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Stockholders' Equity - Weighted-Average Fair Value of Options Granted (Detail)
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (years) 6 years 3 months  
2013 Equity Incentive Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (years) 6 years 3 months 6 years 3 months
Expected volatility 81.70% 90.50%
Risk-free interest rate 1.70% 2.14%
Expected dividend yield 0.00% 0.00%