Acquisitions, Divestitures, and Assets Held for Sale Divestitures (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2015	Jun. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of oil and gas properties			$ 334,988	$ 46,821
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down	$ 66,000		99,900
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			107,800
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down		$ 30,000
Total Expected Exit and Disposal Costs			10,000
Business Exit Costs	$ 5,000		8,500
Mid Continent Divestiture 2015 [Member] | Office Space Leases [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Purchase Commitment, Amount			$ 7,000