Sage Therapeutics (SAGE) 10-Q 10 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	May 03, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Trading Symbol	SAGE
Entity Registrant Name	Sage Therapeutics, Inc.
Entity Central Index Key	1,597,553
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		37,346,289

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 145,460	$ 168,517
Marketable securities	197,104	228,962
Prepaid expenses and other current assets	6,480	5,100
Total current assets	349,044	402,579
Property and equipment, net	1,560	1,388
Restricted cash	564	564
Total assets	351,168	404,531
Current liabilities:
Accounts payable	8,230	12,817
Accrued expenses	23,652	22,352
Total current liabilities	31,882	35,169
Other liabilities	840	845
Total liabilities	32,722	36,014
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at March 31, 2017 and December 31, 2016; no shares issued or outstanding at March 31, 2017 and December 31, 2016
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at March 31, 2017 and December 31, 2016; 37,307,047 and 37,222,518 shares issued at March 31, 2017 and December 31, 2016, respectively; 37,306,701 and 37,222,172 shares outstanding at March 31, 2017 and December 31, 2016, respectively	4	4
Treasury stock, at cost; 346 shares at March 31, 2017 and December 31, 2016	(17)	(17)
Additional paid-in capital	695,645	688,959
Accumulated deficit	(377,105)	(320,327)
Accumulated other comprehensive income (loss)	(81)	(102)
Total stockholders’ equity	318,446	368,517
Total liabilities and stockholders’ equity	$ 351,168	$ 404,531

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2017	Dec. 31, 2016
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	37,307,047	37,222,518
Common stock, shares outstanding	37,306,701	37,222,172
Treasury stock, shares	346	346

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating expenses:
Research and development	$ 45,200	$ 23,581
General and administrative	12,280	7,133
Total operating expenses	57,480	30,714
Loss from operations	(57,480)	(30,714)
Interest income, net	707	175
Other expense, net	(5)	(4)
Net loss	$ (56,778)	$ (30,543)
Net loss per share—basic and diluted	$ (1.52)	$ (0.97)
Weighted average common shares outstanding—basic and diluted	37,269,148	31,643,216
Comprehensive loss:
Net loss	$ (56,778)	$ (30,543)
Other comprehensive items:
Unrealized gain on marketable securities	21
Total other comprehensive gain	21
Total comprehensive loss	$ (56,757)	$ (30,543)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities
Net loss	$ (56,778)	$ (30,543)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,208	3,714
Amortization of premium on marketable securities	33
Depreciation	130	41
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,380)	(1,189)
Accounts payable	(4,643)	(799)
Accrued expenses and other liabilities	1,220	1,341
Net cash used in operating activities	(55,210)	(27,435)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	50,753
Purchases of marketable securities	(18,908)
Purchases of property and equipment	(245)	(341)
Net cash provided by (used in) investing activities	31,600	(341)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	553	302
Payments of offering costs		(599)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts		141,000
Net cash provided by financing activities	553	140,703
Net increase (decrease) in cash and cash equivalents	(23,057)	112,927
Cash and cash equivalents at beginning of period	168,517	186,753
Cash and cash equivalents at end of period	145,460	299,680
Supplemental disclosure of non-cash financing activities
Purchases of property and equipment included in accounts payable or accrued expenses	$ 64	$ 279

