Accounts Payable and Accrued Expenses - Accounts Payable and Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Trade payables	$ 473	$ 1,253
Employee compensation, benefits, and related accruals	573	1,017
Consulting and contracted research	1,026	817
Professional fees	253	252
Other	63	44
Total	$ 2,388	$ 3,383