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Collaboration Agreements - Summary of Operating Expenses Category Related to Collaboration Agreement (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Net reimbursement to (from) Biogen reflected in the condensed consolidated statements of operations and comprehensive loss:        
Total operating costs and expenses $ 214,565 $ 143,219 $ 546,258 $ 397,908
Biogen Collaboration Agreement [Member]        
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]        
Expenses related to the Biogen Collaboration Agreement incurred by Sage 83,276 53,139 210,936 161,111
Net reimbursement to (from) Biogen reflected in the condensed consolidated statements of operations and comprehensive loss:        
Selling, general and administrative expenses 5,835 (480) 16,345 (4,788)
Operating expenses (22,369) (18,347) (51,558) (62,152)
Total operating costs and expenses 60,907 34,792 159,378 98,959
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 $ (28,204) $ (17,867) $ (67,903) $ (57,364)