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Stock Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Stock Based Compensation [Abstract]        
Percentage of eligible earnings to purchase common stock through the employee stock purchase plan (in hundredths) 10.00%   10.00%  
Discount on the average closing price for the employee stock purchase plan (in hundredths)     5.00%  
Stock Based Compensation Expense [Abstract]        
Stock based compensation $ 17,597 $ 16,998 $ 47,035 $ 47,011
Tax effect recognized in net income 6,159 5,949 16,462 16,454
Windfall tax benefit/(shortfall), net $ 2,132 $ 3,126 $ 10,887 $ 14,727
Employee stock options granted during the period (in shares)     585,155 614,500
Fair value per share (in dollars per share)     $ 33.69 $ 35.02
Weighted average assumptions used to value option grants [Abstract]        
Risk-free interest rate (%) (in hundredths)     1.52% 1.63%
Expected option life (years)     5 years 1 month 5 days 5 years 2 months 23 days
Expected volatility (%) (in hundredths)     22.23% 27.27%
Expected dividend yield (%) (in hundredths)     0.62% 0.59%
Cash received from exercise of options     $ 21,300 $ 29,500
Restricted stock awards granted during period (in shares)     349,035 341,810
Weighted average fair value per share of restricted stock awards granted during the period (in dollars per share)     $ 152.80 $ 140.91
Restricted stock awards vested during period (in shares)     61,889  
Weighted average grant date fair value per share (in dollars per share)     $ 129.11  
Weighted average vest date fair value per share (in dollars per share)     $ 166.95  
Shares of stock purchased during the period by participants in the employee stock purchase plan (in shares)     13,437 15,695
Amount paid for stock purchased during the period by participants in the employee stock purchase plan     $ 2,120 $ 2,080