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Assumptions Used in Black Scholes Pricing Model for New Grants (Detail)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility factor, minimum 72.18% 73.04% 69.38%
Volatility factor, maximum 74.47% 78.70% 77.92%
Risk-free interest rates, minimum 1.07% 1.54% 1.81%
Risk-free interest rates, maximum 2.01% 1.93% 2.02%
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 3 years 5 years 6 months 5 years 6 months
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (in years) 6 years 3 months 6 years 3 months 6 years 3 months