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Stock Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Stock Options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Employee stock options granted during the period (in shares) 592,798 633,000
Fair value per share (in dollars per share) $ 40.67 $ 34.45
Cash received from exercise of options $ 40,400 $ 15,900
Restricted Stock Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restricted stock awards granted during period (in shares) 389,117 395,980
Weighted average fair value per share of restricted stock awards granted during the period (in dollars per share) $ 203.02 $ 169.03
Restricted stock awards vested during period (in shares) 138,140  
Weighted average grant date fair value per share (in dollars per share) $ 143.29  
Weighted average vest date fair value per share (in dollars per share) $ 215.77  
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) 15,511 15,076
Amount paid for stock purchased during the period by participants in the employee stock purchase plan $ 3,190 $ 2,580