Segment Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 920	$ 846
Depreciation, depletion and amortization expense	143	106
Operating income	255	266
Interest expense	39	32
Gain (loss) on derivatives	(7)	116
Other income (loss), net	(1)	1
Assets	7,713	7,196	$ 7,521
Capital investments	338	290
Cash and cash equivalents	958	1,382	$ 916	$ 1,423
Change in accrued expenditures	33	(52)
Marketing	253	253
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(613)	(575)
Marketing	(576)	(524)
E&P [Member]
Segment Reporting Information [Line Items]
Revenues	643	566
Depreciation, depletion and amortization expense	117	90
Operating income	238	225
Interest expense	39	32
Gain (loss) on derivatives	(7)	116
Other income (loss), net		2
Assets	5,346	4,413
Capital investments	334	283
E&P [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	6	3
Midstream [Member]
Segment Reporting Information [Line Items]
Revenues	277	280
Depreciation, depletion and amortization expense	26	16
Operating income	17	41
Other income (loss), net	(1)
Assets	1,217	1,268
Capital investments	4	6
Impairment of natural gas and oil properties	10
Marketing	829	777
Midstream [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	(619)	(578)
Marketing	576	524
Other [Member]
Segment Reporting Information [Line Items]
Other income (loss), net		(1)
Assets	1,150	1,515
Capital investments		1
Cash and cash equivalents	$ 958	$ 1,400