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Fair Value of Employee Stock Options Assumptions (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate, minimum 2.63% 1.83% 1.18%
Risk free interest rate, maximum 3.11% 2.13% 1.80%
Volatility, minimum 92.68% 83.32% 77.57%
Volatility, maximum 93.62% 90.39% 82.75%
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected lives 5 years 10 months 24 days 5 years 4 months 28 days 5 years 1 month 17 days
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected lives 7 years 2 months 8 days 5 years 9 months 10 days 5 years 9 months 25 days