Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid research and development project costs	$ 1,545	$ 762
Other receivables	1,457	751
Prepaid insurance	185	555
Income taxes receivable	115	163
Other	459	475
Total prepaid expenses and other current assets	$ 3,761	$ 2,706