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Stock-Based Compensation (RSU Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]    
Beginning Balance 721,924  
Granted 229,667  
Additional shares vested 70,525  
Released from restrictions (349,871)  
Forfeited (56,638)  
Ending Balance 615,607  
Vested or expected vest, at end of period 595,462  
Weighted Average Fair Value    
Beginning Balance (in dollars per share) $ 95.65  
Granted (in dollars per share) 102.71  
Additional shares vested (in dollars per share) 114.08  
Released from restrictions (in dollars per share) 103.09  
Forfeited (in dollars per share) 89.16  
Ending Balance (in dollars per share) 101.73  
Vested or expected to vest, at end of period (in dollars per share) $ 101.89  
Intrinsic Value (in millions)    
Outstanding $ 63 $ 69
Vested or expected to vest, at end of period $ 60