Nature of Operations

Nature of Operations	3 Months Ended
	Mar. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Operations	1. Nature of Operations Sage Therapeutics, Inc. (“Sage” or the “Company”) is a clinical-stage biopharmaceutical company committed to developing and commercializing novel medicines to treat life-altering central nervous system, or CNS, disorders, where there are no approved therapies or existing therapies are inadequate. The Company has a portfolio of product candidates with a current focus on modulating two critical CNS receptor systems, GABA and NMDA. The GABA receptor family, which is recognized as the major inhibitory neurotransmitter in the CNS, mediates downstream neurologic and bodily function via activation of GABA A The Company was incorporated under the laws of the State of Delaware on April 16, 2010, and commenced operations on January 19, 2011 as Sterogen Biopharma, Inc. On September 13, 2011, the Company changed its name to Sage Therapeutics, Inc. under its Second Amended and Restated Certificate of Incorporation. The Company is subject to risks and uncertainties common to companies in the biotech industry, including, but not limited to, the risks associated with developing product candidates at each stage of non-clinical and clinical development; the challenges associated with gaining regulatory approval of such product candidates; the risks associated with commercializing pharmaceutical products, if it is able to obtain regulatory approval; the potential for development by third parties of new technological innovations that may compete with the Company’s products; the dependence on key personnel; the challenges of protecting proprietary technology; the need to comply with government regulations; the high costs of drug development; and the uncertainty of being able to secure additional capital when needed to fund operations. The Company has incurred losses and negative cash flows from operations since its inception. As of March 31, 2017, the Company had an accumulated deficit of $377.1 million. From its inception through March 31, 2017, the Company received net proceeds of $643.3 million from the sales of redeemable convertible preferred stock, the issuance of convertible notes, and the proceeds from its initial public offering (“IPO”) in July 2014 and follow-on underwritten public offerings in April 2015, January 2016 and September 2016. Until such time, if ever, as the Company can generate substantial product revenue, the Company expects to finance its cash needs through a combination of equity offerings, debt financings, collaborations, strategic alliances, licensing arrangements and other sources of funding. If the Company is unable to raise additional funds through equity or debt financings when needed, the Company may be required to delay, limit, reduce or terminate product development or future commercialization efforts or grant rights to develop and market products or product candidates that the Company would otherwise prefer to develop and market itself. Based on its current operating plans, the Company believes its cash, cash equivalents and marketable securities of $342.6 million as of March 31, 2017 will be sufficient to fund its anticipated level of operations and capital expenditures into the second quarter of 2018. In August 2014, the Financial Accounting Standards Board issued Accounting Standards Update, or ASU, 2014-15, Presentation of Financial Statements—Going Concern

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The unaudited interim condensed consolidated financial statements of the Company included herein have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2016. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of the Company’s management, the accompanying unaudited condensed consolidated financial statements contain all adjustments, consisting of only normal recurring adjustments, necessary for a fair presentation of its financial position as of March 31, 2017, its results of operations and comprehensive loss for the three months ended March 31, 2017 and 2016, and its cash flows for the three months ended March 31, 2017 and 2016. The consolidated balance sheet at December 31, 2016 was derived from audited financial statements, but does not include all disclosures required by GAAP. The results for the three months ended March 31, 2017 are not necessarily indicative of the results for the year ending December 31, 2017, or for any future period. Principles of Consolidation The unaudited interim condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying its Annual Report on Form 10-K for the fiscal year ended December 31, 2016. Intercompany accounts and transactions have been eliminated. 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. 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. 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. 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. 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 three months ended March 31, 2017. 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 March 31, 2017 and December 31, 2016 were carried at fair value, determined according to the fair value hierarchy. The carrying amounts reflected in the unaudited consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at March 31, 2017 and December 31, 2016, respectively. Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers 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 In February 2016, the FASB issued ASU No. 2016-02, Leases In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, 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 The standard will be effective 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 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

Fair Value Measurements	3 Months Ended
	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company’s cash equivalents are 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 based on prices obtained from independent pricing sources. Consistent with the fair value hierarchy described above, securities with validated quotes from pricing services are 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 March 31, 2017 and December 31, 2016: March 31, 2017 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 $ 145,460 $ 145,460 $ — $ — Total cash equivalents 145,460 145,460 — — Marketable securities: U.S. government securities 91,970 — 91,970 — U.S. corporate bonds 39,118 — 39,118 — International corporate bonds 15,181 — 15,181 — U.S. commercial paper 21,891 — 21,891 — International commercial paper 28,944 — 28,944 — Total marketable securities 197,104 — 197,104 — Total cash equivalents and marketable securities $ 342,564 $ 145,460 $ 197,104 $ — 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 $ — During the three months ended March 31, 2017 and 2016, there were no transfers among the Level 1, Level 2 and Level 3 categories. Marketable Securities The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of March 31, 2017 and December 31, 2016: March 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Assets: U.S. government securities $ 92,026 $ — $ (56 ) $ 91,970 U.S. corporate bonds 39,138 — (20 ) 39,118 International corporate bonds 15,186 — (5 ) 15,181 U.S. commercial paper 21,891 — — 21,891 International commercial paper 28,944 — — 28,944 $ 197,185 $ — $ (81 ) $ 197,104 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Assets: U.S. government securities $ 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 As of March 31, 2017, all marketable securities held by the Company had remaining contractual maturities of one year or less. There have been no impairments of the Company’s assets measured and carried at fair value during the three months ended March 31, 2017 and the year ended December 31, 2016.

