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Leases - Narrative (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
USD ($)
ft²
campus
Dec. 31, 2018
USD ($)
Jun. 01, 2020
ft²
Dec. 31, 2019
USD ($)
Aug. 31, 2019
ft²
Feb. 28, 2019
ft²
extension_period
Aug. 31, 2016
ft²
extension_period
Lessee, Lease, Description [Line Items]              
Number of campuses | campus 2            
Lease liabilities $ 103,393            
Right-of-use assets, operating leases 90,201     $ 86,414      
Rent expense   $ 19,100          
Amounts representing interest or imputed interest, operating lease 70,354            
Amounts representing interest or imputed interest, finance lease 379,110            
Embedded Leases              
Lessee, Lease, Description [Line Items]              
Lease liabilities 24,300            
Right-of-use assets, operating leases $ 62,300            
Cambridge Leases              
Lessee, Lease, Description [Line Items]              
Area of office space (in sqft) | ft² 175,000            
Cambridge Leases | 200 Technology Square              
Lessee, Lease, Description [Line Items]              
Area of office space (in sqft) | ft²         50,000    
Extension term         2 years    
Cambridge Leases | 500 Technology Square              
Lessee, Lease, Description [Line Items]              
Area of office space (in sqft) | ft²         60,000    
Norwood Leases              
Lessee, Lease, Description [Line Items]              
Area of office space (in sqft) | ft²           200,000  
Extension term           5 years 10 years
Number of extension periods | extension_period           4 2
Percentage subleased           64.00%  
Tenant improvements $ 22,200            
Norwood Leases | Office, laboratory, and light manufacturing space              
Lessee, Lease, Description [Line Items]              
Area of office space (in sqft) | ft²             200,000
Amounts representing interest or imputed interest, finance lease $ 338,900            
Norwood Leases | MTC North [Member]              
Lessee, Lease, Description [Line Items]              
Percentage subleased     12.00%        
Additional space (in sqft) | ft²     28,000        
Incremental borrowing rate 8.20%            
Norwood Leases | MTC South              
Lessee, Lease, Description [Line Items]              
Imputed interest rate 17.20%