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Document and Entity Information - USD ($)	12 Months Ended
Dec. 31, 2016	Feb. 15, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY
Trading Symbol	SAGE
Entity Registrant Name	Sage Therapeutics, Inc.
Entity Central Index Key	0001597553
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		37,270,180
Entity Public Float			$ 670,259,868

Consolidated Balance Sheets - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 168,517,000	$ 186,753,000
Marketable securities	228,962,000	0
Prepaid expenses and other current assets	5,100,000	1,738,000
Total current assets	402,579,000	188,491,000
Property and equipment, net	1,388,000	286,000
Restricted cash	564,000	39,000
Deferred offering costs		200,000
Total assets	404,531,000	189,016,000
Current liabilities:
Accounts payable	12,817,000	5,159,000
Accrued expenses	22,352,000	10,148,000
Total current liabilities	35,169,000	15,307,000
Other liabilities	845,000	14,000
Total liabilities	36,014,000	15,321,000
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at December 31, 2016 and 2015; no shares issued or outstanding at December 31, 2016 and 2015
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at December 31, 2016 and 2015; 37,222,518 and 28,823,549 shares issued at December 31, 2016 and 2015, respectively; 37,222,172 and 28,823,549 shares outstanding at December 31, 2016 and 2015, respectively	4,000	3,000
Treasury stock, at cost; 346 shares at December 31, 2016 and none at December 31, 2015	(17,000)
Additional paid-in capital	688,959,000	335,032,000
Accumulated deficit	(320,327,000)	(161,340,000)
Accumulated other comprehensive income (loss)	(102,000)
Total stockholders’ equity	368,517,000	173,695,000
Total liabilities and stockholders’ equity	$ 404,531,000	$ 189,016,000

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating expenses:
Research and development	$ 120,756	$ 69,357	$ 24,100
General and administrative	39,407	25,293	9,710
Total operating expenses	160,163	94,650	33,810
Loss from operations	(160,163)	(94,650)	(33,810)
Interest income, net	1,211	178	8
Other expense, net	(35)	(23)	(9)
Net loss	(158,987)	(94,495)	(33,811)
Accretion of redeemable convertible preferred stock to redemption value			(2,294)
Net loss attributable to common stockholders	$ (158,987)	$ (94,495)	$ (36,105)
Net loss per share attributable to common stockholders—basic and diluted	$ (4.75)	$ (3.40)	$ (1.67)
Weighted average number of common shares used in net loss per share attributable to common stockholders—basic and diluted	33,492,795	27,778,288	21,574,347
Comprehensive loss:
Net loss	$ (158,987)	$ (94,495)	$ (33,811)
Other comprehensive items:
Unrealized loss on marketable securities	(102)
Total other comprehensive loss	(102)
Total comprehensive loss	$ (159,089)	$ (94,495)	$ (33,811)

Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - 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]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Initial Public Offering [Member]	Accumulated other comprehensive income (loss) [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		$ 93,970		$ 14,970	$ 37,890		$ 1			$ 93,969
Shares issued during period, Shares				9,999,999	8,973,905		5,750,000
Issuance of common stock from exercise of stock options, Amount	40								40
Issuance of common stock from exercise of stock options, Shares						87,475
Vesting of restricted stock, Amount	14								14
Vesting of restricted stock, Shares						138,108
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	2,512								2,512
Accretion of redeemable convertible preferred stock to redemption value			$ 2,294
Accretion of redeemable convertible preferred stock to redemption value	(2,294)								(935)			(1,359)
Conversion of redeemable convertible preferred stock to common stock, Amount	92,863		$ (92,863)			$ 2			92,861
Conversion of redeemable convertible preferred stock to common stock, Shares			(56,723,904)			18,007,575
Net loss	(33,811)											(33,811)
Balances at Dec. 31, 2014	121,885					$ 3			188,727			(66,845)
Balances, Shares at Dec. 31, 2014						25,621,791
Shares issued during period, Amount		129,171								129,171
Shares issued during period, Shares							2,628,571
Issuance of common stock from exercise of stock options, Amount	603								603
Issuance of common stock from exercise of stock options, Shares						417,475
Vesting of restricted stock, Amount	17								17
Vesting of restricted stock, Shares						128,051
Issuance of common stock under employee stock purchase plan	127								127
Issuance of common stock under employee stock purchase plan, Shares						3,852
Issuance of common stock in payment of consultant fees, Amount	1,211								1,211
Issuance of common stock in payment of consultant fees, Shares						23,809
Stock-based compensation expense	15,176								15,176
Net loss	(94,495)											(94,495)
Balances at Dec. 31, 2015	173,695					$ 3			335,032			(161,340)
Balances, Shares at Dec. 31, 2015						28,823,549
Shares issued during period, Amount		$ 329,541					$ 1			$ 329,540
Shares issued during period, Shares							8,220,786
Issuance of common stock from exercise of stock options, Amount	1,011								1,011
Issuance of common stock from exercise of stock options, Shares						124,557
Vesting of restricted stock, Amount	11								11
Vesting of restricted stock, Shares						42,781
Issuance of common stock under employee stock purchase plan	510								510
Issuance of common stock under employee stock purchase plan, Shares						10,499
Purchase of treasury stock, Amount	(17)							$ (17)
Purchase of treasury stock, Shares								346
Stock-based compensation expense	22,855								22,855
Unrealized loss on available-for-sale securities	(102)										$ (102)
Net loss	(158,987)											(158,987)
Balances at Dec. 31, 2016	$ 368,517					$ 4		$ (17)	$ 688,959		$ (102)	$ (320,327)
Balances, Shares at Dec. 31, 2016						37,222,172		346

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (158,987)	$ (94,495)	$ (33,811)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	23,020	15,240	2,512
Non-cash licensing and consulting fees		1,211	127
Premium on marketable securities	(756)
Amortization of premium on marketable securities	286
Depreciation	281	115	51
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,362)	(681)	(715)
Accounts payable	7,796	2,590	441
Accrued expenses and other liabilities	13,044	5,339	4,353
Net cash used in operating activities	(118,678)	(70,681)	(27,042)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	30,499
Purchases of marketable securities	(259,093)
Purchases of property and equipment	(1,421)	(198)	(128)
Increase in restricted cash	(525)
Net cash used in investing activities	(230,540)	(198)	(128)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	1,406	730	40
Payments of offering costs	(599)	(584)	(2,285)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	330,175	129,720	96,255
Net cash provided by financing activities	330,982	129,866	146,870
Net increase (decrease) in cash and cash equivalents	(18,236)	58,987	119,700
Cash and cash equivalents at beginning of period	186,753	127,766	8,066
Cash and cash equivalents at end of period	168,517	186,753	127,766
Supplemental disclosure of non-cash financing activities
Accretion of redeemable convertible preferred stock to redemption value			2,294
Conversion of preferred stock to common stock			92,863
Purchases of property and equipment included in accounts payable	$ 8
Public offering costs included in accounts payable or accrued expenses		$ 165
Series A Preferred Stock [Member]
Cash flows from financing activities
Proceeds from the issuance of preferred stock, net of issuance costs			14,970
Series B 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

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

The following is a summary of significant accounting policies followed in the preparation of these financial statements.

Basis of Presentation

The accompanying consolidated financial statements include those of the Company and its subsidiaries after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”).

Use of Estimates

The preparation of consolidated financial statements in conformity with GAAP 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.

Cash and Cash Equivalents

The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents.

Marketable securities

Marketable securities consist of investments with original maturities greater than ninety days. The Company considers its investment portfolio of investments to be available-for-sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive items in stockholders’ equity. Realized gains and losses and declines in value judged to be other than temporary are included as a component of other expense, net, based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. No declines in value were deemed to be other than temporary during the year ended December 31, 2016.

Restricted Cash

A deposit of $39,000 was restricted from withdrawal as of December 31, 2016 and 2015. The restriction is related to securing the Company’s facility lease and expires in 2022 in accordance with the operating lease agreement. This balance is included in restricted cash on the accompanying consolidated balance sheets.

A deposit of $0.5 million was restricted from withdrawal as of December 31, 2016. The restriction is related to securing the facility lease in May 2016, under which the Company rented 19,805 square feet of additional office space in a separate multi-tenant building beginning in September 2016. The lease for the additional space will expire in February 2022. The restriction expires in 2022, in accordance with the operating lease agreement. This balance is included in restricted cash on the accompanying consolidated balance sheet.