Accrued Expenses

Accrued Expenses	3 Months Ended
	Mar. 31, 2017
Payables And Accruals [Abstract]
Accrued Expenses	4. Accrued Expenses Accrued expenses consist of the following: March 31, 2017 December 31, 2016 (in thousands) Development costs $ 18,792 $ 14,541 Employee-related expenses 2,145 5,948 Professional services 2,715 1,751 Other accrued expenses — 112 $ 23,652 $ 22,352

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2017
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. Future minimum lease payments under non-cancelable operating leases are as follows at March 31, 2017: Years Ending December 31, (in thousands) 2017 $ 2,009 2018 2,729 2019 2,771 2020 2,813 2021 2,855 Thereafter 480 $ 13,657 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. In March 2015, a clinical development milestone was met for the SAGE-547 program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense for the three months ended March 31, 2015 of $0.3 million. In April 2015, an additional clinical development milestone was met for the SAGE-547 program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense for the three months ended June 30, 2015 of $0.5 million. In August 2016, an additional clinical development milestone was met for the SAGE-547 program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense for the three months ended September 30, 2016 of $0.3 million. In December 2016, an additional clinical development milestone was met for the SAGE-547 program under the license agreement with CyDex, and accordingly, the Company recorded research and development expense for the three months ended December 31, 2016 of $0.5 million. For the three months ended March 31, 2017, the Company did not record any expense or make any milestone payments under the license agreement with CyDex. 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. In September 2015, a regulatory milestone was met for one of the programs. Accordingly, the Company recorded research and development expenses and made a cash payment of $50,000. 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 three months ended March 31, 2017, the Company did not record any expense or make any milestone payments under the license agreement with Washington University. University of California License Agreements 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, 2013 and 2014, 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. For the three months ended March 31, 2017, 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 and three months ended March 31, 2017, the Company did not record any expense or make any milestone payments under the consulting agreement with the non-employee advisor.

Sale of Equity Securities

Sale of Equity Securities	3 Months Ended
	Mar. 31, 2017
Other Liabilities Disclosure [Abstract]
Sale of Equity Securities	6. Sale of Equity Securities 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 expenses 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 paid by the Company.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	7. Stock-Based Compensation Stock Option Plans On December 15, 2016, the Board of Directors of the Company approved the 2016 Inducement Equity Plan (the “2016 Stock Option Plan”). The 2016 Stock Option Plan provides for the grant of equity awards to individuals who have not previously been an employee or a non-employee director of the Company to induce them to accept employment and to provide them with a proprietary interest in the Company. As of March 31, 2017, the total number of shares reserved under the 2016 Stock Option Plan and the 2014 Stock Option and Incentive Plan (the “2014 Stock Option Plan”) was 8,283,295, and the Company had 2,798,233 shares available for future issuance under such plans. The 2014 Stock Option Plan provides for an annual increase, to be added on the first day of each year, by up to 4% of the Company’s issued and outstanding shares of common stock on the last day of the prior 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. During the three months ended March 31, 2017 and 2016, the Company granted 449,208 and 74,039 options, respectively, to employees to purchase shares of common stock that contain performance-based vesting criteria, primarily related to the 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 three months ended March 31, 2017 and 2016, the achievement of none of the remaining milestones that are criteria for performance-based stock options were considered probable, nor met, and therefore no expense has been recognized related to these awards for the three months ended March 31, 2017 and 2016. Stock-based compensation expense recognized during the three months ended March 31, 2017 and 2016 was as follows: Three Months Ended March 31, 2017 2016 (in thousands) Research and development $ 3,596 $ 1,611 General and administrative 2,612 2,103 $ 6,208 $ 3,714 The weighted average grant date fair value per share relating to outstanding stock options granted under the Company’s stock option plans during the three months ended March 31, 2017 and 2016 was $34.61 and $21.87, respectively. 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, 2016 4,231,807 $ 29.99 8.24 $ 92,843 Granted 1,370,800 49.74 Exercised (97,513 ) 8.53 Forfeited (51,632 ) 37.12 Outstanding as of March 31, 2017 5,453,462 $ 35.27 8.47 $ 195,365 Exercisable as of March 31, 2017 1,940,608 $ 25.03 7.47 $ 89,484 At March 31, 2017, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $62.7 million, which is expected to be recognized over the remaining weighted average vesting period of 2.99 years. The total fair value of options vested for the three months ended March 31, 2017 and 2016 was $6.1 million and $4.1 million, respectively. In addition, the Company granted 691,654 performance-based stock options that are both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $14.9 million at March 31, 2017. Restricted Stock Units During the three months ended March 31, 2017, the Company granted 32,500 restricted stock units to employees of the Company. The Company did not grant restricted stock units prior to January 1, 2017. During the three months ended March 31, 2017, the Company recorded $0.1 million of stock-based compensation expense related to its restricted stock units. These restricted stock units vest ratably over two years, with a 50% cliff vesting at both the one year and two year anniversary of the grant.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Net Loss Per Share	8. Net Loss Per Share Basic and diluted net loss per share was calculated as follows for the three months ended March 31, 2017 and 2016: Three Months Ended March 31, 2017 2016 Basic net loss per share: Numerator: Net loss (in thousands) $ (56,778 ) $ (30,543 ) Denominator: Weighted average common stock outstanding—basic 37,269,148 31,643,216 Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units — — Weighted average common stock outstanding—diluted 37,269,148 31,643,216 Net loss per share—basic and diluted $ (1.52 ) $ (0.97 ) The following common stock equivalents outstanding as of March 31, 2017 and 2016 were excluded from the computation of diluted net loss per share for the periods presented because including them would have been anti-dilutive: Three Months Ended March 31, 2017 2016 Stock options 4,801,491 3,146,627 Restricted stock units 31,600 — Employee stock purchase plan 4,676 3,851 Restricted stock awards — 24,721 4,837,767 3,175,199

