Sage Therapeutics (SAGE) 10-Q 6 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
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 Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		46,887,174

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 253,123	$ 306,235
Marketable securities	768,278	212,613
Prepaid expenses and other current assets	18,511	6,227
Total current assets	1,039,912	525,075
Property and equipment, net	5,168	4,013
Restricted cash	1,919	849
Total assets	1,046,999	529,937
Current liabilities:
Accounts payable	5,879	9,350
Accrued expenses	48,647	42,601
Total current liabilities	54,526	51,951
Other liabilities	3,968	2,511
Total liabilities	58,494	54,462
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.0001 par value per share; 5,000,000 shares authorized at September 30, 2018 and December 31, 2017; no shares issued or outstanding at September 30, 2018 and December 31, 2017
Common stock, $0.0001 par value per share; 120,000,000 shares authorized at September 30, 2018 and December 31, 2017; 46,832,103 and 42,003,894 shares issued at September 30, 2018 and December 31, 2017, respectively; 46,830,589 and 42,002,934 shares outstanding at September 30, 2018 and December 31, 2017, respectively	5	5
Treasury stock, at cost, 1,514 and 960 shares at September 30, 2018 and December 31, 2017, respectively	(211)	(113)
Additional paid-in capital	1,793,909	1,066,059
Accumulated deficit	(804,941)	(590,447)
Accumulated other comprehensive loss	(257)	(29)
Total stockholders’ equity	988,505	475,475
Total liabilities and stockholders’ equity	$ 1,046,999	$ 529,937

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
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	46,832,103	42,003,894
Common stock, shares outstanding	46,830,589	42,002,934
Treasury stock, shares	1,514	960

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Collaboration revenue			$ 90,000
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating expenses:
Research and development	$ 75,052	$ 58,286	$ 193,302	$ 159,386
General and administrative	53,693	16,087	125,709	43,320
Total operating expenses	128,745	74,373	319,011	202,706
Loss from operations	(128,745)	(74,373)	(229,011)	(202,706)
Interest income, net	5,817	677	14,483	2,056
Other income (expense), net	10	(23)	34	(48)
Net loss	$ (122,918)	$ (73,719)	$ (214,494)	$ (200,698)
Net loss per share—basic and diluted	$ (2.63)	$ (1.97)	$ (4.68)	$ (5.37)
Weighted average number of common shares outstanding—basic and diluted	46,706,770	37,470,912	45,866,676	37,367,802
Comprehensive loss:
Net loss	$ (122,918)	$ (73,719)	$ (214,494)	$ (200,698)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	(112)	52	(228)	87
Total other comprehensive gain (loss)	(112)	52	(228)	87
Total comprehensive loss	$ (123,030)	$ (73,667)	$ (214,722)	$ (200,611)

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net loss	$ (214,494)	$ (200,698)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	70,323	25,289
Premium on marketable securities	(165)	(49)
Amortization of discount on marketable securities	(6,901)	(57)
Depreciation	780	396
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(12,271)	208
Accounts payable	(3,370)	221
Accrued expenses and other liabilities	7,205	14,312
Net cash used in operating activities	(158,893)	(160,378)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	724,132	214,436
Purchases of marketable securities	(1,272,959)	(93,816)
Purchases of property and equipment	(2,052)	(556)
Net cash provided by (used in) investing activities	(550,879)	120,064
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	27,480	6,998
Payment of employee tax obligations related to vesting of restricted stock units	(904)
Payments of offering costs	(340)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	631,494
Net cash provided by financing activities	657,730	6,998
Net decrease in cash, cash equivalents and restricted cash	(52,042)	(33,316)
Cash, cash equivalents and restricted cash at beginning of period	307,084	169,081
Cash, cash equivalents and restricted cash at end of period	255,042	135,765
Supplemental disclosure of non-cash investing activities
Purchases of property and equipment financed with landlord tenant incentive		$ 353
Purchases of property and equipment included in accounts payable	$ 19

