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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2014	Nov. 01, 2014
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	Q3
Trading Symbol	SAGE
Entity Registrant Name	Sage Therapeutics, Inc.
Entity Central Index Key	0001597553
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,792,623

Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 136,727	$ 8,066
Prepaid expenses and other current assets	1,063	341
Total current assets	137,790	8,407
Property and equipment, net	134	86
Restricted cash	39	39
Total assets	137,963	8,532
Current liabilities:
Accounts payable	2,157	1,988
Accrued expenses	2,853	327
Total current liabilities	5,010	2,315
Other liabilities:	34	44
Total liabilities	5,044	2,359
Redeemable convertible preferred stock		37,709
Stockholders' equity (deficit):
Common stock, $0.0001 par value; 70,623,905 and 66,000,000 shares authorized at September 30, 2014 and December 31, 2013, respectively; 25,586,295 and 1,622,761 shares issued and outstanding at September 30, 2014 and December 31, 2013, respectively	3
Additional paid-in capital	187,400	139
Accumulated deficit	(54,484)	(31,675)
Total stockholders' equity (deficit)	132,919	(31,536)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 137,963	$ 8,532

Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	37,750,000
Redeemable convertible preferred stock, shares issued	0	37,750,000
Redeemable convertible preferred stock, shares outstanding	0	37,750,000
Redeemable convertible preferred stock, liquidation preference	$ 0	$ 40,663
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	70,623,905	66,000,000
Common stock, shares issued	25,586,295	1,622,761
Common stock, shares outstanding	25,586,295	1,622,761

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Operating expenses:
Research and development	$ 6,601	$ 3,408	$ 15,155	$ 9,845
General and administrative	2,869	1,111	6,294	2,719
Total operating expenses	9,470	4,519	21,449	12,564
Loss from operations	(9,470)	(4,519)	(21,449)	(12,564)
Interest income (expense), net	3		4
Other income (expense), net	(1)		(5)	1
Net loss and comprehensive loss	(9,468)	(4,519)	(21,450)	(12,563)
Accretion of redeemable convertible preferred stock to redemption value	(391)		(2,294)	(4)
Net loss attributable to common stockholders	$ (9,859)	$ (4,519)	$ (23,744)	$ (12,563)
Net loss per share attributable to common stockholders-basic and diluted	$ (0.50)	$ (2.98)	$ (3.08)	$ (8.56)
Weighted average number of common shares used in net loss per share attributable to common stockholders-basic and diluted	19,581,624	1,514,838	7,711,038	1,467,387

Statement of Changes in Redeemable Convertible Preferred Stock and Stockholder's Equity (Deficit) - 9 months ended Sep. 30, 2014 - USD ($) $ in Thousands	Total	Initial Public Offering [Member]	Series A, B and C Redeemable Convertible Preferred Stock[Member]	Series A, B and C Redeemable Convertible Preferred Stock[Member] Series B Preferred Stock [Member]	Series A, B and C Redeemable Convertible Preferred Stock[Member] Series C Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Initial Public Offering [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Initial Public Offering [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2013	$ (31,536)		$ 37,709					$ 139		$ (31,675)
Balances, Shares at Dec. 31, 2013			37,750,000			1,622,761
Shares issued during period, Amount		$ 93,974		$ 14,970	$ 37,890		$ 1		$ 93,973
Shares issued during period, Shares				9,999,999	8,973,905		5,750,000
Issuance of common stock from exercise of stock options, Amount	39							39
Issuance of common stock from exercise of stock options, Shares						85,157
Vesting of restricted stock, Amount	10							10
Vesting of restricted stock, Shares						104,930
Issuance of common stock in payment of consultant fees, Amount	127							127
Issuance of common stock in payment of consultant fees, Shares						15,872
Stock-based compensation expense	1,186							1,186
Accretion of redeemable convertible preferred stock to redemption value	(2,294)		$ 2,294					(935)		(1,359)
Conversion of Preferred Stock, Amount	92,863		$ (92,863)			$ 2		92,861
Conversion of Preferred Stock, Shares			(56,723,904)			18,007,575
Net loss	(21,450)									(21,450)
Balances at Sep. 30, 2014	$ 132,919					$ 3		$ 187,400		$ (54,484)
Balances, Shares at Sep. 30, 2014						25,586,295

Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Sep. 30, 2014	Sep. 30, 2013
Cash flows from operating activities
Net loss	$ (21,450)	$ (12,563)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	1,186	32
Licensing or consulting fees paid in common stock	127
Depreciation and amortization	35	34
Changes in operating assets and liabilities
Prepaid expenses and other current assets	(722)	(311)
Accounts payable	(6)	(40)
Accrued expenses and other	2,455	371
Net cash used in operating activities	(18,375)	(12,477)
Cash flows from investing activities
Purchase of property and equipment	(83)	(3)
Restricted cash
Net cash used in investing activities	(83)	(3)
Cash flows from financing activities
Proceeds from convertible notes payable
Proceeds from the issuance of common stock and restricted stock, net	39	37
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts	96,255
Payment of offering costs	(2,035)
Net cash provided by financing activities	147,119	22,522
Net increase in cash and cash equivalents	128,661	10,042
Cash and cash equivalents at beginning of period	8,066	2,802
Cash and cash equivalents at end of period	136,727	12,844
Supplemental disclosure of non-cash investing and financing activities
Accretion of redeemable convertible preferred stock to redemption value	2,294	4
Initial public offering costs included in accounts payable or accrued expenses	246
Conversion of preferred stock to common stock	92,863
Series A Preferred Stock [Member]
Cash flows from financing activities
Proceeds from the issuance of preferred stock, net of issuance costs		$ 22,485
Series B Preferred Stock [Member]
Cash flows from financing activities
Proceeds from the issuance of preferred stock, net of issuance costs	14,970
Series C Preferred Stock [Member]
Cash flows from financing activities
Proceeds from the issuance of preferred stock, net of issuance costs	$ 37,890

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2014

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The unaudited interim 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 financial statements as of and for the year ended December 31, 2013 and the three months ended March 31, 2014 and notes thereto, included in the Company’s prospectus filed with the SEC pursuant to Rule 424(b)(4) on July 18, 2014 (the “Prospectus”).

The unaudited interim 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 September 30, 2014, the results of its operations for the three and nine months ended September 30, 2014 and 2013 and its cash flows for the nine months ended September 30, 2014 and 2013. Such adjustments are of a normal and recurring nature. The results for the nine months ended September 30, 2014 are not indicative of the results for the year ending December 31, 2014, 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 Nasdaq Global Market on July 19, 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 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.

In May 2014, the Financial Accounting Standards Board, (“FASB”), issued 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. ASU 2014-09 is 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 impact that adopting this new accounting guidance will have on its financial statements and footnote disclosures.

In June 2014, the FASB issued guidance that removes all incremental reporting requirements for development stage entities, including the removal of the topic development stage entities. These changes eliminate the requirement to report inception-to-date information in the statements of income, cash flows, and shareholder equity. These changes become effective for the Company on January 1, 2015 and early adoption is permitted. The Company elected to early adopt this guidance beginning in the period ended June 30, 2014. The adoption of this guidance resulted in decreased financial statement disclosures, but did not impact the Company’s financial condition, results of operations or cash flows.

In August 2014, the FASB issued Accounting Standards Update, (“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 financial statements.

Use of Estimates

The preparation of 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 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 September 30, 2014 (unaudited) and December 31,2013, the Company’s Level 1 assets consisted of a money market fund totaling $136,727 and $8,066 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 September 30, 2014 (unaudited) and December 31, 2013, 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 September 30, 2014 (unaudited) and December 31, 2013, 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 September 30, 2014 (unaudited).

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, $2,281 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.

Stock-Based Compensation

9 Months Ended

Sep. 30, 2014

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

4.	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 and non-statutory stock options. 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 September 30, 2014, the total number of shares reserved under all equity plans is 3,505,868, and the Company had 1,692,835 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.

