Acquisitions, Divestitures, and Assets Held for Sale Divestitures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of oil and gas properties			$ 335,103	$ 41,868
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down			98,600
Mid Continent Divestiture 2015 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of oil and gas properties			316,500
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			108,400
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down		$ 30,000
Business Exit Costs	$ 1,000		9,500
Mid Continent Divestiture 2015 [Member] | Mid Continent Lease Obligation net of Expected Income [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Mid Continent Lease Obligation net of Expected Income			$ 5,800
Scenario, Forecast [Member] | Mid Continent Divestiture 2015 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Exit Costs					$ 10,000