Divestitures and Assets Held for Sale (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	$ (424,849,000)	$ (55,375,000)	$ (364,522,000)
Assets Held-for-sale, Current
Reasonably certain period within which sale will take place (in years)	1
Oil and gas properties held for sale, net of accumulated depletion, depreciation and amortization of $7,390 in 2013 and $20,676 in 2012	19,072,000	33,620,000
Asset Retirement Obligations Assets Held-for-sale Noncurrent	3,033,000	1,393,000
Depletion, depreciation, amortization, and asset retirement obligation liability accretion	822,872,000	727,877,000	511,103,000
Gain (Loss) on Sale of Proved Property	(27,974,000)	27,018,000	(220,676,000)
Mid Continent and Rocky Mountain Region
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets		(57,900,000)
Estimated gain on divestiture of assets		(7,400,000)
South Texas and Gulf Coast Region
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	(370,300,000)		(35,800,000)
Estimated gain on divestiture of assets	(29,200,000)		(28,500,000)
Rocky Mountain Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	(56,300,000)		(45,500,000)
Estimated gain on divestiture of assets	10,100,000		(27,200,000)
Permian Divestiture
Disposal Group, Not Discontinued Operation, Disposal Disclosures [Abstract]
Cash received on divestiture of non-strategic assets	(14,500,000)
Estimated gain on divestiture of assets	(6,500,000)
Oil and Gas Properties
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