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Stock-Based Compensation (Schedule of Options Exercised, Cash Received, and Related Tax Benefits) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]        
Stock options exercised (in shares) 215,132 125,593 470,114 1,011,315
Proceeds from stock options exercised $ 16 $ 8 $ 33 $ 57
Excess tax benefits recognized for share-based awards $ 3 $ 3 $ 7 $ 19