Stock Based Awards - Summary of Fair Value of Employees Stock Purchase Rights Under ESPP Using Black-Scholes Model with Following Weighted-Average Assumptions (Detail) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | |||
| Expected volatility | 77.90% | 79.70% | 84.90% |
| Risk-free interest rate | 1.50% | 1.70% | 1.90% |
| Dividend yield | 0.00% | 0.00% | 0.00% |
| Expected life (in years) | 6 years | 6 years | 6 years 1 month 6 days |
| Employee Stock Purchase Plan [Member] | |||
| Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | |||
| Expected volatility | 73.60% | 72.10% | 54.90% |
| Risk-free interest rate | 0.51% | 0.26% | 0.06% |
| Dividend yield | 0.00% | 0.00% | 0.00% |
| Expected life (in years) | 6 months | 6 months | 6 months |