Acquisitions, Divestitures, and Assets Held for Sale Divestitures and Assets Held for Sale (Details) - USD ($)		3 Months Ended	12 Months Ended
		Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on divestiture of non-strategic assets			$ 357,938,000	$ 43,858,000	$ 424,849,000
Net gain on divestiture activity			43,031,000	646,000	27,974,000
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down			98,600,000	27,600,000
Contractual Obligation	[1]		$ 964,438,000
Assets Held-for-sale, Current
Reasonably certain period within which sale will take place (in years)			1 year
Oil and gas properties held for sale			$ 641,000	17,891,000
Asset Retirement Obligations Assets Held-for-sale Noncurrent			241,000
Mid Continent Divestiture 2015 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on divestiture of non-strategic assets			316,800,000
Net gain on divestiture activity			108,400,000
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down		$ 30,000,000
Business Exit Costs			9,300,000
Permian Divestiture 2015 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on divestiture of non-strategic assets			25,100,000
Net gain on divestiture activity			2,400,000
Other Divestitures 2015 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down			68,600,000
Rocky Mountain Divestiture 2014 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on divestiture of non-strategic assets				50,100,000
Net gain on divestiture activity				$ 26,900,000
Mid Continent Divestiture 2013 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on divestiture of non-strategic assets					368,500,000
Net gain on divestiture activity					25,300,000
Rocky Mountain Divestiture 2013 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on divestiture of non-strategic assets					57,100,000
Net gain on divestiture activity					13,200,000
Permian Divestiture 2013 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash received on divestiture of non-strategic assets					14,000,000
Net gain on divestiture activity					$ (7,000,000)
Office Space Leases [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contractual Obligation			59,400,000
Office Space Leases [Member] | Mid Continent Divestiture 2015 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Contractual Obligation			$ 4,000,000

[1] During the third quarter of 2015, the Company vacated its office space in Tulsa, Oklahoma. These amounts include lease payments for the Tulsa office, net of sublease income. The Company expects to receive $3.5 million of sublease income as follows: $831,000 in 2016, $953,000 in 2017, $978,000 in 2018, and $741,000 in 2019.