Property and Equipment

Property and equipment are recorded at cost and depreciated over their estimated useful lives using the straight-line method. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income. Repairs and maintenance costs are expensed as incurred.

Impairment of Long-Lived Assets

Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends, and significant changes or planned changes in the use of the assets. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any impairment losses on long-lived assets.

Research and Development

Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries and benefits, overhead costs, depreciation, contract services and other related costs. Research and development costs are expensed to operations as the related obligation is incurred.

Research Contract Costs and Accruals

The Company has entered into various research and development contracts with research institutions and other companies both inside and outside of the United States. These agreements are generally cancelable, and related payments are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

Patent Costs

The Company expenses patent costs as incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations and comprehensive loss.

Stock-Based Compensation

The Company recognizes compensation expense for stock-based awards made to employees and nonemployee directors, including grants of stock options and restricted stock, based on the estimated fair value on the date of grant, over the requisite service period.

For stock-based options and restricted stock issued to nonemployee consultants, the Company recognizes the fair value of the award as an expense over the period in which the related services are received. The fair value of the awards and measurement of related stock-based compensation is subject to periodic adjustments as the awards vest.

For awards that vest upon achievement of a performance condition, the Company recognizes compensation expense when achievement of the performance condition is deemed probable over the implicit service period.

The fair value of each option grant is estimated using the Black-Scholes option-pricing model. Through July 2014, the Company was a private company and lacked sufficient Company-specific historical and implied volatility information. Therefore, in 2016, the Company began estimating its expected volatility using a weighted average of the historical volatility of publicly traded peer companies and the volatility of its common stock, and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its traded stock price for the duration of the expected term. The expected term of the Company’s options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options, while the expected term of its options granted to consultants and nonemployees has been determined based on the contractual term of the options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The expected dividend yield is based on the fact that the Company never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The Company also applies a forfeiture rate in order to calculate stock-based compensation expense. To the extent actual forfeitures differ from the estimates, the difference will be recorded as a cumulative adjustment in the period in which the estimates are revised. The Company recognizes stock-based compensation expense for only the portion of awards that are expected to vest. Expected forfeitures are based on the Company’s historical experience and management’s expectations of future forfeitures.

Treasury Stock

The Company records treasury stock at cost. Treasury stock includes shares received from an employee as consideration for an exercise of stock options.

Basic and Diluted Net Loss Per Share

Upon the closing of the Company’s IPO in July 2014, all of the Company’s outstanding shares of redeemable convertible preferred stock were converted into shares of common stock. Prior to this conversion, the Company followed the two-class method when computing net loss per share as the Company had issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to receive dividends as if all income for the period had been distributed. The Company’s redeemable convertible preferred shares contractually entitled the holders of such shares to participate in dividends, but did not contractually require the holders of such shares to participate in losses of the Company. Accordingly, the two-class method did not apply for periods in which the Company reported a net loss or a net loss attributable to common stockholders resulting from dividends or accretion related to its redeemable convertible preferred shares.

Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options and unvested restricted common shares, as determined using the treasury stock method. For periods in which the Company has reported net losses, diluted net loss per common share attributable to common stockholders is the same as basic net loss per common share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is antidilutive.

The Company reported a net loss attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014.

Risks and Uncertainties

The product candidates developed by the Company require approvals from the U.S. Food and Drug Administration or foreign regulatory agencies prior to commercial sales. There can be no assurance that the Company’s current and future product candidates will receive the necessary approvals. If the Company fails to successfully complete clinical development and generate results sufficient to file for regulatory approval or is denied approval or approval is delayed, it may have a material adverse impact on the Company’s business and its financial statements.

Concentration of Credit Risk and of Significant Suppliers

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. The Company has all cash and cash equivalents balances at two accredited financial institutions, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredients and formulated drugs related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs.

Income Taxes

The Company accounts for income taxes under the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to differences between financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. Valuation allowances are provided if based on the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. During November 2015, the FASB issued ASU 2015-17, Balance Sheet Classification of Deferred Taxes, which simplifies the presentation of deferred income taxes. The Company early adopted ASU 2015-17 effective December 31, 2015 on a prospective basis. Adoption of this ASU resulted in a reclassification of the current deferred tax liability to a non-current deferred tax liability, in the amount of $0.6 million, which is netted with the long-term deferred tax asset in its consolidated balance sheet as of December 31, 2015.

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 classified and disclosed in one of the following three categories:

Level 1	—	Quoted market prices in active markets for identical assets or liabilities.

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.

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.

The Company’s cash equivalents and marketable securities at December 31, 2016 and 2015 were carried at fair value, determined according to the fair value hierarchy; see Footnote 3, Fair Value Measurements.

The carrying amounts reflected in the consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at December 31, 2016 and 2015, respectively.

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 IPO in July 2014, $2.3 million of these costs were recorded in stockholders’ equity as a reduction of additional paid-in capital generated as a result of the IPO. After consummation of the follow-on public offering of common stock in April 2015, $0.5 million of these costs were recorded in stockholders’ equity as a reduction of additional paid-in capital generated as a result of the offering. As of December 31, 2015, the Company had recorded deferred offering costs of $0.2 million, which are shown as a non-current asset, that were for the follow-on public offering that was consummated in January 2016. After consummation of the follow-on public offering of common stock in January 2016, $0.6 million of these costs were recorded in stockholders’ equity as a reduction of additional paid-in capital generated as a result of the offering.

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. All tangible assets are held within the United States.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. For the year ended December 31, 2016, the difference between net loss and comprehensive loss was the unrealized loss on marketable securities. For the years ended December 31, 2015 and 2014, there was no difference between net loss and comprehensive loss.

Public Offerings

On July 23, 2014, the Company completed the sale of 5,750,000 shares of its common stock in its IPO (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 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 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 its common stock at a price to the public of $52.50 per share, resulting in net proceeds to the Company of $129.1 million after deducting underwriting discounts and commissions and offering costs paid by the Company.

On January 12, 2016, the Company completed the sale of 3,157,894 shares of its common stock at a price to the public of $47.50 per share, resulting in net proceeds to the Company of $140.4 million after deducting underwriting discounts and commissions and offering costs paid by the Company.

On September 14, 2016, the Company completed the sale of 5,062,892 shares of its common stock at a price to the public of $39.75 per share, resulting in net proceeds to the Company of $189.2 million after deducting underwriting discounts and commissions and offering costs paid by the Company.

Recently Issued Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. The FASB has continued to issue accounting standards updates to clarify and provide implementation guidance related to Revenue from Contracts with Customers, including ASU 2016-08, Revenue from Contract with Customers: Principal versus Agent Considerations, ASU 2016-10, Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing, and ASU 2016-12, Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients. These amendments address a number of areas, including the entity’s identification of its performance obligations in a contract, collectability, non-cash consideration, presentation of sales tax and an entity’s evaluation of the nature of its promise to grant a license of intellectual property and whether or not that revenue is recognized over time or at a point in time. These new standards will be effective for the Company beginning January 1, 2018. The Company could early adopt the standard for the year ending December 31, 2017. The Company plans to early adopt the standard as of January 1, 2017, although there is no impact of this new guidance on its consolidated financial statements as it does not currently have any revenue generating arrangements.

In February 2016, the FASB issued ASU No. 2016-02, Leases, which will replace the existing guidance in ASC 840, “Leases.” The updated standard aims to increase transparency and comparability among organizations by requiring lessees to recognize leased assets and leased liabilities on the consolidated balance sheets and requiring disclosure of key information about leasing arrangements. The standard will be effective on January 1, 2019, with early adoption permitted. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements.

In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting, which intends to simplify several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, an option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, as well as certain classifications on the statement of cash flows. The standard will be effective on January 1, 2017. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which introduces a new methodology for accounting for credit losses on financial instruments, including available-for-sale debt securities. The guidance establishes a new "expected loss model" that requires entities to estimate current expected credit losses on financial instruments by using all practical and relevant information. Any expected credit losses are to be reflected as allowances rather than reductions in the amortized cost of available-for-sale debt securities. Early adoption is permitted for annual periods beginning after December 15, 2018, and interim periods therein. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements.

