Oil and Gas Operations (Unaudited) (Results of Operations) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Gross revenues		$ 944,908	[1]	$ 957,371	$ 815,465
Production (lifting costs)		257,045		224,901	217,429
Exploration expense		13,110		64,584	10,234
Depreciation, depletion and amortization		240,232		200,179	180,752
Accretion expense		6,837		6,178	4,935
Income tax expense		154,180		166,750	143,691
Results of operations from producing activities		273,504		294,779	258,424
After-tax non-cash mark-to-market loss on derivatives	$ 56,600	$ 37,600
After-tax non-cash mark-to-market loss on derivatives, per diluted shares	$ 0.78

[1] The year ended December 31, 2011 gross revenues includes a pre-tax non-cash mark-to-market loss on derivatives of $37.6 million