Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) - USD ($) $ in Millions		3 Months Ended												12 Months Ended
	Apr. 26, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Condensed Income Statements, Captions [Line Items]
Operating Revenues		$ 1,175	$ 951	$ 816	$ 920	$ 809	$ 737	$ 811	$ 846					$ 3,862	$ 3,203		$ 2,436
Operating expenses														785	671		592
General and administrative expenses														209	233		247
Restructuring charges														39			73
Depreciation, depletion and amortization														560	504		436
Impairments														171			2,321
Gain on sale of assets, net														(17)	(6)
Taxes, other than income taxes														89	94		93
Total Operating Costs and Expenses														3,065	2,472		4,626
Operating Income (Loss)		352	66	124	255	167	110	188	266					797	731		(2,190)
Interest Expense, Net														124	135		88
Gain (Loss) on Derivatives														(118)	422		(339)
Loss on Early Extinguishment of Debt	$ (8)													(17)	(70)		(51)
Other Income (Loss), Net															5		(4)
Income (Loss) Before Income Taxes														538	953		(2,672)
Provision (benefit) for income taxes														1	(93)		(29)
Net Income (Loss)		307	(29)	51	208	334	77	284	351	$ (210)	$ (708)	$ (593)	$ (1,132)	537	1,046	[1]	(2,643)
Mandatory convertible preferred stock dividend															108		108
Participating securities - mandatory convertible preferred stock														2	123
Net Income (Loss) Attributable to Common Stock		307	(29)	51	205	267	43	224	281					535	815		(2,751)
Other comprehensive income (loss)		4	4			(7)	1	1			2	3	4	8	(5)		9
Comprehensive income (loss)		$ 311	$ (25)	$ 51	$ 208	$ 327	$ 78	$ 285	$ 351	$ (210)	$ (706)	$ (590)	$ (1,128)	545	1,041	[1]	(2,634)
Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Equity in Earnings of Subsidiaries														(678)	(1,251)		2,559
Income (Loss) Before Income Taxes														(678)	(1,251)		2,559
Net Income (Loss)														(678)	(1,251)		2,559
Net Income (Loss) Attributable to Common Stock														(678)	(1,251)		2,559
Other comprehensive income (loss)															(12)		(6)
Comprehensive income (loss)														(678)	(1,263)		2,553
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Interest Expense, Net														124	135		88
Loss on Early Extinguishment of Debt														(17)	(70)		(51)
Equity in Earnings of Subsidiaries														678	1,251		(2,504)
Income (Loss) Before Income Taxes														537	1,046		(2,643)
Net Income (Loss)														537	1,046		(2,643)
Mandatory convertible preferred stock dividend															108		108
Participating securities - mandatory convertible preferred stock														2	123
Net Income (Loss) Attributable to Common Stock														535	815		(2,751)
Other comprehensive income (loss)														8	(5)		9
Comprehensive income (loss)														545	1,041		(2,634)
Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														3,862	3,203		2,436
Operating expenses														785	671		592
General and administrative expenses														209	233		247
Restructuring charges														39			73
Depreciation, depletion and amortization														560	504		436
Impairments														171			2,266
Gain on sale of assets, net														(17)	(6)
Taxes, other than income taxes														89	94		93
Total Operating Costs and Expenses														3,065	2,472		4,571
Operating Income (Loss)														797	731		(2,135)
Gain (Loss) on Derivatives														(118)	422		(339)
Other Income (Loss), Net															5		(4)
Equity in Earnings of Subsidiaries																	(55)
Income (Loss) Before Income Taxes														679	1,158		(2,533)
Provision (benefit) for income taxes														1	(93)		(29)
Net Income (Loss)														678	1,251		(2,504)
Net Income (Loss) Attributable to Common Stock														678	1,251		(2,504)
Other comprehensive income (loss)															6		3
Comprehensive income (loss)														678	1,257		(2,501)
Non-Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Impairments																	55
Total Operating Costs and Expenses																	55
Operating Income (Loss)																	(55)
Income (Loss) Before Income Taxes																	(55)
Net Income (Loss)																	(55)
Net Income (Loss) Attributable to Common Stock																	(55)
Other comprehensive income (loss)															6		3
Comprehensive income (loss)															6		(52)
Gas Sales [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														1,998	1,793		1,273
Gas Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														1,998	1,793		1,273
Oil Sales [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														196	102		69
Oil Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														196	102		69
NGL Sales [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														352	206		92
NGL Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														352	206		92
Marketing [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														1,222	972		864
Marketing [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														1,222	972		864
Gas Gathering [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														89	126		138
Gas Gathering [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														89	126		138
Other [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														5	4
Other [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues														5	4
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
Condensed Income Statements, Captions [Line Items]
Marketing purchases														1,229	976		864
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Marketing purchases														$ 1,229	$ 976		$ 864

[1] In 2018 and 2017, deferred tax activity incurred in other comprehensive income was offset by a valuation allowance.