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Schedule of Share-Based Compensation Plan Cost (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation cost $ 73 $ 70 $ 58
RSUs      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation cost 63 58 52
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation cost 8 11 $ 6
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation cost $ 2 $ 1