Subsequent Event

Subsequent Event	3 Months Ended
	Mar. 31, 2017
Subsequent Events [Abstract]
Subsequent Event	9. Subsequent Event In May 2017, the Company entered into the Sixth Amendment to the Lease with ARE-MA Region No. 38 (“Sixth Amendment”). The Company increased the amount of square feet of office space at 215 First Street, Cambridge, Massachusetts, by 32,876 square feet, consisting of (i) 8,200 rentable square feet beginning on or around August 15, 2017, and (ii) 24,676 rentable square feet beginning on or around January 1, 2018. The term for this additional space will expire no later than 84 months after the date on which the Company begins to rent the portion of the lease that is approximately 8,200 square feet. Additionally, the term of the existing lease will be extended from March 1, 2022 until the expiration date of the Sixth Amendment, which is anticipated to be on or around August 15, 2024. The increase in future expected payments under the Sixth Amendment will be approximately $15.8 million.

Summary of Significant Accoun15

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The unaudited interim condensed consolidated financial statements of the Company included herein have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2016. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of the Company’s management, the accompanying unaudited condensed consolidated financial statements contain all adjustments, consisting of only normal recurring adjustments, necessary for a fair presentation of its financial position as of March 31, 2017, its results of operations and comprehensive loss for the three months ended March 31, 2017 and 2016, and its cash flows for the three months ended March 31, 2017 and 2016. The consolidated balance sheet at December 31, 2016 was derived from audited financial statements, but does not include all disclosures required by GAAP. The results for the three months ended March 31, 2017 are not necessarily indicative of the results for the year ending December 31, 2017, or for any future period.
Principles of Consolidation	Principles of Consolidation The unaudited interim condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries as disclosed in Note 2, Summary of Significant Accounting Policies, within the “Notes to Consolidated Financial Statements” accompanying its Annual Report on Form 10-K for the fiscal year ended December 31, 2016. Intercompany accounts and transactions have been eliminated.
Use of Estimates	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.
Research and Development	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. 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.
Stock-Based Compensation	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.
Marketable Securities	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 three months ended March 31, 2017.
Fair Value Measurements	Fair Value Measurements Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Financial assets and liabilities carried at fair value are 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 March 31, 2017 and December 31, 2016 were carried at fair value, determined according to the fair value hierarchy. The carrying amounts reflected in the unaudited consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at March 31, 2017 and December 31, 2016, respectively.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers 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 In February 2016, the FASB issued ASU No. 2016-02, Leases In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, 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 The standard will be effective 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 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 (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2017
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 March 31, 2017 and December 31, 2016: March 31, 2017 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 $ 145,460 $ 145,460 $ — $ — Total cash equivalents 145,460 145,460 — — Marketable securities: U.S. government securities 91,970 — 91,970 — U.S. corporate bonds 39,118 — 39,118 — International corporate bonds 15,181 — 15,181 — U.S. commercial paper 21,891 — 21,891 — International commercial paper 28,944 — 28,944 — Total marketable securities 197,104 — 197,104 — Total cash equivalents and marketable securities $ 342,564 $ 145,460 $ 197,104 $ — 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 $ —
Summary of Gross Unrealized Gains and Losses of Marketable Securities	The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of March 31, 2017 and December 31, 2016: March 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Assets: U.S. government securities $ 92,026 $ — $ (56 ) $ 91,970 U.S. corporate bonds 39,138 — (20 ) 39,118 International corporate bonds 15,186 — (5 ) 15,181 U.S. commercial paper 21,891 — — 21,891 International commercial paper 28,944 — — 28,944 $ 197,185 $ — $ (81 ) $ 197,104 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Assets: U.S. government securities $ 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