Nature of the Business

Nature of the Business	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business 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 (“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. The Company is subject to risks and uncertainties common to companies in the biotech and pharmaceutical industries, 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 approved for marketing and sale; 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. Under Accounting Standards Update, or ASU, 2014-15, Presentation of Financial Statements—Going Concern Until such time, if ever, as the Company can generate substantial product revenue and achieve profitability, 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
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 unaudited condensed consolidated financial statements. 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 unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2017, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017. 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 statement of its financial position as of September 30, 2018, its results of operations and comprehensive loss for the three and nine months ended September 30, 2018 and 2017, and its cash flows for the nine months ended September 30, 2018. The consolidated balance sheet at December 31, 2017 was derived from audited financial statements, but does not include all disclosures required by GAAP. The results for the three and nine months ended September 30, 2018 are not necessarily indicative of the results for the year ending December 31, 2018, 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, 2017. 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 costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research and development 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 estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs. Stock-Based Compensation The Company recognizes compensation expense for stock-based awards, including grants of stock options and restricted stock, made to employees and non-employee directors based on the estimated fair value on the date of grant, over the requisite service period. The Company recognizes compensation expense for stock-based awards granted to non-employee consultants based on the fair value of the award on each date on which the awards vest. Compensation expense is recognized over the vesting period, provided that services are rendered by such non-employee consultants during that time. At the end of each financial reporting period, the fair value of unvested options is re-measured using the then-current fair value of the Company’s common stock and updated assumptions in the Black-Scholes option-pricing model; and the fair value of restricted stock awards is re-measured using the then-current fair value of the Company’s common stock. For awards that vest upon achievement of a performance condition, the Company recognizes compensation expense when achievement of the performance condition is met or during the period from which meeting the condition is deemed probable until the expected date of meeting the performance condition. The fair value of each option grant is estimated using the Black-Scholes option-pricing model. Through December 31, 2015, the Company lacked sufficient Company-specific historical and implied volatility information, and as a result, the Company used the volatility of a group of publicly-traded peer companies in the Black-Scholes calculations. Beginning in 2016, the Company estimated 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 non-employee directors 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 has 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. Expected forfeitures are based on the historical experience of the Company and management’s expectations of future forfeitures. To the extent actual forfeitures differ from the estimates, the difference is 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. Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of 90 days or less at the date of purchase to be cash equivalents. As of September 30, 2018, cash equivalents were comprised of cash equivalents and money market funds. As of December 31, 2017, cash equivalents were comprised of cash equivalents, money market funds and overnight reverse repurchase agreements. Marketable securities Marketable securities consist of investments with original maturities greater than 90 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 several 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. Marketable securities that have remaining contractual maturities of one year or less are classified as short term. No declines in value were deemed to be other than temporary during the three and nine months ended September 30, 2018 and 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 September 30, 2018 and December 31, 2017 were carried at fair value, determined according to the fair value hierarchy; see Footnote 3, Fair Value Measurements herein. The carrying amounts reflected in the unaudited condensed consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at September 30, 2018 and December 31, 2017, respectively. Revenue Recognition Effective January 1, 2017, the Company adopted Accounting Standards Codification (“ASC”), Topic 606, Revenue from Contracts with Customers Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to a customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right may be accounted for as a contract modification or as a continuation of the contract for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct in the evaluation of a collaboration arrangement subject to Topic 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the standalone selling price is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed its revenue-generating arrangement in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangement. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time this is based on the use of an output or input method. Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02, Leases Leases (Topic 842): Targeted Improvements 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 Company adopted the standard on the required effective date of . In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash The Company adopted this standard during the first quarter of 2018. Restricted cash is now included as a component of cash, cash equivalents, and restricted cash on the Company’s unaudited condensed consolidated statements of cash flows. Restricted cash balances are classified as non-current unless, under the terms of the applicable agreements, the funds will be released from restrictions within one year from the balance sheet date. The inclusion of restricted cash increased the beginning balances of the unaudited condensed consolidated statements of cash flows by $0.8 million and $0.6 million, respectively, and the ending balances by $1.9 million and $0.8 million, respectively, for the nine months ended September 30, 2018 and 2017. In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) — Scope of Modification Accounting This guidance did not have a significant impact on the Company’s consolidated financial In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718) – Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), which aligns the accounting for share-based payment awards issued to employees and non-employees. Under the new guidance, the existing guidance regarding employees will apply to share-based transactions with non-employees, as long as the transaction is not effectively a form of financing, with the exception of specific guidance related to the attribution of compensation cost. The cost of non-employee awards will continue to be recorded as if the grantor had paid cash for the goods or services. In addition, the contractual term will be able to be used in lieu of an expected term in the option-pricing model for non-employee awards. The amendments in the new guidance are effective for public entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, including in interim periods, but no earlier than an entity’s adoption of Accounting Standards Codification 606. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements. 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	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018 and December 31, 2017. September 30, 2018 Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Cash equivalents $ 253,123 $ 253,123 $ — $ — Total cash equivalents 253,123 253,123 — — Marketable securities: U.S. government securities 283,570 — 283,570 — U.S. corporate bonds 152,747 — 152,747 — International corporate bonds 89,551 — 89,551 — U.S. commercial paper 103,531 — 103,531 — International commercial paper 138,879 — 138,879 — Total marketable securities 768,278 — 768,278 — Total cash equivalents and marketable securities $ 1,021,401 $ 253,123 $ 768,278 $ — December 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: Cash equivalents $ 306,235 $ 306,235 $ — $ — Total cash equivalents 306,235 306,235 — — Marketable securities: U.S. government securities 49,606 — 49,606 — U.S. corporate bonds 48,959 — 48,959 — U.S. commercial paper 65,583 — 65,583 — International commercial paper 48,465 — 48,465 — Total marketable securities 212,613 — 212,613 — Total cash equivalents and marketable securities $ 518,848 $ 306,235 $ 212,613 $ — During the nine months ended September 30, 2018 and 2017, there were no transfers among the Level 1, Level 2 and Level 3 categories. The following tables summarize the gross unrealized gains and losses of the Company’s marketable securities as of September 30, 2018 and December 31, 2017: September 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Assets: U.S. government securities $ 283,701 $ — $ (131 ) $ 283,570 U.S. corporate bonds 152,848 5 (106 ) 152,747 International corporate bonds 89,610 1 (60 ) 89,551 U.S. commercial paper 103,516 34 (19 ) 103,531 International commercial paper 138,860 30 (11 ) 138,879 $ 768,535 $ 70 $ (327 ) $ 768,278 December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Assets: U.S. government securities $ 49,612 $ — $ (6 ) $ 49,606 U.S. corporate bonds 48,982 2 (25 ) 48,959 U.S. commercial paper 65,583 — — 65,583 International commercial paper 48,465 — — 48,465 $ 212,642 $ 2 $ (31 ) $ 212,613 As of September 30, 2018, 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 nine months ended September 30, 2018 and the year ended December 31, 2017.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Balance Sheet Components Property and Equipment, net Property and equipment, net, consists of the following: September 30, 2018 December 31, 2017 (in thousands) Computer hardware and software $ 1,778 $ 1,090 Furniture and equipment 938 1,029 Leasehold improvements 4,305 2,967 7,021 5,086 Less: Accumulated depreciation (1,853 ) (1,073 ) $ 5,168 $ 4,013 Depreciation expense for the three months ended September 30, 2018 and 2017 was $0.3 million and $0.1 million, respectively. Depreciation expense for the nine months ended September 30, 2018 and 2017 was $0.8 million and $0.4 million, respectively. The useful life for computer hardware and software is three years, furniture and equipment is five years and leasehold improvements is the lesser of the useful life or the term of the respective lease. Accrued Expenses Accrued expenses consist of the following: September 30, 2018 December 31, 2017 (in thousands) Development costs $ 23,364 $ 23,473 Employee-related expenses 13,001 15,838 Professional services 12,110 3,166 Other accrued expenses 172 124 $ 48,647 $ 42,601

