Basis of Presentation (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Mar. 31, 2011 As Reported [Member]	Mar. 31, 2011 Correction [Member]	Dec. 31, 2007 Correction [Member]
Assets
Current deferred income taxes	$ 43,394	$ 43,032	$ 57,993	$ 57,083	$ 910	$ 910
Prepaid expenses	24,574	21,598	25,035	24,300	735	735
Total current assets	918,649	863,100	766,895	765,250	1,645
Total assets	8,216,039	8,229,314	8,300,436	8,298,791	1,645
Liabilities
Other current liabilities	211,286	167,560	209,662	192,986	16,676	16,676
Total current liabilities	481,093	406,253	634,958	618,282	16,676
Noncurrent deferred income taxes	704,166	732,528	807,029	812,878	(5,849)	(5,849)
Total liabilities	4,470,432	4,437,697	4,404,001	4,393,174	10,827
Equity
Retained earnings	1,281,080	1,334,476	1,416,486	1,425,668	(9,182)	(9,182)
Total equity	3,745,607	3,791,617	3,896,435	3,905,617	(9,182)
Total liabilities and equity	$ 8,216,039	$ 8,229,314	$ 8,300,436	$ 8,298,791	$ 1,645