Stock-Based Compensation - Weighted-Average Assumptions Used to Estimate Fair Value of Options Granted (Detail) - $ / shares |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Non-Employee | |||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Volatility | 70.38% | 71.76% | 75.68% |
| Risk free interest rate | 2.10% | 2.44% | 1.53% |
| Dividend yield | 0.00% | 0.00% | 0.00% |
| Term (years) | 8 years 8 months 23 days | 9 years 8 months 5 days | 7 years 8 months 5 days |
| Employee Stock Option | |||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Volatility | 60.69% | 70.17% | 76.60% |
| Risk free interest rate | 1.63% | 1.78% | 1.79% |
| Dividend yield | 0.00% | 0.00% | 0.00% |
| Term (years) | 6 years 1 month 17 days | 5 years 11 months 23 days | 6 years 1 month 21 days |
| Weighted-average fair value of options granted during the period (usd per share) | $ 5.54 | $ 5.29 | $ 9.85 |