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Share-Based Compensation - Equity-Classified Awards (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Restricted stock [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Weighted-average grant-date fair value (in usd per share) $ 58.30 $ 59.92 $ 52.03
Total fair value $ 142 $ 132 $ 114
Total unrecognized compensation cost $ 172    
Unrecognized compensation cost, remaining weighted-average remaining service period 1 year 6 months    
Restricted stock [Member] | Maximum [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Vesting period 3 years    
Stock options [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Vesting period 3 years    
Total unrecognized compensation cost related to stock options $ 25    
Stock options [Member] | Maximum [Member]      
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]      
Expiration date 7 years