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Stock Based Compensation (Details) (USD $)
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Notes to Financial Statements [Abstract]        
Percentage of eligible earnings to purchase common stock through the employee stock purchase plan 10.00%   10.00%  
Discount on the average closing price for the employee stock purchase plan     5.00%  
Stock Based Compensation Expense Table        
Stock based compensation $ 13,307,000 $ 9,749,000 $ 26,284,000 $ 19,704,000
Tax effect recognized in net income 4,658,000 3,412,000 9,199,000 6,896,000
Windfall tax benefit/(shortfall), net 1,245,000 3,551,000 5,557,000 10,966,000
Employee stock options granted during the period     520,850 412,100
Weighted average assumptions used to value option grants Table        
Fair value per share ($)     $ 36.55 $ 29.39
Risk-free interest rate (%)     0.78% 0.82%
Expected option life (years)     5 years 2 months 12 days 5 years 2 months 19 days
Expected volatility (%)     36.22% 36.55%
Expected dividend yield (%)     0.57% 0.59%
Cash received from exercise of options     14,400,000 28,300,000
Restricted stock awards granted during period     346,390 290,307
Weighted average fair value per share of restricted stock awards granted during the period     $ 116.45 $ 94.05
Restricted stock awards vested during period     118,962  
Weighted average grant date fair value per share     $ 62.39  
Weighted average vest date fair value per share     $ 120.52  
Shares of stock purchased during the period by participants in the employee stock purchase plan     10,712 12,329
Amount paid for stock purchased during the period by participants in the employee stock purchase plan     $ 1,250,000 $ 1,120,000