Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued Expenses
Payroll and related costs	$ 1,569	$ 3,108
Clinical and nonclinical trial expenses	5,177	3,495
Professional and consulting fees	3,328	1,317
Other	150	80
Total accrued expenses	$ 10,224	$ 8,000