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The standard reduces the diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The standard will be effective on January 1, 2018. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash that changes the presentation of restricted cash and cash equivalents on the statement of cash flows. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This standard is effective for the Company in the fiscal year beginning January 1, 2018, but early adoption is permissible. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements. After adopting the standard, the amounts of restricted cash shown on the consolidated balance sheets would be included in cash and cash equivalents in the statement of cash flows.

Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption.

Fair Value Measurements

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Fair Value Measurements

3.	Fair Value Measurements

The Company’s cash equivalents are generally classified within Level 1 of the fair value hierarchy. The Company’s investments in marketable securities are classified within Level 2 of the fair value hierarchy.

The fair values of the Company’s marketable securities are generally based on prices obtained from independent pricing sources. Consistent with the fair value hierarchy described above, securities with validated quotes from pricing services are generally reflected within Level 2, as they are primarily based on observable pricing for similar assets or other market observable inputs. Typical inputs used by these pricing services include, but are not limited to, reported trades, benchmark yields, issuer spreads, bids, offers or estimates of cash flow, prepayment spreads and default rates.

The following tables summarize the Company’s money market funds and marketable securities as of December 31, 2016 and 2015:

	December 31, 2016
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Money market funds	$	168,517	$	168,517	$	—	$	—
Total cash equivalents		168,517		168,517		—		—
Marketable securities:
U.S. government securities		100,031		—		100,031		—
U.S. corporate bonds		58,452		—		58,452		—
International corporate bonds		24,190		—		24,190		—
U.S. commercial paper		30,351		—		30,351		—
International commercial paper		15,938		—		15,938		—
Total marketable securities		228,962		—		228,962		—
Total cash equivalents and marketable securities	$	397,479	$	168,517	$	228,962	$	—

	December 31, 2015
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash and cash equivalents:
Money market funds	$	186,753	$	186,753	$	—	$	—
Total cash and cash equivalents	$	186,753	$	186,753	$	—	$	—

During the years ended December 31, 2016 and 2015, there were no transfers among the Level 1, Level 2 and Level 3 categories.

Marketable Securities

The following table summarizes the Company’s marketable securities as of December 31, 2016:

	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities (due within 1 year)	$	100,055	$	2	$	(26	)	$	100,031
U.S. corporate bonds		58,508		—		(56	)		58,452
International corporate bonds		24,212		—		(22	)		24,190
U.S. commercial paper		30,351		—		—			30,351
International commercial paper		15,938		—		—			15,938
	$	229,064	$	2	$	(104	)	$	228,962

The Company held no marketable securities as of December 31, 2015.

As of December 31, 2016, all marketable securities held by the Company had remaining contractual maturities of one year or less.

As of December 31, 2016, the Company held 27 marketable securities which were in a loss position due to fluctuations in interest rates for less than one year.

There have been no impairments of the Company’s assets measured and carried at fair value during the years ended December 31, 2016 and 2015.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2016

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

5.	Commitments and Contingencies

Operating Leases

The Company rents 22,067 square feet of office space in a multi-tenant building under an operating lease that will expire in February 2022. In May 2016, the Company entered into a lease under which, beginning in September 2016, the Company rents 19,805 square feet of additional office space, in a separate multi-tenant building. The lease for the additional space will expire in February 2022.

Rent expense, net of sublease income, for the years ended December 31, 2016, 2015 and 2014, was $2.0 million, $0.4 million, and $0.3 million, respectively.

Future minimum lease payments under non-cancelable operating leases are as follows at December 31, 2016:

Years Ending December 31,	(in thousands)
2017	$	2,666
2018		2,729
2019		2,771
2020		2,813
2021		2,855
Thereafter		480
	$	14,314

License Agreements

CyDex License Agreement

In September 2015, the Company and CyDex Pharmaceuticals, Inc. (“CyDex”) amended and restated their existing commercial license agreement. Under the terms of the commercial license agreement as amended and restated, CyDex has granted to the Company an exclusive license to CyDex’s Captisol drug formulation technology and related intellectual property for the manufacture of pharmaceutical products incorporating the Company’s compounds known as SAGE-547 and SAGE-689, and the development and commercialization of the resulting products in the treatment, prevention or diagnosis of any disease or symptom in humans or animals other than (i) the ocular treatment of any disease or condition with a formulation, including a hormone; (ii) topical ocular treatment of inflammatory conditions; (iii) treatment and prophylaxis of fungal infections in humans; and (iv) any ocular treatment for retinal degeneration.

As consideration for the inclusion of SAGE-689 in the license granted by CyDex, the Company paid to CyDex $0.1 million, which was recorded as research and development expense for the three months ended September 30, 2015 in connection with the execution of the amended and restated license agreement.

The Company is obligated to make milestone payments under the amended and restated license agreement with CyDex based on the achievement of clinical development and regulatory milestones in the amount of up to $0.8 million in clinical milestones and up to $3.8 million in regulatory milestones for each of the first two fields with respect to SAGE-547; up to $1.3 million in clinical milestones and up to $8.5 million in regulatory milestones for each of the third and fourth fields with respect to SAGE-547; and up to $0.8 million in clinical milestones and up to $1.8 million in regulatory milestones for one field with respect to SAGE-689.

For the year ended December 31, 2016, additional clinical development milestones were met for the SAGE-547 program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense and made cash payments totaling $0.8 million.

For the year ended December 31, 2015, additional clinical development milestones were met for the SAGE-547 program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense and made cash payments totaling $0.8 million.

Washington University License Agreement

In November 2013, the Company entered into a license agreement with Washington University whereby the Company was granted exclusive, worldwide rights to develop and commercialize a novel set of neuroactive steroids developed by Washington University. In exchange for development and commercialization rights, the Company paid an upfront, non-refundable payment of $50,000 and is required to pay an annual license maintenance fee of $15,000 on each subsequent anniversary date, until the first Phase 2 clinical trial for a licensed product is initiated. The Company is obligated to make milestone payments to Washington University based on achievement of clinical development and regulatory milestones of up to $0.7 million and $0.5 million, respectively. Additionally, the Company fulfilled its obligation to issue to Washington University 47,619 shares of common stock on December 13, 2013. The fair value of these shares of $0.1 million was recorded as research and development expense in 2013.

The Company is obligated to pay royalties to Washington University at rates in the low single digits on net sales of licensed products covered under patent rights and royalties at rates in the low single digits on net sales of licensed products not covered under patent rights. Additionally, the Company has the right to sublicense and is required to make payments at varying percentages of sublicensing revenue received, initially in the mid-teens and descending to the mid-single digits over time.

For the year ended December 31, 2016, the Company did not record any expense or make any milestone payments under the license agreement with Washington University

For the year ended December 31, 2015, a regulatory milestone was met for one of the programs under the license agreement with Washington University, and accordingly, the Company recorded research and development expenses and made a cash payment of $50,000.

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 paid to The Regents of the University of California clinical development milestones of up to $0.1 million and will be required to pay royalties of less than 1% on net sales for a period of fifteen years following the sale of the first 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.

For the years ended December 31, 2014 and 2013, the Company did not record any expense or make any milestone or royalty payments under the license agreement with the University of California.

In June 2015, the Company entered into an exclusive license agreement with The Regents of the University of California whereby the Company was granted an exclusive license to certain patent rights related to the use of allopregnanolone to treat various diseases. In exchange for such license, the Company paid an upfront payment of $50,000 and will make payments of $15,000 for annual maintenance fees until the calendar year following the first sale, if any, of a licensed product. The Company is obligated to make milestone payments following the achievement of specified regulatory and sales milestones of up to $0.7 million and $2.0 million in the aggregate, respectively. Following the first sale, if any, of a licensed product, the Company is obligated to pay royalties at a low single digit percentage of net sales, if any, of licensed products, subject to specified minimum annual royalty amounts. Unless terminated by operation of law or by acts of the parties under the terms of the agreement, the license agreement will terminate when the last-to-expire patents or last-to-be abandoned patent applications expire, whichever is later.

For the year ended December 31, 2015, three clinical development milestones were met, and accordingly, the Company recorded research and development expenses and made cash payments totaling $0.1 million.

