Divestitures and Assets Held for Sale (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2018 USD ($) a	Dec. 31, 2017 USD ($) a	Dec. 31, 2016 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		$ 748,509	$ 776,719	$ 946,062
Net gain (loss) on divestiture activity		426,917	(131,028)	37,074
Impairment of proved properties		$ 0	3,806	354,614
Assets Held-for-sale
Probable period within which sale will take place (in years)		1 year
Properties held for sale, net		$ 5,280	111,700
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 and Halff East Divestiture 2018 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) before income taxes	[1]	(28,975)	(468,786)	(50,034)
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	[2]	0	24,324	(218,506)
Impairment of proved properties				269,600
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
Disposal group, disposed of by sale, not discontinued operations [Member] | Rocky Mountain divestiture 2016 [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				110,300
Net gain (loss) on divestiture activity				16,400
Disposal group, disposed of by sale, not discontinued operations [Member] | Raven/Bear Den divestiture 2016 [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				755,700
Net gain (loss) on divestiture activity				29,500
Disposal group, disposed of by sale, not discontinued operations [Member] | Permian divestiture 2016 [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				54,700
Net gain (loss) on divestiture activity				$ (10,000)
Disposal group, held-for-sale, not discontinued operations [Member]
Assets Held-for-sale
Properties held for sale, net		$ 5,280	$ 111,700
Net acres held for sale | a			112,000

[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 previously held for sale.

[2] (1) Income (loss) before income taxes reflects oil, gas, and NGL production revenue, less oil, gas, and NGL production expense, and depletion, depreciation, amortization, and asset retirement obligation liability accretion. Additionally, income (loss) before income taxes includes $269.6 million of impairment of proved properties expense for the year ended December 31, 2016.