Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate (Detail) |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Income Tax Disclosure [Abstract] | |||
| Federal statutory income tax rate | (34.00%) | (34.00%) | (34.00%) |
| Federal research and development tax credit | (0.50%) | (0.90%) | (0.70%) |
| State taxes, net of federal benefit | (5.30%) | (4.50%) | (5.60%) |
| Stock-based compensation expense | 0.40% | 1.10% | 0.40% |
| Other | 0.10% | 0.20% | 0.10% |
| Increase in deferred tax asset valuation allowance | 39.30% | 38.10% | 39.80% |
| Effective income tax rate | 0.00% | 0.00% | 0.00% |