Segment Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues from external customers		$ 951	$ 737			$ 2,687	$ 2,394
Depreciation, depletion and amortization expense		151	135			436	364
Impairments		161				161
Operating income		66	110			445	564
Interest expense		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
Benefit for income taxes			(14)				(14)
Assets	7,202	7,058	7,202			7,058	7,202	$ 7,521
Capital investments		298	331			1,039	946
Restructuring charges		2				20
Cash and cash equivalents	989	9	989			9	989	$ 916	$ 1,423
Change in accrued expenditures		(31)	(2)			21	(13)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(594)	(467)			(1,708)	(1,579)
E&P [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		639	476			1,809	1,573
Depreciation, depletion and amortization expense		140	120			383	317
Impairments		15				15
Operating income		175	64			510	435
Interest expense		29	31			100	97
Gain (loss) on derivatives		(65)	45			(108)	295
Other income (loss), net			1			3	5
Benefit for income taxes			(14)				(14)
Assets	4,842	5,732	4,842			5,732	4,842
Capital investments		295	320			1,025	921
Restructuring charges		2				18
Assets held for sale		106				106
E&P [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		6	6			19	14
Midstream [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		312	261			878	821
Depreciation, depletion and amortization expense		11	15			53	47
Impairments		145				145
Operating income		(108)	46			(64)	129
Other income (loss), net		(1)	(3)			(2)	1
Assets	1,240	1,115	1,240			1,115	1,240
Capital investments			9			9	21
Restructuring charges						2
Assets held for sale		738				738
Cash and cash equivalents		2				2
Midstream [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(600)	(473)			(1,727)	(1,593)
Other [Member]
Segment Reporting Information [Line Items]
Impairments		1				1
Operating income		(1)				(1)
Loss on early extinguishment of debt			(59)			(8)	(70)
Assets	1,120	211	1,120			211	1,120
Capital investments		3	2			5	4
Cash and cash equivalents	$ 989	9	989			9	989
Marketing [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		287	233			805	736
Marketing [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(559)	(423)			(1,598)	(1,437)
Marketing [Member] | Midstream [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		$ 559	$ 423			1,598	$ 1,437
Non-Core Gathering Assets [Member] | Midstream [Member]
Segment Reporting Information [Line Items]
Impairments						$ 10