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Stock-Based Compensation - Weighted-Average Assumptions Used to Estimate Fair Value of Non-Employee Stock-Based Awards (Detail)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 0.00% 0.00% 0.00%
Non Employee Stock Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility, minimum 65.00% 53.00% 60.00%
Volatility, maximum 67.00% 64.00% 61.00%
Risk-free interest rate, minimum 1.80% 2.00% 1.60%
Risk-free interest rate, maximum 2.20% 2.50% 2.80%
Dividend yield 0.00% 0.00% 0.00%
Non Employee Stock Awards | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 8 years 2 months 2 days 8 years 3 months 2 days 7 years 9 months 2 days
Non Employee Stock Awards | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 8 years 7 months 2 days 8 years 7 months 2 days 8 years 10 months 2 days