Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accrued preclinical and research costs	$ 1,128	$ 1,146
Accrued clinical costs	528	434
Accrued employee-related costs	554	736
Accrued professional services	90	138
Other	106	224
Total accrued liabilities	$ 2,406	$ 2,678