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Share-Based Compensation (Summary of Stock Option Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Stock Option Activity [Roll Forward]      
Number of Stock Options Outstanding, Beginning of period (in shares) 1,406,371    
Number of Stock Options, Granted (in shares) 268,817    
Number of Stock Options, Exercised (in shares) (40,416) (86,811)  
Number of Stock Options Outstanding, End of period (in shares) 1,634,772   1,406,371
Stock Options, Weighted Average Exercise Price [Abstract]      
Weighted-Average Exercise Price, Outstanding Beginning of period (in dollars per share) $ 22.32    
Weighted-Average Exercise Price, Granted (in dollars per share) 66.68    
Weighted-Average Exercise Price, Exercised (in dollars per share) 20.98    
Weighted-Average Exercise Price, Outstanding End of period (in dollars per share) $ 29.64   $ 22.32
Stock Option Activity, Additional Disclosures [Abstract]      
Options Outstanding, Weighted Average Remaining Contractual Term (in years) 3 years 7 months 13 days   2 years 7 months 24 days
Options Outstanding, Aggregate Intrinsic Value $ 65   $ 63
Options Exercisable, Number of Options (in shares) 1,365,955 2,500,000  
Options Exercisable, Weighted Average Exercise Price Per Share (in dollars per share) $ 22.35 $ 27.87  
Options Exercisable, Weighted Average Remaining Contractual Term (in years) 2 years 5 months 9 days    
Options Exercisable, Aggregate Intrinsic Value $ 64