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	5. Commitments and Contingencies Operating Leases The Company leases office space in two multi-tenant buildings in Cambridge, Massachusetts, consisting, as of March 31, 2018, of 54,943 square feet in the first building under an operating lease that expires on August 15, 2024 and 19,805 square feet in the second building under an operating lease that will expire on February 28, 2022. In April 2018, the Company entered into the First Amendment to the lease for office space in the second building. The Company increased the amount of square feet of office space from 19,805 square feet to 40,419 square feet, an increase of 20,614 square feet, consisting of (i) 13,481 square feet beginning on August 1, 2018, and (ii) 7,133 square feet beginning on October 1, 2018. The term for this additional space will expire on August 31, 2024. Additionally, the term of the existing lease was extended from February 28, 2022 until August 31, 2024. In May 2018, the Company entered into a lease for office space in a multi-tenant building in Raleigh, North Carolina. The amount of square feet of office space is 15,525 square feet and the lease period began on September 1, 2018. The term for this space will expire on November 30, 2024. In October 2018, the Company entered into the Seventh Amendment to the lease for office space in the first building. The Company increased the amount of square feet of office space from 54,943 square feet to 58,442 Future minimum lease payments under non-cancelable operating leases are as follows at September 30, 2018: Years Ending December 31, (in thousands) 2018 $ 1,572 2019 6,657 2020 6,784 2021 6,913 2022 7,106 Thereafter 12,189 $ 41,221 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 brexanolone and the Company’s compound known as 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 of September 30, 2018, the Company has paid to CyDex $1.0 million for licensing fees, which was recorded as research and development expense. 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 brexanolone; 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 brexanolone; 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. As of September 30, 2018, the Company has recorded research and development expense and made cash payments of $2.3 million related to these clinical development and regulatory milestones. For the three months ended September 30, 2018 and 2017, the Company did not record any expense or make any milestone payments related to clinical development or regulatory milestones for the brexanolone program under the license agreement with CyDex. For the nine months ended September 30, 2018, the Company recorded research and development expense and made cash payments of $0.8 million related to regulatory milestones for the brexanolone program under the license agreement with CyDex. For the nine months ended September 30, 2017, the Company did not record any expense or make any milestone payments related to clinical development or regulatory milestones for the brexanolone program under the license agreement with CyDex. 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 under which the Company was granted a non-exclusive license to certain clinical data and clinical material related to brexanolone for use in the development and commercialization of biopharmaceutical products in the licensed field, including status epilepticus and postpartum 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. As of December 31, 2015, the Company paid to The Regents of the University of California clinical development milestones of $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 developed using the data and materials. The license will terminate on the earlier to occur of (i) 27 years after the effective date or (ii) 15 years after the last-derived product is first commercially sold. In 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. As of September 30, 2018, the Company has recorded research and development expense and made cash payments of $0.3 million related to these regulatory and sales milestones. For the three months ended September 30, 2018 and 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. For the nine months ended September 30, 2018, the Company recorded research and development expense and made cash payments of $0.2 million related to regulatory milestones under the license agreements with The Regents of the University of California. For the nine months ended September 30, 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. 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. As of September 30, 2018, the Company has recorded research and development expense and made a cash payment of $50,000 related to these clinical and development milestones. 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 three and nine months ended September 30, 2018 and 2017, the Company did not record any expense or make any milestone payments under the license agreement with Washington University. Consulting Agreement In January 2014, the Company entered into a consulting agreement with a then 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. As of September 30, 2018, the Company recorded research and development expense of $1.8 million, comprised of $0.5 million in cash and $1.3 million related to the issuance of 39,681 shares of the Company’s common stock, related to the achievement of these milestones. For the three and nine months ended September 30, 2018 and 2017, the Company did not record any expense or make any milestone payments under the consulting agreement with the non-employee advisor.

Collaborative Agreement

Collaborative Agreement	9 Months Ended
	Sep. 30, 2018
Collaboration Agreement [Abstract]
Collaborative Agreement	6. Collaboration Agreement Effective June 12, 2018, the Company entered into a strategic collaboration with Shionogi & Co., Ltd., (“Shionogi”) for the clinical development and commercialization of SAGE-217 for the treatment of major depressive disorder (“MDD”) and other indications in Japan, Taiwan and South Korea. Under the terms of the agreement, Shionogi will be responsible for all clinical development, regulatory filings and commercialization of SAGE-217 for MDD, and potentially other indications, in Japan, Taiwan and South Korea. Shionogi was required to make an upfront payment to the Company of $90.0 million, and the Company will be eligible to receive additional payments of up to $485.0 million if certain regulatory and commercial milestones are achieved by Shionogi. The potential future milestone payments include up to $70.0 million for the achievement of specified regulatory milestones, up to $30.0 million for the achievement of specified commercialization milestones, and up to $385.0 million for the achievement of specified net sales milestones. Due to the uncertainty of pharmaceutical development and the high historical failure rates generally associated with drug development, we may not receive any additional milestone payments or any royalty payments from Shionogi. The Company concluded that Shionogi meets the definition to be accounted for as a customer since the Company is delivering intellectual property and know-how rights for the SAGE-217 program in support of territories in which the parties are not jointly sharing the risks and rewards. In addition, the Company determined that the Shionogi collaboration met the requirements to be accounted for as a contract, including that it was probable that the Company will collect the consideration related to the up-front payment to which the Company was entitled in exchange for the goods or services that will be delivered to Shionogi. In determining the appropriate amount of revenue to be recognized under ASC 606, the Company performed the following steps: (i) identified the promised goods or services in the contract; (ii) determined whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measured the transaction price, including the constraint on variable consideration; (iv) allocated the transaction price to the performance obligations; and (v) recognized revenue when (or as) the Company satisfies each performance obligation. The Company determined that, pursuant to the new revenue standard, the performance obligations included the license to SAGE-217, supply of certain clinical materials and manufacturing supply of the active pharmaceutical ingredient (“API”). The performance obligation related to the license to SAGE-217 was determined to be distinct from other performance obligations and therefore was a standalone performance obligation for which control was transferred upon signing. The obligation to provide certain clinical materials was determined to be a separate performance obligation. The agreement related to supplying API was determined to be an option for Shionogi to purchase, rather than a firm obligation since no minimum amount or quantities are specified and, therefore, was not considered a performance obligation within the main agreement. Given this fact pattern, the Company has concluded the agreement has two performance obligations. The Company completed the evaluation of the standalone selling prices of each of the performance obligations and determined that the standalone selling price of the license performance obligation was $90.0 million. The Company recognized the transaction price allocated to the license performance obligation of $90.0 million as revenue during the quarter upon delivery of the license to Shionogi and resulting ability of Shionogi to use and benefit from the license. The remaining transaction price related to the performance obligation for the supply of certain clinical material is not significant. The potential milestone payments that the Company is eligible to receive were excluded from the transaction price, as all milestone amounts were fully constrained based on the probability of achievement. The Company will reevaluate the transaction price at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, and, if necessary, adjust its estimate of the transaction price.

