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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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Net Acreage Divested

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Amount of gain (loss) on sale or disposal of proven and unproven oil and gas properties.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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Amount before tax of income (loss) from an individually significant component disposed of or held-for-sale attributable to parent. Excludes discontinued operations.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
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The cash inflow to dispose of long-lived, physical assets and mineral interests in oil and gas properties used for normal oil and gas operations.

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