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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

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