Leases (Details) $ in Thousands	1 Months Ended		6 Months Ended	12 Months Ended
	Apr. 30, 2017 USD ($)	Aug. 31, 2015 USD ($) ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2019 USD ($) ft² campus	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 01, 2020 USD ($)	Aug. 31, 2019 USD ($) ft²	Feb. 28, 2019 ft² extension_period	Jan. 01, 2019 USD ($)	Aug. 31, 2016 USD ($) ft² extension_period	May 31, 2016 USD ($) ft² extension_period $ / ft²
Lessee, Lease, Description [Line Items]
Rent expense					$ 19,100	$ 18,600
Number of campuses | campus				2
Property and equipment, net			$ 211,977		211,977					$ 209,494
Carrying value of the construction financing obligation			$ 33,489		$ 33,489
Amounts representing interest or imputed interest, operating lease				$ 75,750
Amounts representing interest or imputed interest, finance lease				$ 255,759
Imputed interest rate				17.20%
Cambridge Leases
Lessee, Lease, Description [Line Items]
Area of office space (in sqft) | ft²				200,000
Norwood Leases
Lessee, Lease, Description [Line Items]
Area of office space (in sqft) | ft²									200,000
Finance lease, term of lease											35 years
Extension term									5 years		10 years
Percentage subleased									64.00%
Number of extension periods | extension_period									4		2
200 Technology Square | Cambridge Leases
Lessee, Lease, Description [Line Items]
Area of office space (in sqft) | ft²								50,000				124,760
Tenant allowance (in usd per sqft) | $ / ft²												10.00
Security deposit												$ 1,900
Extension term								2 years				5 years
Tenant improvement allowance								$ 3,500
Lease term												11 years
Number of extension periods | extension_period												2
200 Technology Square, Sublease Agreements | Cambridge Leases
Lessee, Lease, Description [Line Items]
Area of office space (in sqft) | ft²			34,268		34,268
500 Technology Square | Cambridge Leases
Lessee, Lease, Description [Line Items]
Area of office space (in sqft) | ft²		61,618						60,000
Rate per annum		$ 3,900
Security deposit		1,000
Tenant improvement allowance		$ 3,100
Lease term		6 years
Land adjacent to Norwood | Norwood Leases
Lessee, Lease, Description [Line Items]
Rate per annum	$ 300
Lease term	35 years
Office, laboratory, and light manufacturing space | Norwood Leases
Lessee, Lease, Description [Line Items]
Area of office space (in sqft) | ft²											200,000
Security deposit				$ 8,900
Tenant improvement allowance											$ 24,200
Carrying value of the construction financing obligation			$ 33,500		$ 33,500	15,700
Financing obligation in accrued liabilities			10,500		10,500	14,500
Amounts representing interest or imputed interest, operating lease				10,300
Amounts representing interest or imputed interest, finance lease				$ 208,500
Scenario, Forecast | 200 Technology Square | Cambridge Leases
Lessee, Lease, Description [Line Items]
Security deposit							$ 2,200
Construction in progress | Office, laboratory, and light manufacturing space | Norwood Leases
Lessee, Lease, Description [Line Items]
Property and equipment, net			86,400		$ 86,400	$ 75,000
Transfers of property and equipment			161,600
Building | Office, laboratory, and light manufacturing space | Norwood Leases
Lessee, Lease, Description [Line Items]
Transfers of property and equipment			$ 140,400