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Collaboration Agreements - Biogen (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2021
USD ($)
milestone
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
milestone
Sep. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]          
Offset to research and development expense, related party cost reimbursement $ 1,200,000   $ 5,300,000    
Cost sharing reimbursements due from related party 1,194,000   1,194,000   $ 5,674,000
Collaboration revenue from customers 5,285,000 [1] $ 9,388,000 36,143,000 [1] $ 18,751,000  
Biogen Collaboration Agreement | Related Party          
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]          
Contract liability 294,900,000   294,900,000    
Biogen Collaborative Arrangement | Biogen Collaboration Agreement          
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]          
Transaction price, change 0   0    
Offset to research and development expense, related party cost reimbursement 1,200,000   5,300,000    
Cost sharing reimbursements due from related party $ 1,200,000   $ 1,200,000   $ 5,700,000
Number of milestones achieved | milestone 0   0    
Biogen Collaborative Arrangement | Biogen Collaboration Agreement | Option Research Services | Related Party          
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]          
Contract liability $ 6,000,000.0   $ 6,000,000.0    
Biogen Collaborative Arrangement | Biogen Collaboration Agreement | Product          
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]          
Collaboration revenue from customers     0    
ROFN and Option Agreement | Biogen Collaboration Agreement | Related Party          
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]          
Contract liability $ 288,900,000   $ 288,900,000    
[1] Includes related party collaboration revenue from a customer of $0.9 million and $2.5 million for the three and nine months ended September 30, 2021, respectively.