Sale of Equity Securities

Sale of Equity Securities	9 Months Ended
	Sep. 30, 2018
Other Liabilities Disclosure [Abstract]
Sale of Equity Securities	7 . Sale of Equity Securities On November 17, 2017, the Company completed the sale of 4,058,822 shares of its common stock in an underwritten public offering at a price to the public of $85.00 per share, resulting in net proceeds of $325.8 million after deducting commissions and underwriting discounts and offering costs paid by the Company. On February 13, 2018, the Company completed the sale of 4,032,012 shares of its common stock in an underwritten public offering at a price to the public of $164.00 per share, resulting in net proceeds of $631.2 million after deducting commissions and underwriting discounts and offering costs paid by the Company.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Restricted Stock Units During the nine months ended September 30, 2017, the Company granted 32,500 restricted stock units to certain employees of the Company. The Company did not grant restricted stock units prior to January 1, 2017. These restricted stock units vest ratably over two years, with cliff vesting of 50% at both the one-year and two-year anniversary of the grant date, which was in February 2018 and will be in February 2019, respectively. During the three months ended March 31, 2018, the Company granted 33,600 performance restricted stock units to certain employees of the Company. These performance restricted stock units will vest upon the achievement of a certain regulatory milestone, in some cases upon meeting the milestone and, in other cases, upon the first anniversary of achievement of the milestone. During the three months ended June 30, 2018, the Company granted 37,800 performance restricted stock units to certain employees of the Company. These performance restricted stock units will vest upon the achievement of a certain commercial milestone. The fair value of restricted stock units that vested during the nine months ended September 30, 2018 was $2.6 million. No restricted stock units vested during the nine months ended September 30, 2017 or during the three months ended September 30, 2018. The table below summarizes activity relating to restricted stock units: Shares Outstanding as of December 31, 2017 29,100 Granted 71,400 Vested (14,550 ) Forfeited (2,050 ) Outstanding as of September 30, 2018 83,900 Stock Option Plans On July 2, 2014, the stockholders of the Company approved the 2014 Stock Option and Incentive Plan (the “2014 Stock Option Plan”), which became effective immediately prior to the completion of the Company’s 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 no longer grants 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. 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 outstanding shares of common stock as of the last day of the prior year. On January 1, 2018, 1,680,117 shares of common stock, representing 4% of the Company’s outstanding shares of common stock as of December 31, 2017, were added to the 2014 Stock Option Plan. On December 15, 2016, the Board of Directors of the Company (the “Board”) 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. On September 20, 2018, the Board amended the 2016 Stock Option Plan to increase the total number of shares reserved for issuance under such plan by 1,200,000 shares. As of September 30, 2018, the total number of shares reserved under all equity plans is 10,001,048, and 2,422,437 shares were available for future issuance under such plans. During the three months ended March 31, 2018 and 2017, the Company granted 323,753 and 449,208 options, respectively, to certain 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, which consider the inherent risk and uncertainty regarding the future outcomes of the milestones. During the three months ended June 30, 2018 and 2017, the Company granted 200,250 and no options, respectively, to certain employees to purchase shares of common stock that contain performance-based vesting criteria, related to the achievement of certain clinical development, regulatory development and commercial 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, which consider the inherent risk and uncertainty regarding the future outcomes of the milestones. During the three months ended September 30, 2018 and 2017, the Company granted no options, respectively, to employees to purchase shares of common stock that contain performance-based vesting criteria. During the three months ended June 30, 2018, a performance milestone was achieved under a stock option granted to a consultant. The milestone was related to the consummation of a licensing or corporate partnering arrangement. During the three months ended June 30, 2018, the Company recognized stock-based compensation expense related to this milestone of $6.9 million. During the nine months ended September 30, 2018 and 2017, the achievement of the milestones that had not been met that are the criteria for vesting of performance-based stock options was considered not probable, and therefore no expense has been recognized related to these awards for the nine months ended September 30, 2018 and 2017. Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the three and nine months ended September 30, 2018 and 2017 was as follows: Three months ended September 30, Nine months ended September 30, 2018 2017 2018 2017 (in thousands) Research and development $ 13,997 $ 5,399 $ 34,991 $ 14,239 General and administrative 11,481 4,332 35,332 11,050 $ 25,478 $ 9,731 $ 70,323 $ 25,289 Stock-based compensation expense by award type recognized during the three and nine months ended September 30, 2018 and 2017 was as follows: Three months ended September 30, Nine months ended September 30, 2018 2017 2018 2017 (in thousands) Stock options $ 25,015 $ 9,414 $ 69,023 $ 24,565 Restricted stock units 164 176 487 444 Employee stock purchase plan 299 141 813 280 $ 25,478 $ 9,731 $ 70,323 $ 25,289 The weighted average grant date fair value per share relating to outstanding stock options granted under the Company’s stock option plans during the nine months ended September 30, 2018 and 2017 was $113.12 and $40.43, 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, 2017 5,586,593 $ 43.58 8.09 $ 676,717 Granted 2,775,594 168.97 Exercised (711,621 ) 35.13 Forfeited (155,855 ) 79.46 Outstanding as of September 30, 2018 7,494,711 $ 90.07 8.15 $ 459,643 Exercisable as of September 30, 2018 2,460,232 $ 36.17 6.73 $ 258,528 At September 30, 2018, the Company had unrecognized stock-based compensation expense related to its unvested service-based stock option awards of $291.8 million, which is expected to be recognized over the remaining weighted average vesting period of 2.94 years. The intrinsic value of stock options exercised during the nine months ended September 30, 2018 and 2017 was $93.0 million and $14.1 million, respectively. At September 30, 2018, 1,098,536 performance-based stock options were both outstanding and unvested, and the total unrecognized stock-based compensation expense related to those awards was $58.4 million.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Net Loss Per Share	9. Net Loss Per Share Basic and diluted net loss per share was calculated as follows for the three and nine months ended September 30, 2018 and 2017: Three months ended September 30, Nine months ended September 30, 2018 2017 2018 2017 Basic net loss per share: Numerator: Net loss (in thousands) $ (122,918 ) $ (73,719 ) $ (214,494 ) $ (200,698 ) Denominator: Weighted average common stock outstanding —basic 46,706,770 37,470,912 45,866,676 37,367,802 Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units — — — — Weighted average common stock outstanding —diluted 46,706,770 37,470,912 45,866,676 37,367,802 Net loss per share—basic and diluted $ (2.63 ) $ (1.97 ) $ (4.68 ) $ (5.37 ) The following common stock equivalents outstanding as of September 30, 2018 and 2017 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 September 30, Nine months ended September 30, 2018 2017 2018 2017 Stock options 6,396,175 5,259,925 6,396,175 5,259,925 Restricted stock units 13,800 30,100 13,800 30,100 Employee stock purchase plan 6,481 6,500 6,481 6,500 6,416,456 5,296,525 6,416,456 5,296,525 Stock options and restricted stock units that are outstanding and contain performance-based vesting criteria for which the performance conditions have not been met are excluded from the calculation of common stock equivalents outstanding.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
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 unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2017, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017. 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 statement of its financial position as of September 30, 2018, its results of operations and comprehensive loss for the three and nine months ended September 30, 2018 and 2017, and its cash flows for the nine months ended September 30, 2018. The consolidated balance sheet at December 31, 2017 was derived from audited financial statements, but does not include all disclosures required by GAAP. The results for the three and nine months ended September 30, 2018 are not necessarily indicative of the results for the year ending December 31, 2018, 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, 2017. 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 costs are recorded as research and development expenses as incurred. The Company records accruals for estimated ongoing research and development 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 estimates made by the Company. The historical accrual estimates made by the Company have not been materially different from the actual costs.