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 September 30, 2014, no shares have been issued under this plan.

Terms of restricted stock awards and stock option agreements, including vesting requirements, are determined by 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. All awards are exercisable from the date of grant for a period of ten years. Stock-based compensation expense recognized during the three and nine months ended September 30, 2014 and 2013 was as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2014		2013		2014		2013
Stock compensation expense:
Research and development	$	294	$	12	$	553	$	24
General and administrative		396		8		633		8


	$	690	$	20	$	1,186	$	32

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 fair value per share relating to outstanding stock options granted under the Company’s stock option plans during the three and nine months ended September 30, 2014 was $29.80 and $9.08, 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 and nine months ended September 30, 2014 was $19.44 and $11.09.

The fair value of each option granted to employees and directors during the three and nine months ended September 30, 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 September 30, 2014			Nine Months Ended September 30, 2014
Expected dividend yield		0.00	%		0.00	%
Expected volatility		100.43	%		101.07	%
Risk free interest rate		1.99	%		1.90	%
Expected life of option		5.96 years			6.01 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 had been operating as a private company until the completion of its IPO in 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 had been operating as a private company until the completion of its IPO in July 2014, there is not sufficient historical share data to calculate the expected term of the options. Therefore, the Company elected to utilize the “simplified” method to value share 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 and nine months ended September 30, 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 Exercise Price		Weighted-Average Remaining Life (in years)		Aggregate Intrinsic Value

Outstanding as of December 31, 2013		1,203,481		$	0.51		9.60	$	1,038

Granted		756,612			9.08
Exercised		(85,157	)		0.46
Forfeited		(61,903	)		1.36


Outstanding as of September 30, 2014		1,813,033		$	4.05		9.44	$	49,763

Vested or expected to vest as of September 30, 2014		1,599,000

Exercisable as of September 30, 2014		297,426

As of September 30, 2014, the Company had unrecognized stock-based compensation expense related to its unvested stock option awards of $6,292,665, which is expected to be recognized over the remaining weighted average vesting period of 3.12 years. The total fair value of shares vested for the nine months ended September 30, 2014 was $619,839. During the nine months ended September 30, 2014, stock option exercises resulted in proceeds of $39,379. The intrinsic value of stock options exercised during the nine months ended September 30, 2014 was $2,373,628.

Restricted Stock Awards

Historically, the Company has granted restricted stock awards to certain officers, employees, directors, and consultants of the Company. During the three and nine months ended September 30, 2014, the Company recorded $72,277 and $127,459, 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

Unvested restricted common stock as of December 31, 2013		315,718		$	—

Issued		—			—
Vested		(104,930	)		—
Forfeited		—			—
Repurchased		(6,778	)		—

Unvested restricted common stock as of September 30, 2014		204,010			—

As of September 30, 2014, the Company had unrecognized stock-based compensation expense related to its unvested restricted stock awards of $354,311, which is expected to be recognized over the remaining weighted average vesting period of 1.39 years.

Unvested shares are subject to repurchase by the Company, at the issuance price, upon the employee’s termination at the Company’s sole discretion. In the nine months ended September 30, 2014, the Company repurchased 6,778 shares of restricted common stock issued to employees with a value of less than $1 in conjunction with the employees’ termination from the Company.

Net Loss Per Share

9 Months Ended

Sep. 30, 2014

Earnings Per Share [Abstract]

Net Loss Per Share

5.	Net Loss Per Share

The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders for the three and nine months ended September 30, 2014 and 2013:


	Three Months Ended September 30,						Nine Months Ended September 30,
	2014			2013			2014			2013

Numerator:
Net loss attributable to common stockholders	$	(9,859	)	$	(4,519	)	$	(23,744	)	$	(12,563	)


Denominator:
Weighted average common shares outstanding—basic		19,581,624			1,514,838			7,711,038			1,467,387

Dilutive effect of common share equivalents resulting from common share options and preferred common shares (as converted)		—			—			—			—

Weighted average common shares outstanding—diluted		19,581,624			1,514,838			7,711,038			1,467,387


