Accrued Expenses - Components of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables and Accruals [Abstract]
Development costs	$ 2,581	$ 852
Employee compensation	735	352
Professional fees	463	413
Other	34	157
Total accrued expenses	$ 3,813	$ 1,774