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Stock Based Compensation (Details) (USD $)
 3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
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 $ 15,442,000 $ 13,307,000 $ 30,013,000 $ 26,284,000
Tax effect recognized in net income 5,405,000 4,658,000 10,505,000 9,199,000
Windfall tax benefit/(shortfall), net 4,116,000 1,245,000 11,601,000 5,557,000
Employee stock options granted during the period (in shares)     568,500 520,850
Fair value per share (in dollars per share)     $ 35.17 $ 36.55
Weighted average assumptions used to value option grants [Abstract]        
Risk-free interest rate (%) (in hundredths)     1.63% 0.78%
Expected option life (years)     5 years 2 months 26 days 5 years 2 months 8 days
Expected volatility (%) (in hundredths)     27.58% 36.22%
Expected dividend yield (%) (in hundredths)     0.59% 0.57%
Cash received from exercise of options     23,700,000 14,400,000
Restricted stock awards granted during period (in shares)     286,810 346,390
Weighted average fair value per share of restricted stock awards granted during the period (in dollars per share)     $ 139.83 $ 116.45
Restricted stock awards vested during period (in shares)     47,161  
Weighted average grant date fair value per share (in dollars per share)     $ 92.89  
Weighted average vest date fair value per share (in dollars per share)     $ 139.75  
Shares of stock purchased during the period by participants in the employee stock purchase plan (in shares)     11,209 10,712
Amount paid for stock purchased during the period by participants in the employee stock purchase plan     $ 1,460,000 $ 1,250,000