Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Total operating costs and expenses	$ 124,781	$ 98,090
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expenses related to the Biogen Collaboration Agreement incurred by Sage	48,754	49,837
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Selling, general and administrative expenses	(1,474)	(2,698)
Operating expenses	(19,988)	(24,766)
Total operating costs and expenses	28,766	25,071
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	$ (18,514)	$ (22,068)