Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 20, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	SAGE
Entity Registrant Name	Sage Therapeutics, Inc.
Entity Central Index Key	0001597553
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		51,911,120
Entity Public Float			$ 9,273,813,345
Entity Interactive Data Current	Yes
Title of 12(b) Security	common stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
Entity File Number	001-36544
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-4486580
Entity Address, Address Line One	215 First Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	299-8380
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	Part III of this Annual Report on Form 10-K incorporates by reference certain information from the registrant’s definitive Proxy Statement for its 2020 annual meeting of shareholders, which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant’s fiscal year end of December 31, 2019. Except with respect to information specifically incorporated by reference in this Form 10-K, the Proxy Statement is not deemed to be filed as part of this Form 10-K.