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Stock-Based Compensation - Fair Value of Market Performance Units Estimated, Monte Carlo Simulation Model (Detail) - Market Performance Units [Member]
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Risk-free interest rate, Minimum 0.06% 0.16%
Risk-free interest rate, Maximum 0.36% 0.18%
Estimated volatility factor, Minimum 75.00% 66.00%
Estimated volatility factor, Maximum 87.00% 118.00%
Expected dividends 0.00% 0.00%