share equivalents resulting from common share options and preferred common shares (as converted)		—			—
	Weighted average common shares outstanding—diluted		25,655,883			1,652,726
	Net loss per share attributable to common stockholders—basic and diluted	$	(0.66	)	$	(3.70	)
Summary of Common Stock Equivalents Outstanding	The following common stock equivalents outstanding as of March 31, 2015 and 2014 were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been anti-dilutive:
		Three Months Ended March 31,
		2015			2014
	Options to purchase common stock		1,736,871			1,269,418
	Restricted stock		137,184			268,960
	Redeemable convertible preferred stock (presented on a weighted average basis)		—			15,121,600
			1,874,055			16,659,978

Nature_of_the_Business_Additio

Nature of the Business - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	0 Months Ended	1 Months Ended
	Jul. 31, 2014	Mar. 31, 2015	Apr. 20, 2015	Apr. 30, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Nature Of Business [Line Items]
Accumulated deficit		($83,716,000)			($66,845,000)
Net proceeds from sale of common stock	94,000,000
Cash balance		113,162,000			127,766,000	55,425,000	8,066,000
Redeemable Convertible Preferred Stock [Member]
Nature Of Business [Line Items]
Convertible preferred stock proceeds raised		90,600,000
Subsequent Events [Member] | Underwritten Public Offering [Member]
Nature Of Business [Line Items]
Proceeds from underwritten initial public offering of common stock, net proceeds			$129,200,000	$129,200,000

Summary_of_Significant_Account2

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	0 Months Ended		1 Months Ended	3 Months Ended	0 Months Ended	1 Months Ended
	Jul. 23, 2014	Jul. 02, 2014	Jul. 31, 2014	Mar. 31, 2015	Apr. 20, 2015	Apr. 30, 2015	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts			$94,000,000
Reverse stock split description				In connection with preparing for the IPO, the Company's board of directors and stockholders approved a 1-for-3.15 reverse stock split of the Company's common stock
Reverse stock split effective date				2-Jul-14
Reverse stock split ratio		0.3175
Issuance of additional shares of common stock upon conversion	18,007,575
Payment of offering costs			2,285,000	35,000
Deferred offering cots				186,000			10,000
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Summary Of Significant Accounting Policies [Line Items]
Assets fair value				113,162,000			127,766,000
Fair Value, Inputs, Level 2 [Member]
Summary Of Significant Accounting Policies [Line Items]
Assets fair value				0			0
Liabilities fair value				0			0
Fair Value, Inputs, Level 3 [Member]
Summary Of Significant Accounting Policies [Line Items]
Assets fair value				0			0
Liabilities fair value				0			0
Initial Public Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock, shares	5,750,000
Common stock price per share	18
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	94,000,000
Underwritten Public Offering [Member] | Subsequent Events [Member]
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock, shares					2,628,571
Common stock price per share					$52.50
Proceeds from underwritten initial public offering of common stock, net proceeds					$129,200,000	$129,200,000

Accrued_Expenses_Summary_of_Ac

Accrued Expenses - Summary of Accrued Expenses (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Payables and Accruals [Abstract]
Employee related expenses	$619	$1,279
Development costs	4,290	2,788
Professional services	432	574
Other accrued expenses	181	46
Total accrued expenses	$5,522	$4,687

