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Stock Award Plans - Fair Value of Employee Stock Options Assumptions (Detail)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk free interest rate, minimum 0.39% 1.52%
Risk free interest rate, maximum 1.52% 2.51%
Volatility, minimum 93.83% 93.05%
Volatility, maximum 94.25% 94.25%
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected lives 5 years 8 months 1 day 6 years 2 months 12 days
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected lives 7 years 9 years 4 months 13 days