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Stock-Based Compensation (Schedule of PSU Activity) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Excess tax benefits recognized for share-based awards $ 7 $ 10 $ 14 $ 29
PSU        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share units earned (in shares)     154,189 171,080
Shares issued net of withholding taxes (in shares)     94,399 99,805
Excess tax benefits recognized for share-based awards     $ 3 $ 1