Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Net reimbursement from Biogen reflected in the consolidated statements of operations and comprehensive income (loss):
Total operating costs and expenses	$ 554,675	$ 467,217	$ 517,974
Biogen Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expenses related to the Biogen Collaboration Agreement incurred by Sage	213,524	193,776
Net reimbursement from Biogen reflected in the consolidated statements of operations and comprehensive income (loss):
Selling, general and administrative expenses	(2,230)	(11,282)
Operating expenses	(75,457)	(91,130)
Total operating costs and expenses	138,067	102,646
Research and Development [Member] | Biogen Collaboration Agreement [Member]
Net reimbursement from Biogen reflected in the consolidated statements of operations and comprehensive income (loss):
Research and development expenses	$ (73,227)	$ (79,848)