Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 990	$ 920
E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	551	643
Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	439	277
Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	542	637
Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	941	896
Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	493	613
Intersegment Revenues | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	(9)	(6)
Intersegment Revenues | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	502	619
Gas sales
Disaggregation of Revenue [Line Items]
Total operating revenues	430	540
Gas sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	421	535
Gas sales | Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Gas sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	(9)	(5)
Oil sales
Disaggregation of Revenue [Line Items]
Total operating revenues	39	35
Oil sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	39	34
Oil sales | Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Oil sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	(1)
NGL sales
Disaggregation of Revenue [Line Items]
Total operating revenues	81	65
NGL sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	81	65
NGL sales | Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
NGL sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	438	253
Marketing | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0
Marketing | Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	940	829
Marketing | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	502	576
Marketing | Intersegment Revenues | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	502	576
Gas gathering
Disaggregation of Revenue [Line Items]
Total operating revenues	0	24
Gas gathering | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues		0
Gas gathering | Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues		67
Gas gathering | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues		43
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	2	3
Other | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	1	3
Other | Operating Segments | Midstream
Disaggregation of Revenue [Line Items]
Total operating revenues	1	0
Other | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 0	$ 0