Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net reimbursement from Biogen reflected in the consolidated statement of operations and comprehensive loss:
Total operating costs and expenses	$ 467,217	$ 517,974	$ 714,992
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expenses related to the Biogen Collaboration Agreement incurred by Sage	193,776
Net reimbursement from Biogen reflected in the consolidated statement of operations and comprehensive loss:
Selling, general and administrative expenses	(11,282)
Operating expenses	(91,130)
Total operating costs and expenses	102,646
Research and Development [Member] | Biogen Collaboration Agreement [Member]
Net reimbursement from Biogen reflected in the consolidated statement of operations and comprehensive loss:
Research and development expenses	$ (79,848)