Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid research and development project costs	$ 540	$ 515
Prepaid insurance	716	511
Other receivables	96	70
Other	393	247
Total prepaid expenses and other current assets	$ 1,745	$ 1,343