Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Research and development expenses	$ (58,056)	$ (63,610)
Selling, general and administrative expenses	(39,847)	(70,130)
Total operating costs and expenses	98,090	$ 133,910
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expenses related to the Biogen Collaboration Agreement incurred by Sage	49,837
Net reimbursement from Biogen reflected in the condensed consolidated statement of operations and comprehensive loss:
Research and development expenses	(22,068)
Selling, general and administrative expenses	(2,698)
Total operating costs and expenses	$ 25,071