Commitments_and_contingencies_

Commitments and contingencies - Additional Information (Detail) (USD $)	3 Months Ended			1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Jun. 30, 2015	Jan. 31, 2014	Dec. 31, 2014	Oct. 31, 2011	Dec. 31, 2012	Aug. 31, 2013	Apr. 30, 2014	Oct. 31, 2013
Commitments And Contingencies [Line Items]
Research and development expense	$12,900	$4,173
Clinical Development [Member]
Commitments And Contingencies [Line Items]
Research and development expense	250
Clinical Development [Member] | Scenario, Forecast [Member]
Commitments And Contingencies [Line Items]
Research and development expense			500
University of California License Agreement [Member]
Commitments And Contingencies [Line Items]
License payment description	The Company will be required to pay clinical development milestones of up to $100 and pay royalties of less than 1% on net sales for a period of fifteen years following the sale of the first commercial product.
Percentage of net sales	1.00%
University of California License Agreement [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period	27 years
University of California License Agreement [Member] | Minimum [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period	15 years
University of California License Agreement [Member] | Clinical Development [Member]
Commitments And Contingencies [Line Items]
Research and development expense	75
Expected milestone payments										100
Consulting Agreement [Member] | Clinical Development And Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Milestone payment				2,000
Issuance of common stock, shares				126,984
Consulting Agreement [Member] | First Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense					177
Issuance of common stock, shares					15,872
Milestone payments					50
Payments of common stock issuance costs					127
Consulting Agreement [Member] | Second Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense	574
Issuance of common stock, shares	7,936
Milestone payments	150
Payments of common stock issuance costs	424
Consulting Agreement [Member] | Third Clinical Development Milestones [Member] | Scenario, Forecast [Member]
Commitments And Contingencies [Line Items]
Research and development expense			1,087
Issuance of common stock, shares			15,873
Milestone payments			300
Payments of common stock issuance costs			787
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
License agreement term	1 year
Initial licensing fee						200
Additional license fee						100
Research and development expense	200						100
Commercial License and Supply Agreements [Member] | Research and Development [Member]
Commitments And Contingencies [Line Items]
Perpetual license fee							100
License Agreement Terms [Member]
Commitments And Contingencies [Line Items]
Upfront payment								300
License Agreement Terms [Member] | Clinical Development [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments								900
License Agreement Terms [Member] | Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments								3,750
Amended and Restated License Agreement [Member]
Commitments And Contingencies [Line Items]
Upfront payment									200
Amended and Restated License Agreement [Member] | Clinical Development [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments									1,250
Amended and Restated License Agreement [Member] | Clinical Development And Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	750	3,750
Amended and Restated License Agreement [Member] | Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments									$8,500

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended	0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2013	Jan. 01, 2015	Jul. 02, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$1,349,000	$160,000
Share option, forfeiture rate	10.00%	10.00%
Contractual term, option	10 years
2014 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under the plan					282,000
Number of shares issued under the plan	0
Share based compensation granted under plan vest period	4 years		1 year
Share based compensation, vest period	1 year
Share based compensation, vesting percentage	25.00%
Share based compensation, term of plan	10 years
Weighted average grant date fair value per share	$38.28	$4.64
2014 Employee Stock Purchase Plan [Member] | Black Scholes Stock Price Valuation Model [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value per share	$28.76
2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of shares reserved under equity plan	4,240,053
Number of shares available for future issuance under the plan	1,459,354
Percentage of increase on issued and outstanding shares of Common stock				3.00%
Common stock available for issuance under 2014 Stock Option Plan				773,779
2014 Stock Option Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of increase on issued and outstanding shares of Common stock					4.00%
2011 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option grant	0
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	82,000	22,000
Unrecognized stock-based compensation expense	266,000	123,000
Weighted average period of unrecognized compensation costs	10 months 28 days	1 year 10 months 24 days
Stock repurchased, shares	0
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option grant	904,475
Unrecognized stock-based compensation expense	17,223,000
Weighted average period of unrecognized compensation costs	3 years 4 days
Total fair value of shares vested	380,000	81,000
Proceeds from stock options exercised	147,000	2,000
Intrinsic value of options exercised	1,756,000	30,000
Performance Milestone [Member] | 2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under the plan	497,100
Stock-based compensation expense	$0

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Recognized Stock-Based Compensation Expense (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$1,349,000	$160,000
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	523,000	106,000
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$826,000	$54,000

StockBased_Compensation_Summar1

Stock-Based Compensation - Summary of Weighted Average Assumptions Used to Compute Fair Value of Option Granted (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Expected dividend yield	0.00%	0.00%
Expected volatility	93.00%	99.85%
Risk free interest rate	1.46%	1.98%
Expected term	5 years 10 months 21 days	5 years 11 months 23 days

