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Fair Value of Employee Stock Options Assumptions (Detail)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk free interest rate, minimum 1.52% 2.63%
Risk free interest rate, maximum 2.51% 3.11%
Volatility, minimum 93.05% 92.68%
Volatility, maximum 94.25% 93.62%
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected lives 6 years 2 months 12 days 5 years 10 months 24 days
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected lives 9 years 4 months 13 days 7 years 2 months 8 days