Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 816	$ 811	$ 1,736	$ 1,657
E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	520	526	1,157	1,089
Operating Revenues	527	531	1,170	1,097
Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	797	822	1,693	1,680
Operating Revenues	289	280	566	560
Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(501)	(537)	(1,114)	(1,112)
Intersegment Eliminations [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	7	5	13	8
Intersegment Eliminations [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(508)	(542)	(1,127)	(1,120)
Gas Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	407	471	947	974
Gas Sales [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	400	466	935	966
Gas Sales [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	7	5	12	8
Oil Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	44	23	79	46
Oil Sales [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	44	23	78	46
Oil Sales [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues			1
NGL Sales [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	75	37	140	77
NGL Sales [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	75	37	140	77
Marketing [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	265	250	518	503
Marketing [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	728	740	1,557	1,517
Marketing [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(463)	(490)	(1,039)	(1,014)
Marketing [Member] | Intersegment Eliminations [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	463	490	1,039	1,014
Gas Gathering [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	24	30	48	57
Gas Gathering [Member] | Midstream [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	69	82	136	163
Gas Gathering [Member] | Intersegment Eliminations [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	(45)	$ (52)	(88)	$ (106)
Other [Member]
Disaggregation of Revenue [Line Items]
Operating Revenues	1		4
Other [Member] | E&P [Member]
Disaggregation of Revenue [Line Items]
Operating revenues, including intersegment revenue	$ 1		$ 4