Net loss per share attributable to common stockholders—basic and diluted	$	(0.50	)	$	(2.98	)	$	(3.08	)	$	(8.56	)

The following common stock equivalents, 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 September 30,				Nine Months Ended September 30,
	2014		2013		2014		2013

Options to purchase common stock		1,580,223		—		1,580,223		—
Restricted stock		202,986		242,616		202,986		242,616
Redeemable convertible preferred stock (presented on a weighted average basis)		—		8,695,649		—		6,692,246

		1,783,209		8,938,265		1,783,209		6,934,862

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2014

Accounting Policies [Abstract]

Basis of Presentation

Basis of Presentation

Use of Estimates

Use of Estimates

Fair Value Measurements

Fair Value Measurements


Level 1		-		Quoted market prices in active markets for identical assets or liabilities. At September 30, 2014 (unaudited) and December 31,2013, the Company’s Level 1 assets consisted of a money market fund totaling $136,727 and $8,066 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 September 30, 2014 (unaudited) and December 31, 2013, 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 September 30, 2014 (unaudited) and December 31, 2013, the Company had no Level 3 assets or liabilities.

Deferred Offering Costs

Deferred Offering Costs

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2014

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 and nine months ended September 30, 2014 and 2013 was as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2014		2013		2014		2013
Stock compensation expense:
Research and development	$	294	$	12	$	553	$	24
General and administrative		396		8		633		8

	$	690	$	20	$	1,186	$	32

Summary of Weighted Average Assumptions Used to Compute Fair Value of Option Granted


*Black-Scholes Assumptions:*
	Three Months Ended September 30, 2014			Nine Months Ended September 30, 2014
Expected dividend yield		0.00	%		0.00	%
Expected volatility		100.43	%		101.07	%
Risk free interest rate		1.99	%		1.90	%
Expected life of option		5.96 years			6.01 years

Summary of Activity Relating to Stock Options

The table below summarizes activity related to stock options:


	Shares			Weighted-Average Exercise Price		Weighted-Average Remaining Life (in years)		Aggregate Intrinsic Value

Outstanding as of December 31, 2013		1,203,481		$	0.51		9.60	$	1,038

Granted		756,612			9.08
Exercised		(85,157	)		0.46
Forfeited		(61,903	)		1.36


Outstanding as of September 30, 2014		1,813,033		$	4.05		9.44	$	49,763

Vested or expected to vest as of September 30, 2014		1,599,000

Exercisable as of September 30, 2014		297,426

Summary of Activity Relating to Restricted Stock

The table below summarizes activity relating to restricted stock:


	Shares			Weighted Average Grant Date Fair Value

Unvested restricted common stock as of December 31, 2013		315,718		$	—

Issued		—			—
Vested		(104,930	)		—
Forfeited		—			—
Repurchased		(6,778	)		—

Unvested restricted common stock as of September 30, 2014		204,010			—

Net Loss Per Share (Tables)

9 Months Ended

Sep. 30, 2014

Earnings Per Share [Abstract]

Summary of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders

The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders for the three and nine months ended September 30, 2014 and 2013:


	Three Months Ended September 30,						Nine Months Ended September 30,
	2014			2013			2014			2013

Numerator:
Net loss attributable to common stockholders	$	(9,859	)	$	(4,519	)	$	(23,744	)	$	(12,563	)


Denominator:
Weighted average common shares outstanding—basic		19,581,624			1,514,838			7,711,038			1,467,387

Dilutive effect of common share equivalents resulting from common share options and preferred common shares (as converted)		—			—			—			—

Weighted average common shares outstanding—diluted		19,581,624			1,514,838			7,711,038			1,467,387


Net loss per share attributable to common stockholders—basic and diluted	$	(0.50	)	$	(2.98	)	$	(3.08	)	$	(8.56	)

Summary of Common Stock Equivalents


	Three Months Ended September 30,				Nine Months Ended September 30,
	2014		2013		2014		2013

