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Stock Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee stock options granted during the period (in shares) 614 452
Fair value per share (in dollars per share) $ 57.39 $ 40.17
Cash received from exercise of options $ 29.1 $ 12.9
Restricted Stock Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restricted stock awards granted during period (in shares) 298 249
Weighted average fair value per share of restricted stock awards granted during the period (in dollars per share) $ 277.15 $ 188.22
Restricted stock awards vested during period (in shares) 81  
Weighted average grant date fair value per share (in dollars per share) $ 170.85  
Weighted average vest date fair value per share (in dollars per share) $ 277.17  
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) 6 6
Amount paid for stock purchased during the period by participants in the employee stock purchase plan $ 1.6 $ 1.0