Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 527	$ 636	$ 1,529	$ 2,293
E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	308	357	884	1,288
Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	219	279	645	1,005
Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	298	348	853	1,261
Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	495	592	1,432	2,159
Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	266	304	756	1,127
Intersegment Revenues | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	(10)	(9)	(31)	(27)
Intersegment Revenues | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	276	313	787	1,154
Gas sales
Disaggregation of Revenue [Line Items]
Total operating revenues	199	238	611	943
Gas sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	190	230	584	918
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)	(27)	(25)
Oil sales
Disaggregation of Revenue [Line Items]
Total operating revenues	40	67	111	153
Oil sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	39	66	107	151
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	(1)	(1)	(4)	(2)
NGL sales
Disaggregation of Revenue [Line Items]
Total operating revenues	68	52	158	191
NGL sales | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	68	52	158	191
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	219	279	645	1,004
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	495	592	1,432	2,158
Marketing | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	276	313	787	1,154
Other
Disaggregation of Revenue [Line Items]
Total operating revenues	1	$ 0	4	2
Other | Operating Segments | E&P
Disaggregation of Revenue [Line Items]
Total operating revenues	1		4	1
Other | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Total operating revenues	0		0	1
Other | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 0		$ 0	$ 0