Revenue Recognition (Disaggregation of Revenue by Segment) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Operating Revenues	$ 745	$ 636	$ 667	$ 990	$ 1,175	$ 951	$ 816	$ 920	$ 3,038	$ 3,862	$ 3,203
Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues									1,740	2,551	2,105
Marketing
Disaggregation of Revenue [Line Items]
Operating Revenues									1,298	1,311	1,098
Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues									1,703	2,525	2,086
Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating Revenues									2,850	3,745	3,198
Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues									(1,515)	(2,408)	(2,081)
Intersegment Revenues | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues									37	26	19
Intersegment Revenues | Marketing
Disaggregation of Revenue [Line Items]
Operating Revenues									(1,552)	(2,434)	(2,100)
Gas sales
Disaggregation of Revenue [Line Items]
Operating Revenues									1,241	1,998	1,793
Gas sales | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues									1,207	1,974	1,775
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									34	24	18
Oil sales
Disaggregation of Revenue [Line Items]
Operating Revenues									223	196	102
Oil sales | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues									220	193	101
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									3	3	1
NGL sales
Disaggregation of Revenue [Line Items]
Operating Revenues									274	352	206
NGL sales | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues									274	353	206
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	(1)	0
Marketing
Disaggregation of Revenue [Line Items]
Operating Revenues									1,297	1,222	972
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									2,849	3,497	2,867
Marketing | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues									(1,552)	(2,275)	(1,895)
Marketing | Intersegment Revenues | Marketing
Disaggregation of Revenue [Line Items]
Operating Revenues									(1,600)	(2,300)	(1,900)
Gas gathering
Disaggregation of Revenue [Line Items]
Operating Revenues									0	89	126
Gas gathering | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues										0	0
Gas gathering | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating Revenues										248	331
Gas gathering | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues										(159)	(205)
Other
Disaggregation of Revenue [Line Items]
Operating Revenues									3	5	4
Other | Operating Segments | Exploration and Production
Disaggregation of Revenue [Line Items]
Operating Revenues									2	5	4
Other | Operating Segments | Marketing
Disaggregation of Revenue [Line Items]
Operating Revenues									1	0	0
Other | Intersegment Revenues
Disaggregation of Revenue [Line Items]
Operating Revenues									$ 0	$ 0	$ 0