Basis of Presentation (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Jun. 30, 2011 As Reported [Member]	Jun. 30, 2011 Correction [Member]	Dec. 31, 2007 Correction [Member]
Assets
Current deferred income taxes	$ 43,357	$ 43,032	$ 45,704	$ 44,794	$ 910	$ 910
Prepaid expenses	24,840	21,598	22,394	21,659	735	735
Total current assets	951,235	863,100	893,998	892,353	1,645
Total assets	8,180,075	8,229,314	8,387,255	8,385,610	1,645
Liabilities
Other current liabilities	159,481	167,560	178,677	162,001	16,676	16,676
Total current liabilities	616,467	406,253	437,636	420,960	16,676
Noncurrent deferred income taxes	687,337	732,528	756,557	762,406	(5,849)	(5,849)
Total liabilities	4,437,139	4,437,697	4,515,172	4,504,345	10,827
Equity
Retained earnings	1,261,501	1,334,476	1,376,026	1,385,208	(9,182)	(9,182)
Total equity	3,742,936	3,791,617	3,872,083	3,881,265	(9,182)
Total liabilities and equity	$ 8,180,075	$ 8,229,314	$ 8,387,255	$ 8,385,610	$ 1,645