Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands |
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
Dec. 31, 2012 |
|---|---|---|---|---|
| Deferred tax assets: | ||||
| Capitalized research and development expenses | $ 37,765 | $ 24,945 | ||
| Net operating loss carryforwards | 10,224 | 6,292 | ||
| Stock-based compensation | 1,371 | 612 | ||
| Research and development tax credit carryforwards | 1,273 | 1,008 | ||
| Accrued expenses | 339 | 414 | ||
| Other | 56 | 1 | ||
| Total gross deferred tax assets | 51,028 | 33,272 | ||
| Valuation allowance | (51,028) | (33,272) | $ (24,402) | $ (18,138) |
| Net deferred tax assets | $ 0 | $ 0 |