Divestitures and Assets Held for Sale (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	$ 364,522,000	$ 311,504,000	$ 39,898,000
Assets Held-for-sale, Current
Reasonably certain period within which sale will take place (in years)	1
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	511,103,000	336,141,000	304,201,000
Impairment of oil and gas properties	219,037,000	6,127,000	174,813,000
Eagle Ford Shale Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	230,800,000
Estimated gain on divestiture of assets	194,600,000
Mid-Continent Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	35,800,000
Estimated gain on divestiture of assets	28,500,000
Rocky Mountain Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	45,500,000
Estimated gain on divestiture of assets	27,200,000
Permian Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	54,700,000
Estimated gain on divestiture of assets	18,400,000
Legacy Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	125,300,000
Estimated gain on divestiture of assets	66,700,000
Sequel Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	129,100,000
Estimated gain on divestiture of assets	53,100,000
Marcellus shale
Assets Held-for-sale, Current
Disposal Groups, Not Discontinued Operations, Agreed Upon Sales Price	80,000,000
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	14,700,000
Impairment of oil and gas properties	$ 27,512,000