Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid research and development project costs	$ 1,242	$ 549
Prepaid insurance	276	478
Other receivables	328	103
Other	313	247
Total prepaid expenses and other current assets	$ 2,159	$ 1,377