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Income Taxes - Reconciliation of Income Tax Expense Computed at Statutory Federal Income Tax Rate to Income Taxes (Detail)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Income Tax Disclosure [Abstract]    
Federal income tax expense at statutory rate 34.00% 34.00%
State income tax, net of federal benefit 5.00% 4.50%
Permanent differences (0.50%) (3.70%)
Research and development credit 0.90% 0.80%
Stock compensation (0.70%) (0.90%)
Expiration of state net operating loss (0.00%) (1.70%)
Other 0.40% (0.80%)
Change in valuation allowance (39.10%) (32.20%)
Effective income tax rate 0.00% 0.00%