Divestitures and Assets Held for Sale (Details) $ in Thousands			12 Months Ended
		Mar. 26, 2018 a	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from the sale of oil and gas properties			$ 13,059	$ 748,509	$ 776,719
Net gain (loss) on divestiture activity			862	426,917	(131,028)
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 proceeds from the sale of oil and gas properties				492,200
Net gain (loss) on divestiture activity				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]
Net proceeds from the sale of oil and gas properties				252,200
Net gain (loss) on divestiture activity				15,400
Disposal group, disposed of by sale, not discontinued operations [Member] | Divide County Divestiture 2018 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) before income taxes			$ 0	$ (28,975)	(468,786)	[1]
Write-down on disposal group					523,600
Disposal group, disposed of by sale, not discontinued operations [Member] | Non-Operated Eagle Ford Divestiture 2017 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from the sale of oil and gas properties					744,100
Net gain (loss) on divestiture activity					396,800
Income (loss) before income taxes	[1]				24,300
Disposal group, disposed of by sale, not discontinued operations [Member] | Rocky Mountain and Permian Divestitures [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from the sale of oil and gas properties					$ 36,200

[1] ⁽¹⁾  Loss before income taxes reflects oil, gas, and NGL production revenue, less oil, gas, and NGL production expense, depletion, depreciation, amortization, and asset retirement obligation liability accretion expense, impairment expense, and net loss on divestiture activity. During the year ended December 31, 2017, the Company recorded a write-down of $523.6 million on these assets.