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Share-Based Compensation (Summary of Employee Stock Purchase Plan) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation expenses $ 69 $ 70 $ 86
Employee Stock Purchase Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares purchased by employees 205,785 235,859 233,464
Weighted-average price of shares purchased (in dollars per share) $ 66.79 $ 58.26 $ 50.39
Compensation expenses $ 3 $ 3 $ 4