Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Net reimbursement to (from) Biogen reflected in the condensed consolidated statements of operations and comprehensive loss:
Total operating costs and expenses	$ 172,931	$ 129,908	$ 331,695	$ 254,689
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expenses related to the Biogen Collaboration Agreement incurred by Sage	71,524	59,218	127,660	107,972
Net reimbursement to (from) Biogen reflected in the condensed consolidated statements of operations and comprehensive loss:
Selling, general and administrative expenses	7,476	(2,835)	10,510	(4,309)
Operating expenses	(14,942)	(23,818)	(29,190)	(43,806)
Total operating costs and expenses	56,582	35,400	98,470	64,166
Research and Development [Member] | Biogen Collaboration Agreement [Member]
Net reimbursement to (from) Biogen reflected in the condensed consolidated statements of operations and comprehensive loss:
Research and development expenses	$ (22,418)	$ (20,983)	$ (39,700)	$ (39,497)