Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid research and development project costs	$ 1,106	$ 549
Prepaid insurance	815	478
Other receivables	407	103
Income taxes receivable	294
Other	581	247
Total prepaid expenses and other current assets	$ 3,203	$ 1,377