Options to purchase common stock		1,580,223		—		1,580,223		—
Restricted stock		202,986		242,616		202,986		242,616
Redeemable convertible preferred stock (presented on a weighted average basis)		—		8,695,649		—		6,692,246

		1,783,209		8,938,265		1,783,209		6,934,862

Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Thousands			9 Months Ended
	Jul. 23, 2014 USD ($) $ / shares shares	Jul. 02, 2014	Sep. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Net proceeds			$ 96,255
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			Jul. 02, 2014
Reverse stock split ratio		0.3174
Issuance of additional shares of common stock upon conversion \| shares	18,007,575
Deferred offering cots			$ 2,281
Fair Value, Inputs, Level 1 [Member] \| Money Market Funds [Member]
Summary Of Significant Accounting Policies [Line Items]
Money market fund total			$ 136,727	$ 8,066
Initial Public Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock, Shares \| shares	5,750,000
Initial public offering, common stock price per share \| $ / shares	$ 18.00
Net proceeds	$ 94,000

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2014	Dec. 31, 2013
Payables and Accruals [Abstract]
Employee related expenses	$ 837	$ 49
Development & clinical costs	1,392	57
Professional services	471	190
Other accrued expenses	153	31
Total accrued expenses	$ 2,853	$ 327

Stock-Based Compensation - Additional Information (Detail) - USD ($)			3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Jul. 02, 2014	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share option, forfeiture rate			10.00%		10.00%
Contractual term, option					10 years
Stock-based compensation expense			$ 690,000	$ 20,000	$ 1,186,000	$ 32,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense	$ 354,311,000		354,311,000		$ 354,311,000
Weighted average period of unrecognized compensation costs					1 year 4 months 21 days
Stock-based compensation expense			72,277,000		$ 127,459,000
Stock repurchased , shares					6,778
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense	$ 6,292,665,000		$ 6,292,665,000		$ 6,292,665,000
Weighted average period of unrecognized compensation costs					3 years 1 month 13 days
Total fair value of shares vested					$ 619,839,000
Proceeds from stock options exercised					39,379,000
Intrinsic value of options exercised					2,373,628,000
Maximum [Member] \| Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock repurchased, value					$ 1,000
2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under the plan	3,505,868		3,505,868		3,505,868
Number of shares available for future issuance under the plan	1,692,835		1,692,835		1,692,835
Share based compensation granted under plan vest period					4 years		1 year
Share based compensation, vest period					1 year
Share based compensation, term of plan					10 years
Share based compensation, vesting percentage					25.00%
Weighted average grant date fair value per share			$ 29.80		$ 9.08
2014 Stock Option 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			$ 11.09		$ 19.44
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%
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
Share based compensation,number of shares issued	0

Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - Stock Options [Member] - USD ($)	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Shares--Stock options	1,203,481
Granted, Shares--Stock options	756,612
Exercised, Shares--Stock options	(85,157)
Forfeited, Shares--Stock options	(61,903)
Ending balance, Outstanding Shares--Stock options	1,813,033	1,203,481
Vested or expected to vest, Shares--Stock options	1,599,000
Exercisable, Shares--Stock options	297,426
Beginning balance, Weighted Average Exercise Price--Stock options	$ 0.51
Granted, Weighted Average Exercise Price--Stock options	9.08
Exercised, Weighted Average Exercise Price--Stock options	0.46
Forfeited, Weighted Average Exercise Price--Stock options	1.36
Ending balance, Outstanding Weighted Average Exercise Price--Stock options	4.05	$ 0.51
Vested or expected to vest, Weighted Average Exercise Price--Stock options
Exercisable, Weighted Average Exercise Price--Stock options
Outstanding, Weighted Average Remaining Life--Stock options	9 years 5 months 9 days	9 years 7 months 6 days
Vested or expected to vest, Weighted Average Remaining Life--Stock options	0 years
Exercisable, Weighted Average Remaining Life--Stock options	0 years
Outstanding, Aggregate Intrinsic Value--Stock options	$ 49,763	$ 1,038
Vested or expected to vest, Aggregate Intrinsic Value--Stock options
Exercisable, Aggregate Intrinsic Value--Stock options

