Restructuring Costs (Details) $ in Thousands	1 Months Ended	12 Months Ended			18 Months Ended
	Jul. 31, 2018 employee	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)
Restructuring costs
Restructuring costs		$ 181	$ 3,112
Fixed asset impairments		0	500	$ 0
Long-term portion of restructuring accrual			11
Cambridge, Massachusetts facility operations
Restructuring costs
Number of positions eliminated | employee	18
Percentage of positions eliminated	40.00%
Number of positions eliminated since inception | employee	15
Restructuring costs		$ 181	$ 3,534		$ 3,300
Non-cash asset impairments					700
Non-cash gain due to write-off of remaining deferred rent liability					400
Fixed asset impairments					500
Expenses associated with lease termination or prepaid expenses					200
Termination costs in connection with the reduction in workforce, including severance, benefits and related costs					$ 2,800