Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued Liabilities And Other Liabilities [Abstract]
Accrued compensation	$ 1,001	$ 1,837
Accrued contracted research and development costs	3,379	2,552
Accrued professional and consulting fees	512	672
Accrued other and other current liabilities	110	159
Total accrued and other current liabilities	$ 5,002	$ 5,220