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Stock-Based Compensation Expense - Summary of Weighted Average Assumptions Used in the Black-Scholes Option Pricing Model (Detail)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Risk-free interest rate, minimum 1.56% 1.83% 1.10%
Risk-free interest rate, maximum 1.93% 2.29% 1.68%
Expected price volatility, minimum 90.00% 94.50% 95.75%
Expected price volatility, maximum 95.80% 105.40% 96.13%
Expected term (in years) 6 years 2 months 12 days 7 years  
Minimum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (in years)     10 months 24 days
Maximum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (in years)     10 years