Assets Held for Sale and Divestitures Assets Held for Sale and Divestitures (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
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	$ (49,500)	$ 68,300		$ 30,000
Proceeds from Sale of Oil and Gas Property and Equipment					$ 12,967	$ 334,988
Asset retirement obligation associated with oil and gas properties held for sale	32,055				32,055		$ 241
Disposal Group, Including Discontinued Operation, Assets, Current	$ 173,001				$ 173,001		$ 641
Assets Held-for-sale Reasonably Certain Period for Sale					1 year
Mid Continent Divestiture 2015 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal						107,800
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down			$ 66,000
Proceeds from Sale of Oil and Gas Property and Equipment						316,500
Business Exit Costs			$ 5,000			$ 8,500