Stock-Based Compensation	Stock-Based Compensation The Company recognizes compensation expense for stock-based awards, including grants of stock options and restricted stock, made to employees and non-employee directors based on the estimated fair value on the date of grant, over the requisite service period. The Company recognizes compensation expense for stock-based awards granted to non-employee consultants based on the fair value of the award on each date on which the awards vest. Compensation expense is recognized over the vesting period, provided that services are rendered by such non-employee consultants during that time. At the end of each financial reporting period, the fair value of unvested options is re-measured using the then-current fair value of the Company’s common stock and updated assumptions in the Black-Scholes option-pricing model; and the fair value of restricted stock awards is re-measured using the then-current fair value of the Company’s common stock. For awards that vest upon achievement of a performance condition, the Company recognizes compensation expense when achievement of the performance condition is met or during the period from which meeting the condition is deemed probable until the expected date of meeting the performance condition. The fair value of each option grant is estimated using the Black-Scholes option-pricing model. Through December 31, 2015, the Company lacked sufficient Company-specific historical and implied volatility information, and as a result, the Company used the volatility of a group of publicly-traded peer companies in the Black-Scholes calculations. Beginning in 2016, the Company estimated 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 non-employee directors 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 has 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. Expected forfeitures are based on the historical experience of the Company and management’s expectations of future forfeitures. To the extent actual forfeitures differ from the estimates, the difference is 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.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of 90 days or less at the date of purchase to be cash equivalents. As of September 30, 2018, cash equivalents were comprised of cash equivalents and money market funds. As of December 31, 2017, cash equivalents were comprised of cash equivalents, money market funds and overnight reverse repurchase agreements.
Marketable Securities	Marketable securities Marketable securities consist of investments with original maturities greater than 90 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 several 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. Marketable securities that have remaining contractual maturities of one year or less are classified as short term. No declines in value were deemed to be other than temporary during the three and nine months ended September 30, 2018 and 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 September 30, 2018 and December 31, 2017 were carried at fair value, determined according to the fair value hierarchy; see Footnote 3, Fair Value Measurements herein. The carrying amounts reflected in the unaudited condensed consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at September 30, 2018 and December 31, 2017, respectively.
Revenue Recognition	Revenue Recognition Effective January 1, 2017, the Company adopted Accounting Standards Codification (“ASC”), Topic 606, Revenue from Contracts with Customers Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to a customer. Once a contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations. Arrangements that include rights to additional goods or services that are exercisable at a customer’s discretion are generally considered options. The Company assesses if these options provide a material right to the customer and if so, they are considered performance obligations. The exercise of a material right may be accounted for as a contract modification or as a continuation of the contract for accounting purposes. The Company assesses whether each promised good or service is distinct for the purpose of identifying the performance obligations in the contract. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. Promised goods and services are considered distinct provided that: (i) the customer can benefit from the good or service either on its own or together with other resources that are readily available to the customer (that is, the good or service is capable of being distinct) and (ii) the entity’s promise to transfer the good or service to the customer is separately identifiable from other promises in the contract (that is, the promise to transfer the good or service is distinct within the context of the contract). In assessing whether a promised good or service is distinct in the evaluation of a collaboration arrangement subject to Topic 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The transaction price is then determined and allocated to the identified performance obligations in proportion to their standalone selling prices (“SSP”) on a relative SSP basis. SSP is determined at contract inception and is not updated to reflect changes between contract inception and when the performance obligations are satisfied. Determining the SSP for performance obligations requires significant judgment. In developing the SSP for a performance obligation, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the standalone selling price is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the unconstrained amount of estimated variable consideration in the transaction price. The amount included in the transaction price is constrained to the amount for which it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates the estimated variable consideration included in the transaction price and any related constraint, and if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis in the period of adjustment. If an arrangement includes development and regulatory milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered probable of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensees and the transfer of the promised goods or services to the licensees will be one year or less. The Company assessed its revenue-generating arrangement in order to determine whether a significant financing component exists and concluded that a significant financing component does not exist in the arrangement. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time this is based on the use of an output or input method.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-02, Leases Leases (Topic 842): Targeted Improvements 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 Company adopted the standard on the required effective date of . In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash The Company adopted this standard during the first quarter of 2018. Restricted cash is now included as a component of cash, cash equivalents, and restricted cash on the Company’s unaudited condensed consolidated statements of cash flows. Restricted cash balances are classified as non-current unless, under the terms of the applicable agreements, the funds will be released from restrictions within one year from the balance sheet date. The inclusion of restricted cash increased the beginning balances of the unaudited condensed consolidated statements of cash flows by $0.8 million and $0.6 million, respectively, and the ending balances by $1.9 million and $0.8 million, respectively, for the nine months ended September 30, 2018 and 2017. In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) — Scope of Modification Accounting This guidance did not have a significant impact on the Company’s consolidated financial In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718) – Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), which aligns the accounting for share-based payment awards issued to employees and non-employees. Under the new guidance, the existing guidance regarding employees will apply to share-based transactions with non-employees, as long as the transaction is not effectively a form of financing, with the exception of specific guidance related to the attribution of compensation cost. The cost of non-employee awards will continue to be recorded as if the grantor had paid cash for the goods or services. In addition, the contractual term will be able to be used in lieu of an expected term in the option-pricing model for non-employee awards. The amendments in the new guidance are effective for public entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, including in interim periods, but no earlier than an entity’s adoption of Accounting Standards Codification 606. The Company is in the process of evaluating the impact that this new guidance will have on its consolidated financial statements. 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)	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018 and December 31, 2017. September 30, 2018 Total Quoted Prices in Active Markets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) (in thousands) Cash equivalents: Cash equivalents $ 253,123 $ 253,123 $ — $ — Total cash equivalents 253,123 253,123 — — Marketable securities: U.S. government securities 283,570 — 283,570 — U.S. corporate bonds 152,747 — 152,747 — International corporate bonds 89,551 — 89,551 — U.S. commercial paper 103,531 — 103,531 — International commercial paper 138,879 — 138,879 — Total marketable securities 768,278 — 768,278 — Total cash equivalents and marketable securities $ 1,021,401 $ 253,123 $ 768,278 $ — December 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: Cash equivalents $ 306,235 $ 306,235 $ — $ — Total cash equivalents 306,235 306,235 — — Marketable securities: U.S. government securities 49,606 — 49,606 — U.S. corporate bonds 48,959 — 48,959 — U.S. commercial paper 65,583 — 65,583 — International commercial paper 48,465 — 48,465 — Total marketable securities 212,613 — 212,613 — Total cash equivalents and marketable securities $ 518,848 $ 306,235 $ 212,613 $ —
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 September 30, 2018 and December 31, 2017: September 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Assets: U.S. government securities $ 283,701 $ — $ (131 ) $ 283,570 U.S. corporate bonds 152,848 5 (106 ) 152,747 International corporate bonds 89,610 1 (60 ) 89,551 U.S. commercial paper 103,516 34 (19 ) 103,531 International commercial paper 138,860 30 (11 ) 138,879 $ 768,535 $ 70 $ (327 ) $ 768,278 December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Assets: U.S. government securities $ 49,612 $ — $ (6 ) $ 49,606 U.S. corporate bonds 48,982 2 (25 ) 48,959 U.S. commercial paper 65,583 — — 65,583 International commercial paper 48,465 — — 48,465 $ 212,642 $ 2 $ (31 ) $ 212,613