For the year ended December 31, 2016, the Company did not record any expense or make any milestone or royalty payments under either license agreement with The Regents of the University of California.

Consulting Agreement

In January 2014, the Company entered into a consulting agreement with a non-employee advisor whereby the Company is obligated to make cash payments of up to $2.0 million 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 $0.2 million, comprised of $50,000 in cash and $0.1 million related to the issuance of 15,872 shares of the Company’s common stock.

For the year ended December 31, 2015, the second and third clinical development milestones for one of the programs included in the consulting agreement were met. Accordingly, the Company recorded research and development expense for the year ended December 31, 2015 of $1.7 million, comprised of $0.5 million in cash and $1.2 million related to the issuance of 23,809 shares of the Company’s common stock, related to the achievement of these milestones.

For the year ended December 31, 2016, the Company did not record any expense or make any milestone payments under the consulting agreement with the non-employee advisor.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8.	Stock-Based Compensation

Stock Option Plans

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, restricted stock units, 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 no longer grant 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 December 31, 2016, the total number of shares reserved under all equity plans is 4,891,922, and the Company had 660,115 shares available for future issuance under such plans. On December 15, 2016, the Board of Directors of the Company approved the 2016 Inducement Equity Plan, for which no grants were made as of December 31, 2016.

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 last day of the prior fiscal year. On January 1, 2017, 1,488,886 shares of common stock, representing 4% of the Company’s issued and outstanding shares of common stock as of December 31, 2016, were added to the 2014 Stock Option Plan.

Terms of restricted stock awards, restricted stock units, and stock option agreements, including vesting requirements, are determined by the Board of Directors or the Compensation Committee of the Board of Directors, subject to the provisions of the applicable stock option plan. Options and restricted stock awards granted by the Company, that are not performance-based, generally vest based on the continued service of the grantee with the Company during a specified period following grant. These awards, when granted to employees, generally vest ratably over four years, with a 25% cliff vesting at the one year anniversary. All option awards expire in 10 years.

During the years ended December 31, 2016 and 2015, the Company granted 74,039 and 497,100 options, respectively, to employees to purchase shares of common stock that contain performance-based vesting criteria, primarily related to achievement of certain clinical and regulatory development milestones related to 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.

During the year ended December 31, 2015, one milestone was achieved. This milestone represents 35% of the performance-based option grants that were made during the year ended December 31, 2015. During the year ended December 31, 2015, the Company recognized stock-based compensation expense related to this milestone of $4.8 million.

During the year ended December 31, 2016, one milestone was achieved. This milestone represents 50% and 30%, of the performance-based option grants that were made during the years ended December 31, 2016 and 2015, respectively. During the year ended December 31, 2016, the Company recognized stock-based compensation expense related to this milestone of $5.0 million.

The achievement of the remaining milestones was deemed to be not probable as of December 31, 2016, and therefore no expense has been recognized related to these awards for the year ended December 31, 2016.

Stock-based compensation expense recognized during the years ended December 31, 2016, 2015 and 2014 was as follows:

	Year Ended December 31,
	2016		2015		2014
	(in thousands)
Research and development	$	11,197	$	5,924	$	1,093
General and administrative		11,823		9,316		1,419
	$	23,020	$	15,240	$	2,512

During the years ended December 31, 2016 and 2015, the Company recorded $0.2 million and $0.1 million, respectively, of stock-based compensation expense related to the Employee Stock Purchase Plan.

For stock option awards, the fair value 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 for awards to employees and on a graded basis for awards to non-employees 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 years ended December 31, 2016, 2015 and 2014 was $24.97, $34.08 and $14.33, respectively.

The fair value of each option granted to employees and nonemployee directors during the years ended December 31, 2016, 2015 and 2014 under the Company’s stock option plans has been calculated on the date of grant using the following weighted average assumptions:

	Year Ended December 31,
	2016			2015			2014
Expected dividend yield		0	%		0	%		0	%
Expected volatility		80.15	%		90.54	%		98.86	%
Risk-free interest rate		1.47	%		1.59	%		1.95	%
Expected life of option	6.05 years			6.03 years			6.38 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: the Company does not have sufficient history to support a calculation of volatility using only its historical data. Starting in 2016, the Company uses a weighted-average volatility considering the Company’s own volatility since the IPO in July 2014 and the volatilities of a peer group of comparable companies for time periods prior to the IPO. Prior to 2016, the Company used volatilities based on an analysis of reported data for a peer group of comparable companies.

Expected term (in years): the expected term represents the period that the Company’s stock option grants are expected to be outstanding. The Company has been publicly traded since July 2014, and 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 options granted 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 years ended December 31, 2016, 2015 and 2014, forfeiture rates of 9.58%, 10% and 10%, respectively, were applied.

For options granted to non-employees, the expected life of the option used is 10 years, which is the contractual term of each 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 (in thousands)
Outstanding as of December 31, 2015		3,002,809		$	26.67		8.67	$	96,479
Granted		1,454,568			35.98
Exercised		(124,903	)		8.09
Forfeited		(100,667	)		44.76
Outstanding as of December 31, 2016		4,231,807		$	29.99		8.24	$	92,843
Vested and expected to vest as of December 31, 2016		3,727,837		$	29.36		8.21	$	84,395
Vested and exercisable as of December 31, 2016		1,702,819		$	24.11		7.59	$	48,096

As of December 31, 2016, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $43.6 million, which is expected to be recognized over the remaining weighted average vesting period of 2.77 years. The total fair value of options vested for the years ended December 31, 2016, 2015 and 2014 was $24.1 million, $9.2 million, and $1.0 million, respectively.

In addition, the Company granted 245,872 stock options that are both outstanding and unvested that will vest upon the achievement of certain performance criteria in the future. Total unrecognized stock-based compensation expense related to those awards was $5.2 million at December 31, 2016.

The intrinsic value of stock options exercised during the years ended December 31, 2016, 2015 and 2014 was $4.6 million, $28.4 million and $2.4 million, respectively.

Restricted Stock Awards

During the years ended December 31, 2013, 2012 and 2011, the Company granted restricted stock awards to certain officers, employees, directors, and consultants of the Company.

The table below summarizes activity relating to restricted stock:

	Shares
Outstanding as of December 31, 2015		42,781
Issued		—
Vested		(42,781	)
Forfeited		—
Repurchased		—
Outstanding as of December 31, 2016		—

As of December 31, 2016, all of the restricted stock was vested.

2014 Employee Stock Purchase Plan

On July 2, 2014, the Company’s stockholders approved the 2014 Employee Stock Purchase Plan, which had been previously approved by the Board of Directors. 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 December 31, 2016, 14,351 shares have been issued under this plan. At December 31, 2016, accrued expenses includes $67,000 of stock-based compensation expense related to an enrollment period for which the related shares had not been issued as of December 31, 2016.

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

Net Loss Per Share

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Net Loss Per Share

9.	Net Loss Per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the years ended December 31, 2016, 2015 and 2014:

	Year Ended December 31,
	2016			2015			2014
Basic net loss per share attributable to common stockholders:
Numerator:
Net loss attributable to common stockholders (in thousands)	$	(158,987	)	$	(94,495	)	$	(36,105	)
Denominator:
Weighted average common stock outstanding—basic		33,492,795			27,778,288			21,574,347
Dilutive effect of shares of common stock equivalents resulting from common stock options and preferred common stock (as converted)		—			—			—
Weighted average common stock outstanding—diluted		33,492,795			27,778,288			21,574,347
Net loss per share attributable to common stockholders—basic and diluted	$	(4.75	)	$	(3.40	)	$	(1.67	)

The following common stock equivalents outstanding as of December 31, 2016 and 2015 were excluded from the computation of diluted net loss per share for the periods presented because including them would have been anti-dilutive:

	Year Ended December 31,
	2016		2015
Stock options		3,985,935		2,643,833
Employee stock purchase plan		6,784		3,307
Restricted stock		—		42,781
		3,992,719		2,689,921

Income Taxes

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Income Taxes

10.	Income Taxes

There is no provision for income taxes because the Company has historically incurred operating losses and maintains a full valuation allowance against its net deferred tax assets. The reported amount of income tax expense for the years differs from the amount that would result from applying domestic federal statutory tax rates to pretax losses primarily because of changes in valuation allowance.

