Restructuring Costs - Accrual Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		15 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019
Restructuring costs
Charges incurred	$ 5	$ 3,017	$ 181	$ 3,017
Cambridge, Massachusetts facility operations
Restructuring costs
Accrued restructuring balance, beginning of period			1,147
Charges incurred			181		$ 3,300
Cash payments			(1,119)
Accrued restructuring balance, end of period	209		209		209
Cambridge, Massachusetts facility operations | Employee Severance and Benefits
Restructuring costs
Accrued restructuring balance, beginning of period			1,147
Charges incurred			181
Cash payments			(1,119)
Accrued restructuring balance, end of period	$ 209		$ 209		$ 209