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net, consists of the following: September 30, 2018 December 31, 2017 (in thousands) Computer hardware and software $ 1,778 $ 1,090 Furniture and equipment 938 1,029 Leasehold improvements 4,305 2,967 7,021 5,086 Less: Accumulated depreciation (1,853 ) (1,073 ) $ 5,168 $ 4,013
Summary of Accrued Expenses	Accrued expenses consist of the following: September 30, 2018 December 31, 2017 (in thousands) Development costs $ 23,364 $ 23,473 Employee-related expenses 13,001 15,838 Professional services 12,110 3,166 Other accrued expenses 172 124 $ 48,647 $ 42,601

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
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 September 30, 2018: Years Ending December 31, (in thousands) 2018 $ 1,572 2019 6,657 2020 6,784 2021 6,913 2022 7,106 Thereafter 12,189 $ 41,221

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Activity Relating to Restricted Stock Units	The table below summarizes activity relating to restricted stock units: Shares Outstanding as of December 31, 2017 29,100 Granted 71,400 Vested (14,550 ) Forfeited (2,050 ) Outstanding as of September 30, 2018 83,900
Summary of Stock-Based Compensation Expense for Stock Options, Restricted Stock Units and Employee Stock Purchase Plan Recognized	Stock-based compensation expense for stock options, restricted stock units and the employee stock purchase plan recognized during the three and nine months ended September 30, 2018 and 2017 was as follows: Three months ended September 30, Nine months ended September 30, 2018 2017 2018 2017 (in thousands) Research and development $ 13,997 $ 5,399 $ 34,991 $ 14,239 General and administrative 11,481 4,332 35,332 11,050 $ 25,478 $ 9,731 $ 70,323 $ 25,289
Summary of Stock-Based Compensation Expense by Award Type Recognized	Stock-based compensation expense by award type recognized during the three and nine months ended September 30, 2018 and 2017 was as follows: Three months ended September 30, Nine months ended September 30, 2018 2017 2018 2017 (in thousands) Stock options $ 25,015 $ 9,414 $ 69,023 $ 24,565 Restricted stock units 164 176 487 444 Employee stock purchase plan 299 141 813 280 $ 25,478 $ 9,731 $ 70,323 $ 25,289
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, 2017 5,586,593 $ 43.58 8.09 $ 676,717 Granted 2,775,594 168.97 Exercised (711,621 ) 35.13 Forfeited (155,855 ) 79.46 Outstanding as of September 30, 2018 7,494,711 $ 90.07 8.15 $ 459,643 Exercisable as of September 30, 2018 2,460,232 $ 36.17 6.73 $ 258,528

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2018
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 and nine months ended September 30, 2018 and 2017: Three months ended September 30, Nine months ended September 30, 2018 2017 2018 2017 Basic net loss per share: Numerator: Net loss (in thousands) $ (122,918 ) $ (73,719 ) $ (214,494 ) $ (200,698 ) Denominator: Weighted average common stock outstanding —basic 46,706,770 37,470,912 45,866,676 37,367,802 Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units — — — — Weighted average common stock outstanding —diluted 46,706,770 37,470,912 45,866,676 37,367,802 Net loss per share—basic and diluted $ (2.63 ) $ (1.97 ) $ (4.68 ) $ (5.37 )
Summary of Anti-Dilutive Common Stock Equivalents Outstanding	The following common stock equivalents outstanding as of September 30, 2018 and 2017 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 September 30, Nine months ended September 30, 2018 2017 2018 2017 Stock options 6,396,175 5,259,925 6,396,175 5,259,925 Restricted stock units 13,800 30,100 13,800 30,100 Employee stock purchase plan 6,481 6,500 6,481 6,500 6,416,456 5,296,525 6,416,456 5,296,525

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	102 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Nature Of Business [Line Items]
Accumulated deficit	$ (804,941)	$ (590,447)
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	$ 1,600,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accounting Standards Update 2016-18 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents increased by amount due to inclusion of restricted cash	$ 1.9	$ 0.8	$ 0.8	$ 0.6

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Company's Money Market Funds and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	$ 253,123	$ 306,235
Total marketable securities	768,278	212,613
Total cash equivalents and marketable securities	1,021,401	518,848
Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	253,123	306,235
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	253,123	306,235
Total cash equivalents and marketable securities	253,123	306,235
Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents	253,123	306,235
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	768,278	212,613
Total cash equivalents and marketable securities	768,278	212,613
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	283,570	49,606
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	283,570	49,606
U.S. Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	152,747	48,959
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	152,747	48,959
International Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	89,551
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	89,551
U.S. Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	103,531	65,583
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	103,531	65,583
International Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total marketable securities	138,879	48,465
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	$ 138,879	$ 48,465

