Restructuring Costs (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		15 Months Ended
	Jul. 31, 2018 employee	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Restructuring costs
Restructuring costs		$ 5	$ 3,017	$ 181	$ 3,017
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		$ 3,300
Termination costs in connection with the reduction in workforce, including severance, benefits and related costs						2,800
Non-cash asset impairments						700
Non-cash gain due to write-off of remaining deferred rent liability						400
Expenses associated with lease termination or prepaid expenses						200
Fixed asset impairments						$ 500