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Share-Based Compensation (Summary of Stock Option Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Number of Stock Options          
Number of Stock Options, Outstanding at Beginning of period (in shares)     571,380    
Number of Stock Options, Exercised (in shares) (11,953) (40,160) (99,953) (1,074,321)  
Number of Stock Options, Forfeited (in shares)     (3,182)    
Number of Stock Options, Outstanding at End of period (in shares) 468,245   468,245   571,380
Stock options, exercisable (in shares) 289,033 300,000 289,033 300,000  
Weighted-Average Exercise Price          
Weighted-Average Exercise Price, Outstanding at Beginning of period (in dollars per share)     $ 43.84    
Weighted-Average Exercise Price, Exercised (in dollars per share)     24.52    
Weighted-Average Exercise Price, Forfeited (in dollars per share)     26.86    
Weighted-Average Exercise Price, Outstanding at End of period (in dollars per share) $ 48.08   48.08   $ 43.84
Options exercisable, weighted average exercise price (in dollars per share) $ 36.55 $ 23.77 $ 36.55 $ 23.77  
Weighted-Average Remaining Contractual Term (in years)     5 years 5 months 22 days   5 years 4 months 25 days
Weighted-Average Remaining Contractual Term, Exercisable (in years)     3 years 10 months    
Aggregate Intrinsic Value (in millions) $ 18   $ 18   $ 19
Aggregate Intrinsic Value, Exercisable (in millions) $ 14   $ 14