Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued research and development expenses	$ 355	$ 609
Accrued employee bonus compensation	261	763
Employee withholdings	32	29
Other accrued expenses	289	304
Total accrued expenses	$ 937	$ 1,705