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
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 - Sum_2

Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 768,535	$ 212,642
Gross Unrealized Gains	70	2
Gross Unrealized Losses	(327)	(31)
Fair Value	768,278	212,613
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	283,701	49,612
Gross Unrealized Losses	(131)	(6)
Fair Value	283,570	49,606
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	152,848	48,982
Gross Unrealized Gains	5	2
Gross Unrealized Losses	(106)	(25)
Fair Value	152,747	48,959
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	89,610
Gross Unrealized Gains	1
Gross Unrealized Losses	(60)
Fair Value	89,551
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	103,516	65,583
Gross Unrealized Gains	34
Gross Unrealized Losses	(19)
Fair Value	103,531	65,583
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	138,860	48,465
Gross Unrealized Gains	30
Gross Unrealized Losses	(11)
Fair Value	$ 138,879	$ 48,465

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 7,021	$ 5,086
Less: Accumulated depreciation	(1,853)	(1,073)
Property and equipment, net	5,168	4,013
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,778	1,090
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	938	1,029
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 4,305	$ 2,967

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 0.3	$ 0.1	$ 0.8	$ 0.4
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

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Development costs	$ 23,364	$ 23,473
Employee-related expenses	13,001	15,838
Professional services	12,110	3,166
Other accrued expenses	172	124
Total accrued expenses	$ 48,647	$ 42,601

Commitments and Contingencies -

Commitments and Contingencies - Operating Leases - Additional Information (Detail) - ft²	Dec. 01, 2018	Oct. 01, 2018	Aug. 01, 2018	Oct. 31, 2018	May 31, 2018	Apr. 30, 2018	Mar. 31, 2018
Operating Lease One [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease							54,943
Lease expire date							Aug. 15, 2024
Operating Lease Two [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease						40,419	19,805
Lease expire date						Aug. 31, 2024	Feb. 28, 2022
Increased office space rent			13,481			20,614
Operating Lease Two [Member] | Subsequent Event [Member]
Commitments And Contingencies [Line Items]
Increased office space rent		7,133
Operating Lease Three [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease					15,525
Lease expire date					Nov. 30, 2024
Operating Lease Four [Member] | Scenario, Forecast [Member]
Commitments And Contingencies [Line Items]
Increased office space rent	3,499
Operating Lease Four [Member] | Subsequent Event [Member]
Commitments And Contingencies [Line Items]
Office space rent under operating lease				58,442
Lease expire date				Aug. 31, 2024
Increased office space rent				54,943

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Lease Payments, Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Sep. 30, 2018USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 1,572
2,019	6,657
2,020	6,784
2,021	6,913
2,022	7,106
Thereafter	12,189
Total Operating leases, future minimum payments	$ 41,221

Commitments and Contingencies_3

Commitments and Contingencies - CyDex License Agreement - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments And Contingencies [Line Items]
Research and development expense	$ 75,052,000	$ 58,286,000	$ 193,302,000	$ 159,386,000
CyDex License Agreement [Member]
Commitments And Contingencies [Line Items]
Research and development expense			1,000,000
Research and development expense related to milestone expense	0	0	800,000	0
Milestone payments	0	$ 0	800,000	$ 0
CyDex License Agreement [Member] | First and Second Clinical Development Milestones [Member] | Brexanolone [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000		800,000
CyDex License Agreement [Member] | First and Second Regulatory Milestones [Member] | Brexanolone [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	3,800,000		3,800,000
CyDex License Agreement [Member] | Third and Fourth Clinical Development Milestones [Member] | Brexanolone [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	1,300,000		1,300,000
CyDex License Agreement [Member] | Third and Fourth Regulatory Milestones [Member] | Brexanolone [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	8,500,000		8,500,000
CyDex License Agreement [Member] | Clinical Development [Member] | SAGE-689 [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	800,000		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		1,800,000
CyDex License Agreement [Member] | Clinical Development and Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense			2,300,000
Milestone payments			$ 2,300,000

Commitments and Contingencies_4

Commitments and Contingencies - University of California License Agreements - Additional Information (Detail) - USD ($)	Dec. 31, 2015	Jun. 30, 2015	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2015
Regulatory and Sales Milestones [Member]
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense					$ 300,000
Milestone payments					300,000
University of California License Agreements [Member]
Commitments And Contingencies [Line Items]
Upfront payment		$ 50,000
Annual license maintenance fee							$ 15,000
Research and development expense related to milestone expense			$ 0	$ 0	200,000	$ 0
Milestone payments			$ 0	$ 0	$ 200,000	$ 0
University of California License Agreements [Member] | After The Effective Date [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period, maximum	27 years
University of California License Agreements [Member] | After The First Sale [Member]
Commitments And Contingencies [Line Items]
Licenses Expiration period, maximum	15 years
University of California License Agreements [Member] | Clinical Development [Member]
Commitments And Contingencies [Line Items]
Milestone payments	$ 100,000
University of California License Agreements [Member] | Clinical Development [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Percentage of net sales paid as royalties	1.00%
University of California License Agreements [Member] | Regulatory Milestones [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		700,000
University of California License Agreements [Member] | Sales Milestones [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments		$ 2,000,000

Commitments and Contingencies_5

Commitments and Contingencies - Washington University License Agreement - Additional Information (Detail) - Washington University License Agreement [Member] - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Nov. 30, 2013	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	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	$ 0	$ 0
Milestone payments		$ 0	$ 0	0	$ 0
Clinical Development [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 Contingencies_6

Commitments and Contingencies - Consulting Agreement - Additional Information (Detail) - Consulting Agreement [Member] - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2014	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commitments And Contingencies [Line Items]
Research and development expense related to milestone expense		$ 0	$ 0	$ 0	$ 0
Milestone payments		$ 0	$ 0	0	$ 0
Clinical Development and Regulatory Milestones [Member]
Commitments And Contingencies [Line Items]
Shares of common stock for attaining milestones	126,984
Research and development expense related to milestone expense				1,800,000
Milestone payments				500,000
Payments for stock issued upon reaching a milestone				$ 1,300,000
Milestone based share compensation, shares issued				39,681
Clinical Development and Regulatory Milestones [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Expected milestone payments	$ 2,000,000

