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Earnings Per Share (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Earnings:        
Net income (numerator for basic and diluted calculations) $ 141,111 $ 123,944 $ 264,429 $ 272,864
Shares:        
Weighted average outstanding shares (denominator for basic calculation) 73,718 73,971 73,646 73,593
Equivalent shares from outstanding stock options 336 559 402 591
Denominator for diluted calculation 74,054 74,530 74,048 74,184
Earnings Per Share:        
Basic earning per share $ 1.91 $ 1.68 $ 3.59 $ 3.71
Diluted earnings per share $ 1.91 $ 1.66 $ 3.57 $ 3.68