Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			6 Months Ended
	Sep. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		$ 816		$ 811	$ 1,736		$ 1,657
Marketing purchases		265		253	520		504
Operating expenses		193		164	382		311
General and administrative expenses		59		58	114		108
Restructuring charges		18			18
Depreciation, depletion and amortization		142		123	285		229
Taxes, other than income taxes		15		25	38		51
Total Operating Costs and Expenses		692		623	1,357		1,203
Operating Income		124		188	379		454
Interest Expense, Net		32		34	71		66
Gain (Loss) on Derivatives		(36)		134	(43)		250
Loss on Early Extinguishment of Debt	$ (59)	(8)		(10)	(8)		(11)
Other Income, Net		3		6	2		8
Income (Loss) Before Income Taxes		51		284	259		635
Provision (Benefit) for Income Taxes
Net Income		51	[1]	284	259	[1]	635
Mandatory convertible preferred stock dividend				27			54
Participating securities - mandatory convertible preferred stock				33	2		76
Net Income Attributable to Common Stock		51		224	257		505
Other comprehensive income				1			1
Comprehensive income		51	[1]	285	259	[1]	636
Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Equity in Earnings of Subsidiaries		(91)		(328)	(338)		(712)
Income (Loss) Before Income Taxes		(91)		(328)	(338)		(712)
Provision (Benefit) for Income Taxes
Net Income		(91)		(328)	(338)		(712)
Mandatory convertible preferred stock dividend
Participating securities - mandatory convertible preferred stock
Net Income Attributable to Common Stock		(91)		(328)	(338)		(712)
Comprehensive income		(91)		(328)	(338)		(712)
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Interest Expense, Net		32		34	71		66
Loss on Early Extinguishment of Debt		(8)		(10)	(8)		(11)
Equity in Earnings of Subsidiaries		91		328	338		712
Income (Loss) Before Income Taxes		51		284	259		635
Provision (Benefit) for Income Taxes
Net Income		51		284	259		635
Mandatory convertible preferred stock dividend				27			54
Participating securities - mandatory convertible preferred stock				33	2		76
Net Income Attributable to Common Stock		51		224	257		505
Other comprehensive income				1			1
Comprehensive income		51		285	259		636
Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		816		811	1,736		1,657
Marketing purchases		265		253	520		504
Operating expenses		193		164	382		311
General and administrative expenses		59		58	114		108
Restructuring charges		18			18
Depreciation, depletion and amortization		142		123	285		229
Taxes, other than income taxes		15		25	38		51
Total Operating Costs and Expenses		692		623	1,357		1,203
Operating Income		124		188	379		454
Gain (Loss) on Derivatives		(36)		134	(43)		250
Other Income, Net		3		6	2		8
Income (Loss) Before Income Taxes		91		328	338		712
Provision (Benefit) for Income Taxes
Net Income		91		328	338		712
Mandatory convertible preferred stock dividend
Participating securities - mandatory convertible preferred stock
Net Income Attributable to Common Stock		91		328	338		712
Comprehensive income		91		328	338		712
Non-Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Provision (Benefit) for Income Taxes
Mandatory convertible preferred stock dividend
Participating securities - mandatory convertible preferred stock
Gas Sales [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		407		471	947		974
Gas Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		407		471	947		974
Oil Sales [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		44		23	79		46
Oil Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		44		23	79		46
NGL Sales [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		75		37	140		77
NGL Sales [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		75		37	140		77
Marketing [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		265		250	518		503
Marketing [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		265		250	518		503
Gas Gathering [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		24		30	48		57
Gas Gathering [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		24		$ 30	48		$ 57
Other [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		1			4
Other [Member] | Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Revenues from external customers		$ 1			$ 4

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