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Share-Based Compensation - Summary of Changes of Restricted Stock and Restricted Stock Units (Detail) - $ / shares
shares in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Restricted stock outstanding, beginning of period (in shares) 4.8    
Restricted stock units granted (in shares) 2.0    
Restricted stock units grant adjustment (in shares) (0.5)    
Restricted stock units vested (in shares) (1.2)    
Restricted stock units forfeited (in shares) (0.8)    
Restricted stock outstanding, end of period (in shares) 4.3 4.8  
Restricted stock units expected to vest, end of period (in shares) 4.8    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Weighted-average grant date fair value per share, outstanding, beginning of period (per share) $ 25    
Granted, weighted-average grant date fair value (per share) 21 $ 17 $ 29
Grant adjustment, weighted-average grant date fair value (per share) 48    
Vested, weighted-average grant date fair value (per share) 26    
Forfeited, weighted-average grant date fair value (per share) 21    
Weighted-average grant date fair value per share, outstanding, end of period (per share) 21 $ 25  
Weighted-average grant date fair value expected to vest, end of period (per share) $ 20