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Fair Values of Stock Options Estimated Using Black-Scholes Valuation Model (Detail)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Employee Stock Option    
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Black Scholes Method Used [Line Items]    
Expected term 6 years 1 month 21 days 5 years 6 months 22 days
Expected volatility 76.59% 68.64%
Risk-free interest rate 1.53% 1.34%
Expected dividend yield 0.00% 0.00%
Non Employee Stock Option    
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Black Scholes Method Used [Line Items]    
Expected term 9 years 8 months 27 days 5 years 7 months 21 days
Expected volatility 101.12% 69.98%
Risk-free interest rate 2.39% 1.30%