Accrued Expenses (Tables)

Accrued Expenses (Tables)	3 Months Ended
	Mar. 31, 2017
Payables And Accruals [Abstract]
Summary of Accrued Expenses	Accrued expenses consist of the following: March 31, 2017 December 31, 2016 (in thousands) Development costs $ 18,792 $ 14,541 Employee-related expenses 2,145 5,948 Professional services 2,715 1,751 Other accrued expenses — 112 $ 23,652 $ 22,352

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments, Under Non-Cancelable Operating Leases	Future minimum lease payments under non-cancelable operating leases are as follows at March 31, 2017: Years Ending December 31, (in thousands) 2017 $ 2,009 2018 2,729 2019 2,771 2020 2,813 2021 2,855 Thereafter 480 $ 13,657

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Recognized Stock-Based Compensation Expense	Stock-based compensation expense recognized during the three months ended March 31, 2017 and 2016 was as follows: Three Months Ended March 31, 2017 2016 (in thousands) Research and development $ 3,596 $ 1,611 General and administrative 2,612 2,103 $ 6,208 $ 3,714
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, 2016 4,231,807 $ 29.99 8.24 $ 92,843 Granted 1,370,800 49.74 Exercised (97,513 ) 8.53 Forfeited (51,632 ) 37.12 Outstanding as of March 31, 2017 5,453,462 $ 35.27 8.47 $ 195,365 Exercisable as of March 31, 2017 1,940,608 $ 25.03 7.47 $ 89,484

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Summary of Basic and Diluted Net Loss Per Share	Basic and diluted net loss per share was calculated as follows for the three months ended March 31, 2017 and 2016: Three Months Ended March 31, 2017 2016 Basic net loss per share: Numerator: Net loss (in thousands) $ (56,778 ) $ (30,543 ) Denominator: Weighted average common stock outstanding—basic 37,269,148 31,643,216 Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units — — Weighted average common stock outstanding—diluted 37,269,148 31,643,216 Net loss per share—basic and diluted $ (1.52 ) $ (0.97 )
Summary of Anti-Dilutive Common Stock Equivalents Outstanding	The following common stock equivalents outstanding as of March 31, 2017 and 2016 were excluded from the computation of diluted net loss per share for the periods presented because including them would have been anti-dilutive: Three Months Ended March 31, 2017 2016 Stock options 4,801,491 3,146,627 Restricted stock units 31,600 — Employee stock purchase plan 4,676 3,851 Restricted stock awards — 24,721 4,837,767 3,175,199

Nature of Operations - Addition

Nature of Operations - Additional Information (Detail) - USD ($) $ in Thousands	84 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Nature Of Operations [Line Items]
Accumulated deficit	$ (377,105)	$ (320,327)
Cash, cash equivalents and marketable securities	342,564
Redeemable Convertible Preferred Stock [Member] | Convertible Notes [Member] | Initial Public Offering [Member]
Nature Of Operations [Line Items]
Net proceeds from sale of equity and notes	$ 643,300

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 145,460	$ 168,517
Total marketable securities	197,104	228,962
Total cash equivalents and marketable securities	342,564	397,479
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	145,460	168,517
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	145,460	168,517
Total cash equivalents and marketable securities	145,460	168,517
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	145,460	168,517
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	197,104	228,962
Total cash equivalents and marketable securities	197,104	228,962
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	91,970	100,031
U.S. Government Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	91,970	100,031
U.S. Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	39,118	58,452
U.S. Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	39,118	58,452
International Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	15,181	24,190
International Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	15,181	24,190
U.S. Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	21,891	30,351
U.S. Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	21,891	30,351
International Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	28,944	15,938
International Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	$ 28,944	$ 15,938

