Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Gas sales	$ 540	$ 503
Oil sales	35	23
NGL sales	65	40
Marketing	253	253
Gas gathering	24	27
Other	3
Total Operating Revenues	920	846
E&P [Member]
Disaggregation of Revenue [Line Items]
Gas sales	535	500
Oil sales	34	23
NGL sales	65	40
Other	3
Total	637	563
Total Operating Revenues	643	566
Midstream [Member]
Disaggregation of Revenue [Line Items]
Marketing	829	777
Gas gathering	67	81
Total	896	858
Total Operating Revenues	277	280
Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Gas sales	5	3
Oil sales	1
Marketing	(576)	(524)
Gas gathering	(43)	(54)
Total Operating Revenues	(613)	(575)
Intersegment Eliminations [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Total Operating Revenues	6	3
Intersegment Eliminations [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Marketing	576	524
Total Operating Revenues	$ (619)	$ (578)