Divestitures and Assets Held for Sale (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	$ 55,375,000	$ 364,522,000	$ 311,504,000
Assets Held-for-sale, Current
Reasonably certain period within which sale will take place (in years)	1
Assets Held-for-sale, Property, Plant and Equipment	33,620,000	246,000
Asset Retirement Obligations Assets Held-for-sale Noncurrent	1,393,000	1,277,000
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	727,877,000	511,103,000	336,141,000
Gain (Loss) on Sale of Proved Property	(27,018,000)	220,676,000	155,277,000
Mid Continent and Rocky Mountain Region [Member]
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	57,400,000
Estimated gain on divestiture of assets	6,900,000
South Texas and Gulf Coast Region [Member]
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets		230,700,000
Estimated gain on divestiture of assets		193,800,000
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
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
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
Oil and Gas Properties [Member]
Assets Held-for-sale, Current
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	1,700,000
Gain (Loss) on Sale of Proved Property	33,900,000
Sale Of Long Lived Assets [Member] | Rocky Mountain Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	9,200,000
Estimated gain on divestiture of assets	$ 2,500,000