Earnings Per Share (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings [Abstract]
Net income	$ 222,989,000	$ 144,475,000	$ 141,111,000	$ 123,318,000	$ 138,579,000	$ 134,602,000	$ 123,944,000	$ 148,920,000	$ 631,893,000	$ 546,045,000	$ 535,742,000
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted Average Number of Shares Outstanding, Basic									73,737,000	73,586,000	73,157,000
Equivalent shares from outstanding stock options									416,000	522,000	1,537,000
Weighted Average Number of Shares Outstanding, Diluted									74,153,000	74,108,000	74,694,000
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 3.02	$ 1.96	$ 1.91	$ 1.68	$ 1.89	$ 1.82	$ 1.68	$ 2.03	$ 8.57	$ 7.42	$ 7.32
Earnings Per Share, Diluted	$ 3.00	$ 1.95	$ 1.91	$ 1.67	$ 1.88	$ 1.81	$ 1.66	$ 2.02	$ 8.52	$ 7.37	$ 7.17