Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands |
Jun. 30, 2015 |
Dec. 31, 2014 |
|---|---|---|
| Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] | ||
| Prepaid research and development project costs | $ 414 | $ 515 |
| Other | 368 | 46 |
| Total prepaid expenses and other current assets | $ 782 | $ 561 |