Balance Sheet Components (Details) - Accrued liabilities - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued liabilities [Abstract]
Accrued compensation	$ 1,076	$ 1,320
Accrued clinical and manufacturing expenses	359	69
Accrued professional and consulting services	254	113
Convertible preferred stock warrant liability		67
Other	450	36
Total	$ 2,139	$ 1,605