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Share-Based Compensation (Summary of Employee Purchase Plan) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Compensation expense (in millions) $ 86 $ 68 $ 62
Employee Stock Purchase Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares purchased by employees (in shares) 233,464 247,444 256,772
Weighted-average price of shares purchased (in dollars per share) $ 50.39 $ 40.95 $ 33.06
Compensation expense (in millions) $ 4 $ 4 $ 4