Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 779	$ 527	$ 410	$ 592	$ 745	$ 636	$ 667	$ 990	$ 2,308	$ 3,038	$ 3,862
Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									1,391	1,740	2,551
Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									917	1,298	1,311
Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									1,348	1,703	2,525
Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									2,145	2,850	3,745
Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating revenues									(1,185)	(1,515)	(2,408)
Intersegment Revenues | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									43	37	26
Intersegment Revenues | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									(1,228)	(1,552)	(2,434)
Gas sales
Disaggregation of Revenue [Line Items]
Operating revenues									967	1,241	1,998
Gas sales | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									928	1,207	1,974
Gas sales | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	0
Gas sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating revenues									39	34	24
Oil sales
Disaggregation of Revenue [Line Items]
Operating revenues									154	223	196
Oil sales | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									150	220	193
Oil sales | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	0
Oil sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating revenues									4	3	3
NGL sales
Disaggregation of Revenue [Line Items]
Operating revenues									265	274	352
NGL sales | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									265	274	353
NGL sales | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	0
NGL sales | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	(1)
Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									917	1,297	1,222
Marketing | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	0
Marketing | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									2,145	2,849	3,497
Marketing | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating revenues									(1,228)	(1,552)	(2,275)
Marketing | Intersegment Revenues | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									(1,200)	(1,600)	(2,300)
Gas gathering
Disaggregation of Revenue [Line Items]
Operating revenues									0	0	89
Gas gathering | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues											0
Gas gathering | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues											248
Gas gathering | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating revenues											(159)
Other
Disaggregation of Revenue [Line Items]
Operating revenues									5	3	5
Other | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating revenues									5	2	5
Other | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating revenues									0	1	0
Other | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating revenues									$ 0	$ 0	$ 0