Income Taxes - Summary of Changes in the Valuation Allowance for Deferred Tax Assets (Detail) - USD ($) $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| Income Tax Disclosure [Abstract] | |||
| Valuation allowance as of beginning of year | $ 74,541 | $ 42,398 | $ 27,831 |
| Decreases recorded as benefit to income tax provision | 0 | 0 | 0 |
| Increases recorded to income tax provision | 23,179 | 32,143 | 14,567 |
| Valuation allowance as of end of year | $ 97,720 | $ 74,541 | $ 42,398 |