Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Net reimbursement to (from) Biogen reflected in the condensed consolidated statements of operations and comprehensive loss:
Total operating costs and expenses	$ 158,764	$ 124,781
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expenses related to the Biogen Collaboration Agreement incurred by Sage	56,136	48,754
Net reimbursement to (from) Biogen reflected in the condensed consolidated statements of operations and comprehensive loss:
Selling, general and administrative expenses	3,034	(1,474)
Operating expenses	(14,248)	(19,988)
Total operating costs and expenses	41,888	28,766
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	$ (17,282)	$ (18,514)