Collaboration Agreement - Addit

Collaboration Agreement - Additional Information (Detail) - USD ($)	Jun. 12, 2018	Sep. 30, 2018
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 90,000,000
Shionogi Collaboration Agreement [Member] | Clinical Development [Member] | SAGE-217 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement effective date	Jun. 12, 2018
Upfront payment	$ 90,000,000
Average percentage on tiered royalties	20.00%
Standalone selling price of license performance obligation		90,000,000
Revenue		$ 90,000,000
Shionogi Collaboration Agreement [Member] | Clinical Development [Member] | SAGE-217 [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional milestone payment receivable	$ 485,000,000
Shionogi Collaboration Agreement [Member] | Clinical Development [Member] | SAGE-217 [Member] | Maximum [Member] | Regulatory Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	70,000,000
Shionogi Collaboration Agreement [Member] | Clinical Development [Member] | SAGE-217 [Member] | Maximum [Member] | Commercial Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	30,000,000
Shionogi Collaboration Agreement [Member] | Clinical Development [Member] | SAGE-217 [Member] | Maximum [Member] | Sales Milestones [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future milestone payments	$ 385,000,000

Sale of Equity Securities - Add

Sale of Equity Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Feb. 13, 2018	Nov. 17, 2017
Equity [Abstract]
Issuance of common stock, shares	4,032,012	4,058,822
Common stock price per share	$ 164	$ 85
Proceeds from public offering of common stock, net of commissions and underwriting discounts and offering costs	$ 631.2	$ 325.8

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Jan. 01, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 20, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of options outstanding		10,001,048						10,001,048
Common stock available for issuance under stock option plan		2,422,437						2,422,437
Stock-based compensation expense		$ 25,478,000			$ 9,731,000			$ 70,323,000	$ 25,289,000
Weighted average grant date fair value per share								$ 113.12	$ 40.43
2011 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted								0
2016 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total number of shares reserved for issuance											1,200,000
2014 Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares annual increase added to plan	1,680,117
2014 Stock Option Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of increase on outstanding shares of Common stock										4.00%
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted								71,400	32,500
Share based compensation granted under plan vest period								2 years
Fair value of restricted stock units vested								$ 2,600,000
Restricted stock units vested		0						14,550	0
Stock-based compensation expense		$ 164,000			$ 176,000			$ 487,000	$ 444,000
Restricted Stock Units [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 units vesting period month and year								2018-02
Restricted Stock Units [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%
Restricted stock units vesting period month and year								2019-02
Performance Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted			37,800	33,600
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		0	200,250	323,753	0	0	449,208
Total unrecognized stock-based compensation expense		$ 58,400,000						$ 58,400,000
Number of shares outstanding and unvested stock options		1,098,536						1,098,536
Performance-Based Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense								$ 0	0
Performance-Based Stock Options [Member] | Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 6,900,000
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted								2,775,594
Total number of options outstanding		7,494,711						7,494,711		5,586,593
Stock-based compensation expense		$ 25,015,000			$ 9,414,000			$ 69,023,000	24,565,000
Total unrecognized stock-based compensation expense		$ 291,800,000						$ 291,800,000
Weighted average period of unrecognized compensation costs								2 years 11 months 8 days
Intrinsic value of options exercised								$ 93,000,000	$ 14,100,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Activity Relating to Restricted Stock Units (Detail) - Restricted Stock Units [Member] - shares	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, Shares at beginning balance-Restricted stock units		29,100
Granted, Shares-Restricted stock units		71,400	32,500
Vested, Shares-Restricted stock units	0	(14,550)	0
Forfeited, Shares-Restricted stock units		(2,050)
Outstanding, Shares at ending balance-Restricted stock units	83,900	83,900

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-Based Compensation Expense for Stock Options, Restricted Stock Units and Employee Stock Purchase Plan Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 25,478	$ 9,731	$ 70,323	$ 25,289
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	13,997	5,399	34,991	14,239
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 11,481	$ 4,332	$ 35,332	$ 11,050

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Stock-Based Compensation Expense by Award Type Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 25,478	$ 9,731	$ 70,323	$ 25,289
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	25,015	9,414	69,023	24,565
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	164	176	487	444
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 299	$ 141	$ 813	$ 280

Stock-Based Compensation - Su_4

Stock-Based Compensation - Summary of Activity Relating to Stock Options (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Ending balance, Outstanding Shares	10,001,048
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning balance, Outstanding Shares	5,586,593
Granted, Shares	2,775,594
Exercised, Shares	(711,621)
Forfeited, Shares	(155,855)
Ending balance, Outstanding Shares	7,494,711	5,586,593
Exercisable, Shares	2,460,232
Beginning balance, Outstanding Weighted Average Exercise Price | $ / shares	$ 43.58
Granted, Weighted Average Exercise Price | $ / shares	168.97
Exercised, Weighted Average Exercise Price | $ / shares	35.13
Forfeited, Weighted Average Exercise Price | $ / shares	79.46
Ending balance, Outstanding Weighted Average Exercise Price | $ / shares	90.07	$ 43.58
Exercisable, Weighted Average Exercise Price | $ / shares	$ 36.17
Outstanding, Weighted Average Remaining Life	8 years 1 month 24 days	8 years 1 month 2 days
Exercisable, Weighted Average Remaining Life	6 years 8 months 23 days
Outstanding, Aggregate Intrinsic Value | $	$ 459,643	$ 676,717
Exercisable, Aggregate Intrinsic Value | $	$ 258,528

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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net loss	$ (122,918)	$ (73,719)	$ (214,494)	$ (200,698)
Denominator:
Weighted average common stock outstanding —basic	46,706,770	37,470,912	45,866,676	37,367,802
Dilutive effect of shares of common stock equivalents resulting from common stock options and restricted stock units	0	0	0	0
Weighted average common stock outstanding —diluted	46,706,770	37,470,912	45,866,676	37,367,802
Net loss per share—basic and diluted	$ (2.63)	$ (1.97)	$ (4.68)	$ (5.37)

Net Loss Per Share - Summary _2

Net Loss Per Share - Summary of Anti-Dilutive Common Stock Equivalents Outstanding (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	6,416,456	5,296,525	6,416,456	5,296,525
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	6,396,175	5,259,925	6,396,175	5,259,925
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	13,800	30,100	13,800	30,100
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive common stock equivalents	6,481	6,500	6,481	6,500