Restructuring Costs - Accrual Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended			18 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
Restructuring costs
Charges incurred		$ 181	$ 3,112
Cambridge, Massachusetts facility operations
Restructuring costs
Accrued restructuring balance, beginning of period	$ 113	1,147
Charges incurred		181	3,534	$ 3,300
Cash payments	(113)	(1,215)	(1,605)
Non-cash settlements			(698)
Adjustments			(84)
Accrued restructuring balance, end of period		113	1,147	113
Cambridge, Massachusetts facility operations | Employee Severance and Benefits
Restructuring costs
Accrued restructuring balance, beginning of period	113	1,147
Charges incurred		181	2,635
Cash payments	$ (113)	(1,215)	(1,380)
Non-cash settlements			(24)
Adjustments			(84)
Accrued restructuring balance, end of period		$ 113	1,147	$ 113
Cambridge, Massachusetts facility operations | Contract Termination Costs
Restructuring costs
Charges incurred			225
Cash payments			(225)
Cambridge, Massachusetts facility operations | Asset Impairments
Restructuring costs
Charges incurred			674
Non-cash settlements			$ (674)