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Debt Instrument Fair Value Carrying Value [Line Items]
Transfers among the Level 1, Level 2 and Level 3 categories	$ 0		$ 0
Impairment of assets	$ 0	$ 0
Maximum [Member]
Debt Instrument Fair Value Carrying Value [Line Items]
Marketable securities, remaining contractual maturities	1 year

Fair Value Measurements - Sum24

Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 197,185	$ 229,064
Gross Unrealized Gains		2
Gross Unrealized Losses	(81)	(104)
Fair Value	197,104	228,962
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	92,026	100,055
Gross Unrealized Gains		2
Gross Unrealized Losses	(56)	(26)
Fair Value	91,970	100,031
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,138	58,508
Gross Unrealized Losses	(20)	(56)
Fair Value	39,118	58,452
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,186	24,212
Gross Unrealized Losses	(5)	(22)
Fair Value	15,181	24,190
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,891	30,351
Fair Value	21,891	30,351
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,944	15,938
Fair Value	$ 28,944	$ 15,938

Accrued Expenses - Summary of A

Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Development costs	$ 18,792	$ 14,541
Employee-related expenses	2,145	5,948
Professional services	2,715	1,751
Other accrued expenses		112
Total accrued expenses	$ 23,652	$ 22,352

Commitments and Contingencies -

Commitments and Contingencies - Operating Leases - Additional Information (Detail) - ft²	1 Months Ended	3 Months Ended
	May 31, 2016	Mar. 31, 2017
Operating Lease One [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease		22,067
Lease expire date		Feb. 28, 2022
Operating Lease Two [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease	19,805
Lease expire date	Feb. 28, 2022
Commencement date of lease	Sep. 30, 2016

Commitments and Contingencies27

Commitments and Contingencies - Schedule of Future Minimum Lease Payments, Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Mar. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,017	$ 2,009
2,018	2,729
2,019	2,771
2,020	2,813
2,021	2,855
Thereafter	480
Total Operating leases, future minimum payments	$ 13,657

Commitments and Contingencies28

Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015
Commitments And Contingencies [Line Items]
Research and development expense	$ 45,200,000			$ 23,581,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense					$ 100,000
Research and development expense related to milestone expense	0
Milestone payments	0
CyDex License Agreement [Member] | First and Second Clinical Development Milestones [Member] | SAGE-547 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000
CyDex License Agreement [Member] | First and Second Regulatory Milestones [Member] | SAGE-547 [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] | SAGE-547 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	1,300,000
CyDex License Agreement [Member] | Third and Fourth Regulatory Milestones [Member] | SAGE-547 [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		$ 500,000	$ 300,000			$ 500,000	$ 300,000
CyDex License Agreement [Member] | Clinical Development [Member] | SAGE-689 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000
CyDex License Agreement [Member] | Regulatory Milestones [Member] | SAGE-689 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 1,800,000

Commitments and Contingencies29

Commitments and Contingencies - Washington University License Agreement - Additional Information (Detail) - Washington University License Agreement [Member] - USD ($)	1 Months Ended		3 Months Ended	12 Months Ended
	Sep. 30, 2015	Nov. 30, 2013	Mar. 31, 2017	Dec. 31, 2016	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 issue of common stock obligation					$ 100,000
Research and development expense related to milestone expense			$ 0	$ 0
Milestone payments			$ 0	$ 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] | Clinical Development [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		700,000
Maximum [Member] | Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		$ 500,000

Commitments and Contingencies30

Commitments and Contingencies - University of California License Agreements - Additional Information (Detail) - University of California License Agreements [Member]	1 Months Ended		3 Months Ended	12 Months Ended
	Jun. 30, 2015USD ($)	May 31, 2014USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)Milestone	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			$ 0	$ 0		$ 0	$ 0
Milestone payments			$ 0	$ 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 Contingencies31

