Divestitures and Assets Held for Sale (Details) - USD ($) $ in Thousands		2 Months Ended	3 Months Ended			12 Months Ended
		Feb. 14, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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						$ 776,719	$ 946,062	$ 357,938
Net gain (loss) on divestiture activity						(131,028)	37,074	43,031
Income (loss) before income taxes						(343,813)	(1,201,916)	(722,861)
Impairment of proved properties						$ 3,806	354,614	468,679
Assets Held-for-sale
Probable period within which sale will take place (in years)						1 year
Oil and gas properties held for sale, net			$ 111,700			$ 111,700	372,621
Asset retirement obligations associated with oil and gas properties held for sale			11,369			11,369	26,241
Disposal group, disposed of by sale, not discontinued operations [Member] | Outside-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]					24,324	(218,506)	71,556
Impairment of proved properties							269,600
Disposal group, disposed of by sale, not discontinued operations [Member] | Other divestitures 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			$ 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, disposed of by sale, not discontinued operations [Member] | Mid Continent divestiture 2015 [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							310,300
Net gain (loss) on divestiture activity							$ 108,400
Disposal group, disposed of by sale, not discontinued operations [Member] | Permian divestiture 2015 [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								25,100
Net gain (loss) on divestiture activity								2,300
Disposal group, disposed of by sale, not discontinued operations [Member] | Other divestitures 2015 [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Write-down on disposal group, not discontinued operations								$ 68,600
Disposal group, held-for-sale, not discontinued operations [Member] | Divide County disposal group [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of proved properties						$ 526,500
Write-down on disposal group, not discontinued operations				$ 166,900	$ 359,600
Subsequent Event [Member] | Disposal group, held-for-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 proceeds from the sale of oil and gas properties		$ 500,000

[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 approximately $269.6 million of impairment of proved properties expense for the year ended December 31, 2016.