Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued expenses
Accrued employee compensation and benefits	$ 753	$ 89
Accrued clinical trial costs	4,601	3,582
Accrued professional fees	1,594	390
Accrued lease liability	353	362
Other	513	285
Total accrued expenses	$ 7,814	$ 4,708