Supplemental Oil and Natural Gas Disclosures (Unaudited) (Results of Operations from Oil and Natural Gas Producing Activities) (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Results of operations from oil and natural gas producing activities excluding corporate overhead and interest costs
Oil, natural gas, and related product sales	$ 2,409,867	$ 2,269,151	$ 1,793,292
Lease operating costs	532,359	507,397	470,364
Marketing expenses	52,836	26,047	31,036
Taxes other than income	160,016	147,534	120,541
Derivatives expense (income)			(23,833)
Net operating income	1,189,099	1,256,250	777,749
Income tax provision	457,803	477,375	295,545
Results of operations from oil and natural gas producing activities	731,296	778,875	482,204
Depletion, depreciation and amortization per BOE	18.69	16.42	15.82
Oil and Gas [Member]
Results of operations from oil and natural gas producing activities excluding corporate overhead and interest costs
Depletion, depreciation and amortization	448,424	369,075	391,782
Co2 Property And Pipelines [Member]
Results of operations from oil and natural gas producing activities excluding corporate overhead and interest costs
Depletion, depreciation and amortization	42,064	24,460	29,206
Oil And Gas Producing Activities [Member]
Results of operations from oil and natural gas producing activities excluding corporate overhead and interest costs
Taxes other than income	149,919	138,419	114,569
Derivatives expense (income)	$ (4,834)	$ (52,497)	$ (21,414)