Quarterly Operations (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Information [Line Items]
Revenues	$ 670,314	$ 617,211	$ 631,607	$ 746,822	$ 670,296	$ 487,527	$ 573,096	$ 758,994	$ 2,665,954	$ 2,489,913	$ 2,179,018
Operating income	144,278	110,526	115,022	108,032	133,186	76,407	131,744	119,278	477,858	460,615	346,089
Net earnings from continuing operations									255,424	242,648	179,580
Loss from discontinued operations					(18,100)	0	0	0	0	(18,100)	0
Net earnings	76,957	58,032	59,773	60,662					255,424	224,548	179,580
Net earnings available for common stockholders	76,957	58,032	59,773	60,662	55,868	36,632	69,424	54,289	255,424	216,213	170,897
Basic earnings per share [Abstract]
Net earnings available for common stockholders from continuing operations					$ 0.59	$ 0.29	$ 0.56	$ 0.44	$ 2.05	$ 1.88	$ 1.38
Net earnings available for common stockholders	$ 0.62	$ 0.47	$ 0.48	$ 0.49	$ 0.45	$ 0.29	$ 0.55	$ 0.43	$ 2.05	$ 1.74	$ 1.38
Diluted Earnings Per Share [Abstract]
Net earnings available for common stockholders from continuing operations					$ 0.59	$ 0.29	$ 0.56	$ 0.44	$ 2.02	$ 1.87	$ 1.37
Net earnings available for common stockholders	$ 0.61	$ 0.46	$ 0.47	$ 0.48	$ 0.45	$ 0.29	$ 0.55	$ 0.43	$ 2.02	$ 1.73	$ 1.37
Quarterly Financial Information [Member]
Quarterly Financial Information [Line Items]
Net earnings from continuing operations					$ 73,968	$ 37,331	$ 74,889	$ 56,460