Unaudited Quarterly Financial Data (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
Unaudited Quarterly Financial Data
Revenues	$ 748,357	$ 878,177	$ 889,496	$ 806,389	$ 840,966	$ 799,724	$ 822,603	$ 859,681	$ 3,322,419	$ 3,322,974	$ 3,631,284
Operating income	218,276	350,277	367,596	319,265	335,710	317,180	345,807	426,677	1,255,414	1,425,374	1,903,213
Income before income tax expense	203,547	334,849	344,026	296,849	310,512	298,566	320,926	406,012	1,179,271	1,336,016	1,868,431
Net income	$ 188,490	$ 256,854	$ 266,586	$ 250,612	$ 241,687	$ 198,524	$ 224,393	$ 290,853	$ 962,542	$ 955,457	$ 1,376,219
Earnings per share:
Basic	$ 1.36	$ 1.85	$ 1.92	$ 1.80	$ 1.74	$ 1.43	$ 1.61	$ 2.09	$ 6.92	$ 6.87	$ 9.90
Diluted	$ 1.36	$ 1.85	$ 1.92	$ 1.80	$ 1.74	$ 1.43	$ 1.61	$ 2.09	$ 6.92	$ 6.87	$ 9.89