Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid research and development project costs	$ 1,330	$ 549
Income taxes receivable	378
Other receivables	343	103
Prepaid insurance	99	478
Other	273	247
Total prepaid expenses and other current assets	$ 2,423	$ 1,377