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Fair Value Options Weighted-Average Assumptions (Detail) - Employee and Nonemployee Stock Option
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Dividend yield 0.00% 0.00%
Expected volatility 99.90% 96.50%
Risk-free interest rate 2.50% 2.70%
Expected life in years 5 years 9 months 3 days 5 years 10 months 6 days