Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended						6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		$ 951			$ 737				$ 2,687	$ 2,394
Operating expenses		206			170				588	481
General and administrative expenses		51			62				165	170
Restructuring charges		2							20
Depreciation, depletion and amortization		151			135				436	364
Impairments		161							161
Taxes, other than income taxes		26			24				64	75
Total Operating Costs and Expenses		885			627				2,242	1,830
Operating Income		66			110				445	564
Interest Expense, Net		29			31				100	97
Gain (Loss) on Derivatives		(65)			45				(108)	295
Loss on Early Extinguishment of Debt	$ (59)				(59)	$ (10)		$ (11)	(8)	(70)
Other Income (Loss), Net		(1)			(2)				1	6
Income (Loss) Before Income Taxes		(29)			63				230	698
Provision (Benefit) for Income Taxes					(14)					(14)
Net Income (Loss)		(29)	$ 51	$ 208	77	284	$ 351		230	712
Mandatory convertible preferred stock dividend					27					81
Participating securities - mandatory convertible preferred stock					7				1	83
Net Income (Loss) Attributable to Common Stock		(29)			43				229	548
Other comprehensive income		4			1	1			4	2
Comprehensive income		(25)	$ 51	$ 208	78	$ 285	$ 351		234	714
Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Equity in Earnings of Subsidiaries					(167)				(338)	(879)
Income (Loss) Before Income Taxes					(167)				(338)	(879)
Net Income (Loss)					(167)				(338)	(879)
Net Income (Loss) Attributable to Common Stock					(167)				(338)	(879)
Comprehensive income					(167)				(338)	(879)
Parent Company [Member]
Condensed Income Statements, Captions [Line Items]
Interest Expense, Net		29			31				100	97
Loss on Early Extinguishment of Debt					(59)				(8)	(70)
Equity in Earnings of Subsidiaries					167				338	879
Income (Loss) Before Income Taxes		(29)			77				230	712
Net Income (Loss)		(29)			77				230	712
Mandatory convertible preferred stock dividend					27					81
Participating securities - mandatory convertible preferred stock					7				1	83
Net Income (Loss) Attributable to Common Stock		(29)			43				229	548
Other comprehensive income		4			1				4	2
Comprehensive income		(25)			78				234	714
Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		951			737				2,687	2,394
Operating expenses		206			170				588	481
General and administrative expenses		51			62				165	170
Restructuring charges		2							20
Depreciation, depletion and amortization		151			135				436	364
Impairments		161							161
Taxes, other than income taxes		26			24				64	75
Total Operating Costs and Expenses		885			627				2,242	1,830
Operating Income		66			110				445	564
Gain (Loss) on Derivatives		(65)			45				(108)	295
Other Income (Loss), Net		(1)			(2)				1	6
Income (Loss) Before Income Taxes					153				338	865
Provision (Benefit) for Income Taxes					(14)					(14)
Net Income (Loss)					167				338	879
Net Income (Loss) Attributable to Common Stock					167				338	879
Comprehensive income					167				338	879
Gas Sales [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		465			394				1,412	1,368
Gas Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		465			394				1,412	1,368
Oil Sales [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		62			27				141	73
Oil Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		62			27				141	73
NGL Sales [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		112			55				252	132
NGL Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		112			55				252	132
Marketing [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		287			233				805	736
Marketing [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		287			233				805	736
Gas Gathering [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		25			28				73	85
Gas Gathering [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		25			28				73	85
Other [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers									4
Other [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers									4
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
Condensed Income Statements, Captions [Line Items]
Marketing purchases		288			236				808	740
Natural Gas, Gathering, Transportation, Marketing and Processing [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Marketing purchases		$ 288			$ 236				$ 808	$ 740