A reconciliation of U.S. statutory rate to the Company’s effective tax rate is as follows:

	Year Ended December 31,
	2016			2015			2014
Tax at Statutory Rate		34.0	%		34.0	%		34.0	%
State Taxes, net of federal benefit		4.2			4.0			4.5
Permanent Items		(1.0	)		(0.9	)		(1.0	)
Orphan Drug Credit Addback		(2.9	)		(4.2	)		—
Foreign Rate Differential		(3.1	)		(3.1	)		—
Federal and State Credits		10.3			13.2			8.5
Change in Valuation Allowance		(41.5	)		(43.1	)		(46.0	)
Other		—			0.1			—
		0.0	%		0.0	%		0.0	%

Significant components of the Company’s net deferred tax asset at December 31, 2016 and 2015 are as follows:

	December 31,
	2016			2015
	(in thousands)
Net operating losses	$	84,374		$	44,172
Capitalized start-up costs		2,155			2,335
Accounting method change		(673	)		(1,347	)
Tax credit carryforwards		33,477			17,013
Accrued expenses		2,287			1,062
Depreciation and amortization		999			716
Stock options		12,103			5,097
Others		350			21
Total net deferred tax asset before valuation allowance		135,072			69,069
Valuation allowance		(135,072	)		(69,069	)
Net deferred tax asset	$	-		$	-

As of December 31, 2016, the Company had federal and state net operating loss carryforwards of $235.4 million and $234.3 million, respectively, which begin to expire in 2031. As of December 31, 2016, the Company had federal and state research and development tax credits carryforwards of $4.1 million and $1.6 million, respectively, which begin to expire in 2031 and 2027, respectively. As of December 31, 2016, the Company had federal orphan drug tax credit carry forwards of $29.8 million, which begin to expire in 2034. At December 31, 2016, the Company has excess equity based compensation tax deductions related to net operating losses for federal and state purposes of $20.4 million and $20.4 million respectively. The Company has excess equity based compensation related to credits for federal and state purposes of $1.3 million and $0.2 million, respectively. These excess tax benefits have not been included in the net deferred tax assets before valuation allowance since these benefits would be credited directly to additional paid in capital if subsequently recognized through a reduction in taxes payable.

The deferred tax assets above exclude $8.0 million of net operating losses and $1.5 million of federal and state research and development credits related to tax deductions from the exercise of stock options subsequent to the adoption of the 2006 accounting standard on stock-based compensation. This amount represents an excess tax benefit and has not been included in the gross deferred tax assets. The Company will adopt ASU 2016-09, Improvements to Employee Share-Based Payment Accounting, for the quarter ended March 31, 2017. As a result of adoption, the deferred tax assets associated with net operating losses will increase by $8.0 million and the deferred tax assets associated with federal and state research credits will increase by $1.5 million. These amounts will be offset by a corresponding increase in the valuation allowance. The adoption of ASU 2016-09 will have no impact to the Company’s income statement, balance sheet, or retained earnings.

As of December 31, 2016, net deferred tax assets increased approximately $66.0 million primarily due to the operating loss and tax credits incurred during the year. This increase in net deferred tax assets was offset by a corresponding increase in the valuation allowance.

Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards and tax credit carryforwards. Under the applicable accounting standards, management has considered the Company’s history of losses and concluded that it is more likely than not that the Company will not recognize the benefits of federal and state deferred tax assets. Accordingly, a full valuation allowance of $135.1 million and $69.1 million has been established at December 31, 2016 and 2015, respectively.

Pursuant to Section 382 of the Internal Revenue Code, certain substantial changes in the Company’s ownership may result in a limitation on the amount of net operating loss carryforwards and tax carryforwards that may be used in future years. Utilization of the net operating loss (“NOL”) and tax credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership change limitations that have occurred previously or that could occur in the future. These ownership changes may limit the amount of NOL and tax credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. The Company has not completed a study to assess whether an ownership change has occurred, or whether there have been multiple ownership changes since its formation, due to significant complexity and related costs associated with such a study. There could also be additional ownership changes in the future which may result in additional limitations on the utilization of NOL carryforwards and credits. Further, until a study is completed and any limitation is known, no amounts are being presented as an uncertain tax position.

The Company applies the authoritative guidance on accounting for and disclosure of uncertainty in tax positions, which requires the Company to determine whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority.

The following is a rollforward of the Company’s unrecognized tax benefits:

	Year Ended December 31,
	2016		2015		2014
	(in thousands)
Unrecognized tax benefits—as of the beginning of the year	$	—	$	—	$	2,880
Gross increases—current period tax positions		—		—		—
Gross decreases—tax positions of prior periods		—		—		(2,880	)
Unrecognized tax benefits—as of the end of the year	$	—	$	—	$	—

During 2014, the Company filed an application for change in accounting method with the IRS to capitalize start-up costs that were historically deducted and included as part of the NOL carryforward through December 31, 2013. As a result, the Company’s unrecognized tax benefits, which historically related to start-up costs, are zero at December 31, 2016. The Company has not, as of yet, conducted a study of its R&D credit carryforwards. This study may result in an adjustment to the Company’s R&D credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position under Topic 740. A full valuation allowance has been provided against the Company’s R&D credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the consolidated balance sheets or statements of operations if an adjustment were required.

The Company will recognize interest and penalties related to uncertain tax positions in income tax expense when in a taxable income position. As of December 31, 2016 and 2015, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s statement of operations.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending tax examinations, and the Company’s tax returns are open under statute from 2013 to the present. The tax attributes prior to 2013 may still be adjusted upon examination. The Company’s policy is to record interest and penalties related to income taxes as part of the tax provision.

During November 2015, the FASB issued ASU 2015-17, Balance Sheet Classification of Deferred Taxes, which simplifies the presentation of deferred income taxes. This ASU requires that deferred tax assets and liabilities be classified as non-current in a statement of financial position. The standard is effective for public companies for fiscal years beginning after December 31, 2016, including interim periods within that reporting period. Early adoption is permitted for any interim and annual financial statements that have not yet been issued. The Company early adopted ASU 2015-17 effective December 31, 2015 on a prospective basis. Adoption of this ASU resulted in a reclassification of the current deferred tax liability to a non-current deferred tax liability, in the amount of $0.6 million, which is netted with the long-term deferred tax asset in its consolidated balance sheet as of December 31, 2015. No prior periods were retrospectively adjusted.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data (Unaudited)

12.	Selected Quarterly Financial Data (Unaudited)

The following table contains quarterly financial information for 2016 and 2015. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.

	2016
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total operating expenses	$	30,714		$	35,006		$	38,064		$	56,379		$	160,163
Loss from operations		(30,714	)		(35,006	)		(38,064	)		(56,379	)		(160,163	)
Net loss		(30,543	)		(34,747	)		(37,796	)	$	(55,901	)		(158,987	)
Net loss per share attributable to common stockholders—basic and diluted	$	(0.97	)	$	(1.08	)	$	(1.15	)	$	(1.50	)	$	(4.75	)

	2015
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total operating expenses	$	16,897		$	25,059		$	24,082		$	28,612		$	94,650
Loss from operations		(16,897	)		(25,059	)		(24,082	)		(28,612	)		(94,650	)
Net loss		(16,871	)		(25,027	)		(24,035	)		(28,562	)		(94,495	)
Net loss per share attributable to common stockholders—basic and diluted	$	(0.66	)	$	(0.90	)	$	(0.84	)	$	(0.99	)	$	(3.40	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Cash and Cash Equivalents

The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents.

Marketable Securities

Marketable securities

Restricted Cash

Property and Equipment

Impairment of Long-Lived Assets

Research and Development

Research Contract Costs and Accruals

Patent Costs

The Company expenses patent costs as incurred and classifies such costs as general and administrative expenses in the accompanying statements of operations and comprehensive loss.

Stock-Based Compensation

Treasury Stock

The Company records treasury stock at cost. Treasury stock includes shares received from an employee as consideration for an exercise of stock options.

Basic and Diluted Net Loss Per Share

The Company reported a net loss attributable to common stockholders for the years ended December 31, 2016, 2015 and 2014.

Risks and Uncertainties

Concentration of Credit Risk and of Significant Suppliers

Income Taxes

Fair Value Measurements

Level 1	—	Quoted market prices in active markets for identical assets or liabilities.

