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Share-Based Compensation (Share-Based Compensation Expense) (Details) - 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]      
Share-based compensation expense $ 166 $ 138 $ 158
Tax benefit related to share-based compensation expense 58 48 56
Total unrecognized compensation cost related to non-vested awards $ 94    
Weighted-average period to recognize compensation cost 2 years 6 months    
Stock option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 32 34 39
Restricted stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense 24 24 22
Restricted stock units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense 65 76 83
Liability awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 45 $ 4 $ 14