Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 410	$ 667	$ 1,002	$ 1,657
E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	223	380	576	931
Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	187	287	426	726
Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	211	371	555	913
Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	389	626	937	1,567
Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	190	330	490	823
Intersegment Revenues | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	(12)	(9)	(21)	(18)
Intersegment Revenues | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	202	339	511	841
Gas sales
Disaggregation of Revenue [Line Items]
Total operating revenues	164	275	412	705
Gas sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	155	267	394	688
Gas sales | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0	0	0
Gas sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	(9)	(8)	(18)	(17)
Oil sales
Disaggregation of Revenue [Line Items]
Total operating revenues	19	47	71	86
Oil sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	16	46	68	85
Oil sales | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0	0	0
Oil sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	(3)	(1)	(3)	(1)
NGL sales
Disaggregation of Revenue [Line Items]
Total operating revenues	40	58	90	139
NGL sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	40	58	90	139
NGL sales | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0	0	0
NGL sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0	0	0
Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	187	287	426	725
Marketing | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	0	0	0	0
Marketing | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	389	626	937	1,566
Marketing | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	202	339	511	841
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 0	$ 0	3	2
Other | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues			3	1
Other | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues			0	1
Other | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues			$ 0	$ 0