Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Total operating costs and expenses	$ 129,908	$ 109,664		$ 254,689	$ 207,754
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expenses related to the Biogen Collaboration Agreement incurred by Sage	59,218	48,866		107,972	98,703
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Selling, general and administrative expenses	(2,835)	(3,531)		(4,309)	(6,229)
Operating expenses	(23,818)	(23,672)	$ (24,800)	(43,806)	(48,438)
Total operating costs and expenses	35,400	25,194		64,166	50,265
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	$ (20,983)	$ (20,141)		$ (39,497)	$ (42,209)