Restructuring Costs (Details) $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Jul. 31, 2018 employee	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Restructuring costs
Restructuring costs		$ 45	$ 176
Long-term portion of restructuring accrual		$ 12	12	$ 12	$ 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			$ 173	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