Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid research and development project costs	$ 549	$ 515
Prepaid insurance	478	511
Other receivables	103	70
Other	247	247
Total prepaid expenses and other current assets	$ 1,377	$ 1,343