Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 951	$ 737	$ 2,687	$ 2,394
E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	633	470	1,790	1,559
Operating Revenues	639	476	1,809	1,573
Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	912	734	2,605	2,414
Operating Revenues	312	261	878	821
Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(594)	(467)	(1,708)	(1,579)
Intersegment Eliminations [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	6	6	19	14
Intersegment Eliminations [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(600)	(473)	(1,727)	(1,593)
Gas Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	465	394	1,412	1,368
Gas Sales [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	460	388	1,395	1,354
Gas Sales [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	5	6	17	14
Oil Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	62	27	141	73
Oil Sales [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	61	27	139	73
Oil Sales [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	1		2
NGL Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	112	55	252	132
NGL Sales [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	112	55	252	132
Marketing [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	287	233	805	736
Marketing [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	846	656	2,403	2,173
Marketing [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(559)	(423)	(1,598)	(1,437)
Marketing [Member] | Intersegment Eliminations [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	559	423	1,598	1,437
Gas Gathering [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	25	28	73	85
Gas Gathering [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	66	78	202	241
Gas Gathering [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	$ (41)	$ (50)	(129)	$ (156)
Other [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues			4
Other [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue			$ 4