Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Total operating costs and expenses	$ 143,219	$ 132,334	$ 397,908	$ 340,088
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expenses related to the Biogen Collaboration Agreement incurred by Sage	53,139	50,950	161,111	149,653
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Selling, general and administrative expenses	(480)	(2,551)	(4,788)	(8,780)
Operating expenses	(18,347)	(24,185)	(62,152)	(72,623)
Total operating costs and expenses	34,792	26,765	98,959	77,030
Research and Development [Member] | Biogen Collaboration Agreement [Member]
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Research and development expenses	$ (17,867)	$ (21,634)	$ (57,364)	$ (63,843)