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.

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.

Deferred Offering Costs

Segment Data

Comprehensive Loss

Public Offerings

Recently Issued Accounting Pronouncements

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Summary of Company's Money Market Funds and Marketable Securities

The following tables summarize the Company’s money market funds and marketable securities as of December 31, 2016 and 2015:

	December 31, 2016
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash equivalents:
Money market funds	$	168,517	$	168,517	$	—	$	—
Total cash equivalents		168,517		168,517		—		—
Marketable securities:
U.S. government securities		100,031		—		100,031		—
U.S. corporate bonds		58,452		—		58,452		—
International corporate bonds		24,190		—		24,190		—
U.S. commercial paper		30,351		—		30,351		—
International commercial paper		15,938		—		15,938		—
Total marketable securities		228,962		—		228,962		—
Total cash equivalents and marketable securities	$	397,479	$	168,517	$	228,962	$	—

	December 31, 2015
	Total		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
	(in thousands)
Cash and cash equivalents:
Money market funds	$	186,753	$	186,753	$	—	$	—
Total cash and cash equivalents	$	186,753	$	186,753	$	—	$	—

Summary of Marketable Securities

The following table summarizes the Company’s marketable securities as of December 31, 2016:

	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Assets:
U.S. government securities (due within 1 year)	$	100,055	$	2	$	(26	)	$	100,031
U.S. corporate bonds		58,508		—		(56	)		58,452
International corporate bonds		24,212		—		(22	)		24,190
U.S. commercial paper		30,351		—		—			30,351
International commercial paper		15,938		—		—			15,938
	$	229,064	$	2	$	(104	)	$	228,962

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Recognized Stock-Based Compensation Expense

Stock-based compensation expense recognized during the years ended December 31, 2016, 2015 and 2014 was as follows:

	Year Ended December 31,
	2016		2015		2014
	(in thousands)
Research and development	$	11,197	$	5,924	$	1,093
General and administrative		11,823		9,316		1,419
	$	23,020	$	15,240	$	2,512

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

	Year Ended December 31,
	2016			2015			2014
Expected dividend yield		0	%		0	%		0	%
Expected volatility		80.15	%		90.54	%		98.86	%
Risk-free interest rate		1.47	%		1.59	%		1.95	%
Expected life of option	6.05 years			6.03 years			6.38 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 (in thousands)
Outstanding as of December 31, 2015		3,002,809		$	26.67		8.67	$	96,479
Granted		1,454,568			35.98
Exercised		(124,903	)		8.09
Forfeited		(100,667	)		44.76
Outstanding as of December 31, 2016		4,231,807		$	29.99		8.24	$	92,843
Vested and expected to vest as of December 31, 2016		3,727,837		$	29.36		8.21	$	84,395
Vested and exercisable as of December 31, 2016		1,702,819		$	24.11		7.59	$	48,096

Summary of Activity Relating to Restricted Stock

The table below summarizes activity relating to restricted stock:

	Shares
Outstanding as of December 31, 2015		42,781
Issued		—
Vested		(42,781	)
Forfeited		—
Repurchased		—
Outstanding as of December 31, 2016		—

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Summary of Basic and Diluted Net Loss Per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the years ended December 31, 2016, 2015 and 2014:

	Year Ended December 31,
	2016			2015			2014
Basic net loss per share attributable to common stockholders:
Numerator:
Net loss attributable to common stockholders (in thousands)	$	(158,987	)	$	(94,495	)	$	(36,105	)
Denominator:
Weighted average common stock outstanding—basic		33,492,795			27,778,288			21,574,347
Dilutive effect of shares of common stock equivalents resulting from common stock options and preferred common stock (as converted)		—			—			—
Weighted average common stock outstanding—diluted		33,492,795			27,778,288			21,574,347
Net loss per share attributable to common stockholders—basic and diluted	$	(4.75	)	$	(3.40	)	$	(1.67	)

Summary of Common Stock Equivalents Outstanding

	Year Ended December 31,
	2016		2015
Stock options		3,985,935		2,643,833
Employee stock purchase plan		6,784		3,307
Restricted stock		—		42,781
		3,992,719		2,689,921

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate

A reconciliation of U.S. statutory rate to the Company’s effective tax rate is as follows:

	Year Ended December 31,
	2016			2015			2014
Tax at Statutory Rate		34.0	%		34.0	%		34.0	%
State Taxes, net of federal benefit		4.2			4.0			4.5
Permanent Items		(1.0	)		(0.9	)		(1.0	)
Orphan Drug Credit Addback		(2.9	)		(4.2	)		—
Foreign Rate Differential		(3.1	)		(3.1	)		—
Federal and State Credits		10.3			13.2			8.5
Change in Valuation Allowance		(41.5	)		(43.1	)		(46.0	)
Other		—			0.1			—
		0.0	%		0.0	%		0.0	%

Summary of Significant Components of Company's Net Deferred Tax Asset

Significant components of the Company’s net deferred tax asset at December 31, 2016 and 2015 are as follows:

	December 31,
	2016			2015
	(in thousands)
Net operating losses	$	84,374		$	44,172
Capitalized start-up costs		2,155			2,335
Accounting method change		(673	)		(1,347	)
Tax credit carryforwards		33,477			17,013
Accrued expenses		2,287			1,062
Depreciation and amortization		999			716
Stock options		12,103			5,097
Others		350			21
Total net deferred tax asset before valuation allowance		135,072			69,069
Valuation allowance		(135,072	)		(69,069	)
Net deferred tax asset	$	-		$	-

Summary of Rollforward of Company's Unrecognized Tax Benefits

The following is a rollforward of the Company’s unrecognized tax benefits:

	Year Ended December 31,
	2016		2015		2014
	(in thousands)
Unrecognized tax benefits—as of the beginning of the year	$	—	$	—	$	2,880
Gross increases—current period tax positions		—		—		—
Gross decreases—tax positions of prior periods		—		—		(2,880	)
Unrecognized tax benefits—as of the end of the year	$	—	$	—	$	—

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Summary of Quarterly Financial Information

	2016
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total operating expenses	$	30,714		$	35,006		$	38,064		$	56,379		$	160,163
Loss from operations		(30,714	)		(35,006	)		(38,064	)		(56,379	)		(160,163	)
Net loss		(30,543	)		(34,747	)		(37,796	)	$	(55,901	)		(158,987	)
Net loss per share attributable to common stockholders—basic and diluted	$	(0.97	)	$	(1.08	)	$	(1.15	)	$	(1.50	)	$	(4.75	)

	2015
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter			Total
	(in thousands, except per share amounts)
Total operating expenses	$	16,897		$	25,059		$	24,082		$	28,612		$	94,650
Loss from operations		(16,897	)		(25,059	)		(24,082	)		(28,612	)		(94,650	)
Net loss		(16,871	)		(25,027	)		(24,035	)		(28,562	)		(94,495	)
Net loss per share attributable to common stockholders—basic and diluted	$	(0.66	)	$	(0.90	)	$	(0.84	)	$	(0.99	)	$	(3.40	)

Summary of Significant Accounting Policies - Additional Information (Detail)						1 Months Ended			12 Months Ended
	Sep. 14, 2016 USD ($) $ / shares shares	Jan. 12, 2016 USD ($) $ / shares shares	Apr. 20, 2015 USD ($) $ / shares shares	Jul. 23, 2014 USD ($) $ / shares shares	Jul. 02, 2014	May 31, 2016 ft²	Jan. 31, 2016 USD ($)	Apr. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit									$ 564,000	$ 39,000
Payment of offering costs				$ 2,300,000			$ 600,000	$ 500,000	$ 599,000	584,000	$ 2,285,000
Deferred offering costs										200,000
Issuance of common stock, shares \| shares	5,062,892	3,157,894	2,628,571
Common stock price per share \| $ / shares	$ 39.75	$ 47.50	$ 52.50
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					0.3175
Issuance of additional shares of common stock upon conversion \| shares				18,007,575
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering costs	$ 189,200,000	$ 140,400,000	$ 129,100,000
Initial Public Offering [Member]
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock, shares \| shares				5,750,000
Common stock price per share \| $ / shares				$ 18.00
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts				$ 94,000,000
ASU 2015-17, Balance Sheet Classification of Deferred Taxes [Member] \| ASU Early Adoption [Member]
Summary Of Significant Accounting Policies [Line Items]
Decrease in deferred tax liability current										(600,000)
Increase in deferred tax liability non-current										600,000
Operating Lease One [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit									$ 39,000	$ 39,000
Operating Lease Two [Member] \| Multi Tenant Building [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted cash deposit									$ 500,000
Additional Office space rented under operating lease \| ft²						19,805			22,067
Lease expire date						Feb. 28, 2022			Feb. 28, 2022

Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total cash equivalents	$ 168,517,000
Total marketable securities	228,962,000	$ 0
Total cash equivalents and marketable securities	397,479,000
Total cash and cash equivalents	168,517,000	186,753,000	$ 127,766,000	$ 8,066,000
Money Market Funds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total cash equivalents	168,517,000
Total cash and cash equivalents		186,753,000
Fair Value, Inputs, Level 1 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total cash equivalents	168,517,000
Total cash equivalents and marketable securities	168,517,000
Total cash and cash equivalents		186,753,000
Fair Value, Inputs, Level 1 [Member] \| Money Market Funds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total cash equivalents	168,517,000
Total cash and cash equivalents		$ 186,753,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	228,962,000
Total cash equivalents and marketable securities	228,962,000
U.S. Government Securities [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	100,031,000
U.S. Government Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	100,031,000
U.S. Corporate Bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	58,452,000
U.S. Corporate Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	58,452,000
International Corporate Bonds [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	24,190,000
International Corporate Bonds [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	24,190,000
U.S. Commercial Paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	30,351,000
U.S. Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	30,351,000
International Commercial Paper [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	15,938,000
International Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]
Total marketable securities	$ 15,938,000

Stock-Based Compensation - Additional Information (Detail)		12 Months Ended
	Jan. 01, 2017 shares	Dec. 31, 2016 USD ($) Milestone $ / shares shares	Dec. 31, 2015 USD ($) Milestone $ / shares shares	Dec. 31, 2014 USD ($) $ / shares	Jul. 02, 2014 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of options outstanding		4,891,922
Share based compensation granted under plan vest period		4 years
Share based compensation, vest period		1 year
Share based compensation, term of plan		10 years
Stock-based compensation expense \| $		$ 23,020,000	$ 15,240,000	$ 2,512,000
Milestone expense \| $		$ 0
Weighted average grant date fair value per share \| $ / shares		$ 24.97	$ 34.08	$ 14.33
Share option, forfeiture rates		9.58%	10.00%	10.00%
Non-employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, term of plan		10 years
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option grant		1,454,568
Total number of options outstanding		4,231,807	3,002,809
Total unrecognized stock-based compensation expense \| $		$ 43,600,000
Weighted average period of unrecognized compensation costs		2 years 9 months 7 days
Total fair value of options vested \| $		$ 24,100,000	$ 9,200,000	$ 1,000,000
Intrinsic value of options exercised \| $		$ 4,600,000	$ 28,400,000	$ 2,400,000
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option grant		245,872
Total unrecognized stock-based compensation expense \| $		$ 5,200,000
Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, vesting percentage		25.00%
2011 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option grant		0
2016 Inducement Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option grant		0
2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for issuance under stock option plan		660,115
Percentage of increase on issued and outstanding shares of Common stock		4.00%
Milestones achieved \| Milestone		1	1
Percentage of performance based grants that were achieved		50.00%
2014 Stock Option Plan [Member] \| Milestone One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants that were achieved			35.00%
2014 Stock Option Plan [Member] \| Milestone Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based grants that were achieved			30.00%
2014 Stock Option Plan [Member] \| Performance Milestone [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options with performance-based criteria		74,039	497,100
Stock-based compensation expense \| $		$ 5,000,000	$ 4,800,000
2014 Stock Option Plan [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares annual increase added to plan	1,488,886
2014 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense \| $		$ 200,000	$ 100,000
Shares of common stock initially authorized for issuance under this plan					282,000
Number of shares issued under the plan		14,351
Accrued expenses \| $		$ 67,000

Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Ending balance, Outstanding Shares-Stock options	4,891,922
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Outstanding Shares-Stock options	3,002,809
Granted, Stock options	1,454,568
Exercised, Stock options	(124,903)
Forfeited, Stock options	(100,667)
Ending balance, Outstanding Shares-Stock options	4,231,807	3,002,809
Vested and expected to vest as of December 31, 2016-Stock options	3,727,837
Vested and exercisable as of December 31, 2016-Stock options	1,702,819
Beginning balance, Outstanding Weighted Average Exercise Price-Stock options	$ 26.67
Granted, Weighted Average Exercise Price-Stock options	35.98
Exercised, Weighted Average Exercise Price-Stock options	8.09
Forfeited, Weighted Average Exercise Price-Stock options	44.76
Ending balance, Outstanding Weighted Average Exercise Price-Stock options	29.99	$ 26.67
Vested and expected to vest, Weighted Average Exercise Price-Stock options	29.36
Vested and exercisable, Weighted Average Exercise Price-Stock options	$ 24.11
Outstanding, Weighted Average Remaining Life-Stock options	8 years 2 months 27 days	8 years 8 months 1 day
Vested and expected to vest as of December 31, 2016-Stock options	8 years 2 months 16 days
Vested and exercisable as of December 31, 2016-Stock options	7 years 7 months 2 days
Outstanding, Aggregate Intrinsic Value-Stock options	$ 92,843	$ 96,479
Vested and expected to vest as of December 31, 2016-Stock options	84,395
Vested and exercisable as of December 31, 2016-Stock options	$ 48,096

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Income Tax Disclosure [Line Items]
Provision for income taxes		$ 0
Income tax position description		For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority.
Federal operating loss carryforwards		$ 235,400,000
State operating loss carryforwards		$ 234,300,000
Federal and state net operating loss carryforwards expiration year		2031
Federal orphan drug tax credit carry forwards		$ 29,800,000
Tax credit carry forwards expiration period		2034
Change in amount of deferred tax assets		$ 66,000,000
Valuation allowance		135,072,000	$ 69,069,000
Unrecognized tax benefit related to start-up costs		0		$ 2,880,000
Accrued interest or penalties related to uncertain tax positions		0	0
Uncertain tax positions		0
ASU 2016-09 [Member] \| Scenario, Forecast [Member]
Income Tax Disclosure [Line Items]
Increase in deferred tax assets associated with net operating losses	$ 8,000,000
Increase in deferred tax assets associated with federal and state research credits	$ 1,500,000
ASU 2015-17, Balance Sheet Classification of Deferred Taxes [Member] \| ASU Early Adoption [Member]
Income Tax Disclosure [Line Items]
Decrease in deferred tax liability current			(600,000)
Increase in deferred tax liability non-current			$ 600,000
Tax Deductions from Exercise of Stock Options [Member]
Income Tax Disclosure [Line Items]
Net operating losses exclude from deferred tax assets		8,000,000
Federal and state research and development credits exclude from deferred tax assets		1,500,000
Domestic Tax Authority [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards		$ 4,100,000
Research and development tax credits carryforwards expiration year		2031
Excess equity based compensation, tax deductions		$ 20,400,000
Excess equity based compensation, tax credits		1,300,000
State and Local Jurisdiction [Member]
Income Tax Disclosure [Line Items]
Research and development tax credits carryforwards		$ 1,600,000
Research and development tax credits carryforwards expiration year		2027
Excess equity based compensation, tax deductions		$ 20,400,000
Excess equity based compensation, tax credits		$ 200,000

Income Taxes - Summary of Reconciliation of U.S. Statutory Rate to Company's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Tax at Statutory Rate	34.00%	34.00%	34.00%
State Taxes, net of federal benefit	4.20%	4.00%	4.50%
Permanent Items	(1.00%)	(0.90%)	(1.00%)
Orphan Drug Credit Addback	(2.90%)	(4.20%)
Foreign Rate Differential	(3.10%)	(3.10%)
Federal and State Credits	10.30%	13.20%	8.50%
Change in Valuation Allowance	(41.50%)	(43.10%)	(46.00%)
Other		0.10%
Total effective tax rate	0.00%	0.00%	0.00%