StockBased_Compensation_Summar2

Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) (Stock Options [Member], USD $)	3 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Shares-Stock options	1,996,615
Granted, Shares-Stock options	904,475
Exercised, Shares-Stock options	-39,594
Forfeited, Shares-Stock options	-80,797
Ending balance, Outstanding Shares-Stock options	2,780,699		1,996,615
Vested or expected to vest, Shares-Stock options	2,462,001
Exercisable, Shares-Stock options	458,113
Beginning balance, Weighted Average Exercise Price-Stock options	$7.01
Granted, Weighted Average Exercise Price-Stock options	$38.28
Exercised, Weighted Average Exercise Price-Stock options	$3.72
Forfeited, Weighted Average Exercise Price-Stock options	$26.01
Ending balance, Outstanding Weighted Average Exercise Price-Stock options	$16.67		$7.01
Vested or expected to vest, Weighted Average Exercise Price-Stock options	$15.89
Exercisable, Weighted Average Exercise Price-Stock options	$1.06
Outstanding, Weighted Average Remaining Life-Stock options	9 years 26 days		8 years 11 months 23 days
Vested or expected to vest, Weighted Average Remaining Life-Stock options	9 years 15 days
Exercisable, Weighted Average Remaining Life-Stock options	8 years 5 months 9 days
Exercised, Aggregate Intrinsic Value-Stock options	$1,756	$30
Forfeited, Aggregate Intrinsic Value-Stock options	1,595
Outstanding, Aggregate Intrinsic Value-Stock options	93,152		59,362
Vested or expected to vest, Aggregate Intrinsic Value-Stock options	84,546
Exercisable, Aggregate Intrinsic Value-Stock options	$22,370

StockBased_Compensation_Summar3

Stock-Based Compensation - Summary of Activity Relating to Restricted Stock (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]
Unvested restricted common stock, Shares at beginning balance-Restricted stock	170,832
Issued, Shares-Restricted stock	0
Vested, Shares-Restricted stock	-33,175
Forfeited, Shares-Restricted stock	0
Repurchased, Shares-Restricted stock	0
Unvested restricted common stock, Shares at ending balance-Restricted stock	137,657
Unvested restricted common stock , Weighted Average Grant Date Fair Value Per Share, beginning balance-Restricted stock	$0
Issued, Weighted Average Grant Date Fair Value Per Share-Restricted stock	$0
Vested, Weighted Average Grant Date Fair Value Per Share-Restricted stock	$0
Forfeited, Weighted Average Grant Date Fair Value Per Share-Restricted stock	$0
Repurchased, Weighted Average Grant Date Fair Value Per Share-Restricted stock	$0
Unvested restricted common stock, Weighted Average Grant Date Fair Value Per Share, ending balance-Restricted stock	$0

Net_Loss_Per_Share_Schedule_of

Net Loss Per Share - Schedule of Basic and Diluted Net Income (Loss) Per Share Attributable to Common Stockholders (Detail) (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Numerator:
Net loss	($16,871)	($6,116)
Denominator:
Weighted average common shares outstanding-basic	25,655,883	1,652,726
Dilutive effect of common share equivalents resulting from common share options and preferred common shares (as converted)	0	0
Weighted average common shares outstanding-diluted	25,655,883	1,652,726
Net loss per share attributable to common stockholders-basic and diluted	($0.66)	($3.70)

Net_Loss_Per_Share_Summary_of_

Net Loss Per Share - Summary of Common Stock Equivalents Outstanding (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	1,874,055	16,659,978
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	1,736,871	1,269,418
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	137,184	268,960
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents		15,121,600

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
State income tax benefit	$0	$0
Federal income tax benefit	$0	$0

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Detail) (Third Rock Ventures LLC [Member], USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Third Rock Ventures LLC [Member]
Related Party Transaction [Line Items]
Payment for consulting and management services	$8	$99
Accrued expenses to related parties			5
Accounts payable to related parties	$4
Percentage of ownership held with related party transactions	37.80%