Stock-Based Compensation - Summary of Activity Relating to Restricted Stock (Detail) - Restricted Stock [Member]	9 Months Ended
	Sep. 30, 2014 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock, Shares at beginning balance--Restricted stock \| shares	315,718
Issued, Shares--Restricted stock \| shares
Vested, Shares--Restricted stock \| shares	(104,930)
Forfeited, Shares--Restricted stock \| shares
Repurchased, Shares--Restricted stock \| shares	(6,778)
Unvested restricted common stock, Shares at ending balance--Restricted stock \| shares	204,010
Unvested restricted common stock , Weighted Average Grant Date Fair Value Per Share, beginning balance--Restricted stock \| $ / shares
Issued, Weighted Average Grant Date Fair Value Per Share--Restricted stock \| $ / shares
Vested, Weighted Average Grant Date Fair Value Per Share--Restricted stock \| $ / shares
Forfeited, Weighted Average Grant Date Fair Value Per Share--Restricted stock \| $ / shares
Repurchased, Weighted Average Grant Date Fair Value Per Share--Restricted stock \| $ / shares
Unvested restricted common stock, Weighted Average Grant Date Fair Value Per Share, ending balance--Restricted stock \| $ / shares

Net Loss Per Share - Schedule of Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator:
Net loss attributable to common stockholders	$ (9,859)	$ (4,519)	$ (23,744)	$ (12,563)
Denominator:
Weighted average common shares outstanding - basic	19,581,624	1,514,838	7,711,038	1,467,387
Dilutive effect of common share equivalents resulting from common share options and preferred common shares (as converted)
Weighted average common shares outstanding - diluted	19,581,624	1,514,838	7,711,038	1,467,387
Net loss per share attributable to common stockholders - basic and diluted	$ (0.50)	$ (2.98)	$ (3.08)	$ (8.56)

Statement of Changes in Redeemable Convertible Preferred Stock and Stockholder's Equity (Deficit) (Parenthetical) $ in Thousands	9 Months Ended
	Sep. 30, 2014 USD ($)
Series B Preferred Stock [Member]
Preferred Stock, issuance costs	$ 30
Series C Preferred Stock [Member]
Preferred Stock, issuance costs	$ 110

Stock-Based Compensation - Summary of Activity Relating to Restricted Stock (Detail) - Restricted Stock [Member]	9 Months Ended
	Sep. 30, 2014 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted common stock, Shares at beginning balance--Restricted stock \| shares	315,718
Issued, Shares--Restricted stock \| shares
Vested, Shares--Restricted stock \| shares	(104,930)
Forfeited, Shares--Restricted stock \| shares
Repurchased, Shares--Restricted stock \| shares	(6,778)
Unvested restricted common stock, Shares at ending balance--Restricted stock \| shares	204,010
Unvested restricted common stock , Weighted Average Grant Date Fair Value Per Share, beginning balance--Restricted stock \| $ / shares
Issued, Weighted Average Grant Date Fair Value Per Share--Restricted stock \| $ / shares
Vested, Weighted Average Grant Date Fair Value Per Share--Restricted stock \| $ / shares
Forfeited, Weighted Average Grant Date Fair Value Per Share--Restricted stock \| $ / shares
Repurchased, Weighted Average Grant Date Fair Value Per Share--Restricted stock \| $ / shares
Unvested restricted common stock, Weighted Average Grant Date Fair Value Per Share, ending balance--Restricted stock \| $ / shares

Related Party Transactions - Additional Information (Detail) - Third Rock Ventures LLC [Member] - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Related Party Transaction [Line Items]
Payment for consulting and management services	$ 255	$ 564
Accounts payable to related parties	$ 0		$ 125
Minimum [Member]
Related Party Transaction [Line Items]
Ownership percentage in company's common stock	5.00%

Income Taxes - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Income Tax Disclosure [Abstract]
State income tax benefit	$ 0	$ 0
Federal income tax benefit	$ 0	$ 0