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
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	$ 0.0001	$ 0.0001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	37,222,518	28,823,549
Common stock, shares outstanding	37,222,172	28,823,549
Treasury stock, shares	346	0

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	81 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Nature Of Business [Line Items]
Accumulated deficit	$ (320,327)	$ (161,340)
Cash, cash equivalents and marketable securities	397,479
Redeemable Convertible Preferred Stock [Member] \| Convertible Notes [Member] \| Initial Public Offering [Member]
Nature Of Business [Line Items]
Net proceeds from sale of equity and notes	$ 643,300

Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) MarketableSecurity	Dec. 31, 2015 USD ($)
Debt Instrument Fair Value Carrying Value [Line Items]
Transfers among the Level 1, Level 2 and Level 3 categories	$ 0	$ 0
Marketable securities	$ 228,962,000	0
Number of marketable securities loss position due to fluctuations in interest rates \| MarketableSecurity	27
Impairment of assets	$ 0	$ 0
Maximum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities, remaining contractual maturities	1 year

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,930	$ 547
Less: Accumulated depreciation	(542)	(261)
Property and equipment, net	1,388	286
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	708	400
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	695	$ 147
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 527

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 0.3	$ 0.1	$ 0.1
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years

Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2014 USD ($)
Series B Preferred Stock [Member]
Preferred Stock, issuance costs	$ 30
Series C Preferred Stock [Member]
Preferred Stock, issuance costs	$ 110

Fair Value Measurements - Summary of Marketable Securities (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 229,064
Gross Unrealized Gains	2
Gross Unrealized Losses	(104)
Fair Value	228,962
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	100,055
Gross Unrealized Gains	2
Gross Unrealized Losses	(26)
Fair Value	100,031
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,508
Gross Unrealized Losses	(56)
Fair Value	58,452
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,212
Gross Unrealized Losses	(22)
Fair Value	24,190
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,351
Fair Value	30,351
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,938
Fair Value	$ 15,938

Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Development costs	$ 14,541	$ 6,466
Employee-related expenses	5,948	2,718
Professional services	1,751	935
Other accrued expenses	112	29
Total accrued expenses	$ 22,352	$ 10,148

Commitments and Contingencies - Schedule of Future Minimum Lease Payments, Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2016 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2017	$ 2,666
2018	2,729
2019	2,771
2020	2,813
2021	2,855
Thereafter	480
Total Operating leases, future minimum payments	$ 14,314

Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Commitments And Contingencies [Line Items]
Research and development expense		$ 120,756,000	$ 69,357,000	$ 24,100,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense	$ 100,000
CyDex License Agreement [Member] \| First and Second Clinical Development Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		800,000
CyDex License Agreement [Member] \| First and Second Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		3,800,000
CyDex License Agreement [Member] \| Third and Fourth Clinical Development Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		1,300,000
CyDex License Agreement [Member] \| Third and Fourth Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		8,500,000
CyDex License Agreement [Member] \| Clinical Development [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense		800,000	800,000
Milestone payments		800,000	$ 800,000
CyDex License Agreement [Member] \| Clinical Development [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		800,000
CyDex License Agreement [Member] \| Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		$ 1,800,000

Commitments and Contingencies - Washington University License Agreement - Additional Information (Detail) - Washington University License Agreement [Member] - USD ($)	1 Months Ended	12 Months Ended
	Nov. 30, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Commitments And Contingencies [Line Items]
Upfront non-refundable payment	$ 50,000
Annual license maintenance fee				$ 15,000
Issuance of common stock, shares				47,619
Research and development expense related to milestone expense		$ 0		$ 100,000
Milestone payments		$ 0
Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			$ 50,000
Milestone payments			$ 50,000
Maximum [Member] \| Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	700,000
Maximum [Member] \| Clinical Development [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 500,000

Commitments and Contingencies - University of California License Agreement - Additional Information (Detail) - University of California License Agreement [Member]	1 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	May 31, 2014 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Milestone	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			$ 0		$ 0	$ 0
Milestone payments			$ 0		$ 0	$ 0
Upfront payment	$ 50,000
Annual license maintenance fee				$ 15,000
After The Effective Date [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period, maximum		27 years
After The First Sale [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period, maximum		15 years
Clinical Development [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense				100,000
Milestone payments				$ 100,000
Milestones achieved \| Milestone				3
Clinical Development [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Milestone payments		$ 100,000
Percentage of net sales paid as royalties		1.00%
Regulatory Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	700,000
Sales Milestones [Member] \| Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 2,000,000

Commitments and Contingencies - Consulting Agreement - Additional Information (Detail) - Consulting Agreement [Member]	1 Months Ended		12 Months Ended
	Mar. 31, 2014 Milestone	Jan. 31, 2014 USD ($) Milestone shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Milestone shares	Dec. 31, 2014 USD ($) shares
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			$ 0
Milestone payments			$ 0
Clinical Development and Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Milestone payment		$ 2,000,000
Shares of common stock for attaining milestones \| shares		126,984
First Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Milestones achieved \| Milestone	2	2
Research and development expense related to milestone expense					$ 200,000
Milestone payments					50,000
Payments for stock issued upon reaching a milestone					$ 100,000
Milestone based share compensation, shares issued \| shares					15,872
Second Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Milestones achieved \| Milestone				1
Research and development expense related to milestone expense				$ 1,700,000
Milestone payments				500,000
Payments for stock issued upon reaching a milestone				$ 1,200,000
Milestone based share compensation, shares issued \| shares				23,809
Third Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Milestones achieved \| Milestone				1
Research and development expense related to milestone expense				$ 1,700,000
Milestone payments				500,000
Payments for stock issued upon reaching a milestone				$ 1,200,000
Milestone based share compensation, shares issued \| shares				23,809

Preferred Stock - Additional Information (Detail) - shares	Dec. 31, 2016	Dec. 31, 2015
Temporary Equity Disclosure [Abstract]
Preferred stock, shares authorized	5,000,000	5,000,000

Common Stock - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Common stock, shares authorized	120,000,000	120,000,000
Common stock, par value	$ 0.0001	$ 0.0001
Dividends declared	$ 0	$ 0
Voting rights	Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders.
Purchase of treasury stock, Shares	346	0
Total cost of shares held in treasury	$ 17,000

Stock-Based Compensation - Summary of Activity Relating to Restricted Stock (Detail) - Restricted Stock [Member]	12 Months Ended
	Dec. 31, 2016 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, Shares at beginning balance-Restricted stock	42,781
Vested, Shares-Restricted stock	(42,781)
Outstanding, Shares at ending balance-Restricted stock

Net Loss Per Share - Summary of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss attributable to common stockholders									$ (158,987)	$ (94,495)	$ (36,105)
Denominator:
Weighted average common stock outstanding—basic									33,492,795	27,778,288	21,574,347
Dilutive effect of shares of common stock equivalents resulting from common stock options and preferred common stock (as converted)									0	0	0
Weighted average common stock outstanding—diluted									33,492,795	27,778,288	21,574,347
Net loss per share attributable to common stockholders—basic and diluted	$ (1.50)	$ (1.15)	$ (1.08)	$ (0.97)	$ (0.99)	$ (0.84)	$ (0.90)	$ (0.66)	$ (4.75)	$ (3.40)	$ (1.67)

Net Loss Per Share - Summary of Common Stock Equivalents Outstanding (Detail) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	3,992,719	2,689,921
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	3,985,935	2,643,833
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	6,784	3,307
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents		42,781

Income Taxes - Summary of Significant Components of Company's Net Deferred Tax Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Net operating losses	$ 84,374	$ 44,172
Capitalized start-up costs	2,155	2,335
Accounting method change	(673)	(1,347)
Tax credit carryforwards	33,477	17,013
Accrued expenses	2,287	1,062
Depreciation and amortization	999	716
Stock options	12,103	5,097
Others	350	21
Total net deferred tax asset before valuation allowance	135,072	69,069
Valuation allowance	$ (135,072)	$ (69,069)

Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Millions		12 Months Ended
	Nov. 01, 2014	Dec. 31, 2016	Dec. 31, 2015
Postemployment Benefits [Abstract]
Employer matching contribution, percent of match	50.00%
Maximum annual contributions per employee, percent	6.00%
Employer contribution		$ 0.4	$ 0.2