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Stock Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee stock options granted during the period (in shares) 686 614
Fair value per share (in dollars per share) $ 67.61 $ 57.39
Cash received from exercise of options $ 36.8 $ 29.1
Restricted Stock Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restricted stock awards granted during period (in shares) 258 298
Weighted average fair value per share of restricted stock awards granted during the period (in dollars per share) $ 309.02 $ 277.15
Restricted stock awards vested during period (in shares) 147  
Weighted average grant date fair value per share (in dollars per share) $ 180.62  
Weighted average vest date fair value per share (in dollars per share) $ 303.78  
Employee Stock Purchase Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Percentage of eligible earnings to purchase common stock through the employee stock purchase plan 10.00%  
Discount on the average closing price for the employee stock purchase plan 5.00%  
Shares of stock purchased during the period by participants in the employee stock purchase plan (in shares) 8 6
Amount paid for stock purchased during the period by participants in the employee stock purchase plan $ 2.2 $ 1.6