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Share-Based Compensation (Summary of Share-Based Compensation Expense) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]        
Share-based compensation expense before income taxes $ 18 $ 18 $ 51 $ 52
Income tax benefit (5) (7) (14) (21)
Share-based compensation expense after income taxes $ 13 $ 11 $ 37 $ 31