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Stockholders' Equity - Assumptions for Fair Values of Stock Options Granted are Calculated Related to Stock Options Granted to Nonemployees (Detail)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 0.00% 0.00% 0.00%
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility 67.00% 67.00% 69.00%
Weighted-average expected life (in years) 5 years 6 months 5 years 8 months 12 days 5 years 6 months
Risk-free interest rate 1.60% 2.70% 1.90%
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Volatility 87.00% 68.00% 70.00%
Weighted-average expected life (in years) 6 years 9 years 10 months 24 days 10 years
Risk-free interest rate 2.20% 3.00% 2.50%