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Divestitures and Assets Held for Sale (Details) $ in Thousands |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 26, 2018 a | Mar. 31, 2019 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Net divestiture proceeds |  | $ 6,114 | $ 490,780 |
Net gain on divestiture activity |  | $ 61 | 385,369 |
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | 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 divestiture proceeds |  |  | 490,800 | $ 492,200
Net gain on divestiture activity |  |  | 409,200 | $ 410,600
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Divide County Divestiture and Halff East Divestiture 2018 [Member] |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |
Write-down of disposal group |  |  | $ 24,100 |

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