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Stock-Based Compensation (RSU Activity) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]    
Beginning Balance 615,607  
Granted 252,885  
Released from restrictions (194,329)  
Forfeited (42,432)  
Ending Balance 631,731  
Vested or expected vest, at end of period 616,894  
Weighted Average Fair Value    
Beginning Balance (in dollars per share) $ 101.73  
Granted (in dollars per share) 112.98  
Released from restrictions (in dollars per share) 99.80  
Forfeited (in dollars per share) 106.15  
Ending Balance (in dollars per share) 102.80  
Vested or expected to vest, at end of period (in dollars per share) $ 102.67  
Intrinsic Value (in millions)    
Outstanding $ 65 $ 63
Vested or expected to vest, at end of period $ 63