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Divestitures and Assets Held for Sale (Details) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Mar. 26, 2018 a | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Net gain on divestiture activity |  | $ 91 | $ 862 | $ 426,917
Net proceeds from the sale of oil and gas properties |  | $ 92 | $ 13,059 | 748,509
Disposal group, disposed of by sale, not discontinued operations | PRB Divestiture 2018 [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Net Acreage Divested | a | 112,000 |  |  |
Net gain on divestiture activity |  |  |  | 410,600
Net proceeds from the sale of oil and gas properties |  |  |  | 492,200
Disposal group, disposed of by sale, not discontinued operations | Divide County Divestiture and Halff East Divestiture 2018 [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Net gain on divestiture activity |  |  |  | 15,400
Net proceeds from the sale of oil and gas properties |  |  |  | 252,200
Disposal group, disposed of by sale, not discontinued operations | Divide County Divestiture 2018 [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Loss before income taxes |  |  |  | $ (29,000)

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