Commitments and Contingencies - Consulting Agreement - Additional Information (Detail) - Consulting Agreement [Member]	1 Months Ended		3 Months Ended	12 Months Ended
	Mar. 31, 2014Milestone	Jan. 31, 2014USD ($)Milestoneshares	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)Milestoneshares	Dec. 31, 2014USD ($)shares
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			$ 0	$ 0
Milestone payments			$ 0	$ 0
Clinical Development and Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Shares of common stock for attaining milestones | shares		126,984
Clinical Development and Regulatory Milestones [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		$ 2,000,000
First Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Milestones achieved | Milestone	1	1
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 issuance of shares | shares						15,872
Second and Third Clinical Development Milestones [Member]
Commitments And Contingencies [Line Items]
Milestones achieved | Milestone					2
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 issuance of shares | shares					23,809

Sale of Equity Securities - Add

Sale of Equity Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Sep. 14, 2016	Jan. 12, 2016
Equity [Abstract]
Issuance of common stock, shares	5,062,892	3,157,894
Common stock price per share	$ 39.75	$ 47.50
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering expenses	$ 189.2	$ 140.4

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Jan. 02, 2016	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 6,208,000	$ 3,714,000
Weighted average grant date fair value per share		$ 34.61	$ 21.87
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		449,208	74,039
Total unrecognized stock-based compensation expense		$ 14,900,000
Number of shares outstanding and unvested stock options		691,654
Performance-Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 0	$ 0
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		1,370,800
Total unrecognized stock-based compensation expense		$ 62,700,000
Weighted average period of unrecognized compensation costs		2 years 11 months 27 days
Total fair value of options vested		$ 6,100,000	$ 4,100,000
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense		$ 100,000
Restricted stock units granted		32,500
Restricted stock units vesting period		2 years
Restricted Stock Awards [Member] | Restricted Stock Units Vest One Year Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units cliff vesting percentage		50.00%
Restricted Stock Awards [Member] | Restricted Stock Units Vest Two Year Anniversary [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units cliff vesting percentage		50.00%
2016 Stock Option Plan and 2014 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of shares reserved under option plans		8,283,295
Common stock available for issuance under stock option plans		2,798,233
2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares annual increase added to plan	1,488,886
2014 Stock Option Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of increase on issued and outstanding shares of Common stock				4.00%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Recognized Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 6,208	$ 3,714
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	3,596	1,611
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 2,612	$ 2,103

Stock-Based Compensation - Su35

Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) - Stock Options [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Outstanding Shares	4,231,807
Granted, Shares	1,370,800
Exercised, Shares	(97,513)
Forfeited, Shares	(51,632)
Ending balance, Outstanding Shares	5,453,462	4,231,807
Exercisable, Shares	1,940,608
Beginning balance, Outstanding Weighted Average Exercise Price	$ 29.99
Granted, Weighted Average Exercise Price	49.74
Exercised, Weighted Average Exercise Price	8.53
Forfeited, Weighted Average Exercise Price	37.12
Ending balance, Outstanding Weighted Average Exercise Price	35.27	$ 29.99
Exercisable, Weighted Average Exercise Price	$ 25.03
Outstanding, Weighted Average Remaining Life	8 years 5 months 19 days	8 years 2 months 27 days
Exercisable, Weighted Average Remaining Life	7 years 5 months 19 days
Outstanding, Aggregate Intrinsic Value	$ 195,365	$ 92,843
Exercisable, Aggregate Intrinsic Value	$ 89,484

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net loss	$ (56,778)	$ (30,543)
Denominator:
Weighted average common stock outstanding—basic	37,269,148	31,643,216
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units	0	0
Weighted average common stock outstanding—diluted	37,269,148	31,643,216
Net loss per share—basic and diluted	$ (1.52)	$ (0.97)

Net Loss Per Share - Summary 37

Net Loss Per Share - Summary of Anti-Dilutive Common Stock Equivalents Outstanding (Detail) - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	4,837,767	3,175,199
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	4,801,491	3,146,627
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	31,600
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	4,676	3,851
Restricted Stock Awards [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents		24,721

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) $ in Millions	Jan. 01, 2018ft²	Aug. 15, 2017ft²	May 09, 2017USD ($)ft²
Sixth Amendment to Lease [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Increased office space rent			32,876
Lease expiration date			Aug. 15, 2024
Increase in future expected payments | $			$ 15.8
Sixth Amendment to Lease [Member] | Scenario Forecast [Member]
Subsequent Event [Line Items]
Increased office space rent	24,676	8,200
Sixth Amendment to Lease [Member] | Scenario Forecast [Member] | Maximum [Member]
Subsequent Event [Line Items]
Additional space expiration term		84 months
Before Sixth Amendment to Lease [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Lease expiration date			Mar. 1, 2022