Earnings Per Share (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings:
Net income (numerator for basic and diluted calculations)	$ 144,475,000	$ 134,602,000	$ 408,904,000	$ 407,466,000
Shares:
Weighted average outstanding shares (denominator for basic calculation)	73,776,000	73,856,000	73,690,000	73,680,000
Equivalent shares from outstanding stock options	362,000	398,000	388,000	527,000
Denominator for diluted calculation	74,138,000	74,254,000	74,078,000	74,207,000
Earnings Per Share:
Basic earning per share	$ 1.96	$ 1.82	$ 5.55	$ 5.53
Diluted earnings per share	$ 1.95	$ 1.81	$ 5.52	$ 5.49