Segment Information (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
Segment Reporting Information [Line Items]
Revenues from external customers	$ 779	$ 527	$ 410	$ 592	$ 745	$ 636	$ 667	$ 990	$ 2,308	$ 3,038	$ 3,862
Depreciation, depletion and amortization expense									357	471	560
Impairments									2,830	16	171
Operating loss	(244)	$ (381)	$ (756)	$ (1,490)	64	$ (29)	$ 22	$ 213	(2,871)	270	797
Interest expense									94	65	124
Gain (loss) on derivatives									224	274	(118)
Gain (Loss) on Early Extinguishment of Debt									35	8	(17)
Other income, net									1	(7)	0
Provision (benefit) for income taxes									407	(411)	1
Assets	5,160				6,717				5,160	6,717	5,797
Capital investments									899	1,140	1,248
Restructuring charges									16	11	39
Montage merger-related expenses									41	0	0
Increase (decrease) in accrued expenditures between periods									(3)	34	(53)
Marketing
Segment Reporting Information [Line Items]
Revenues from external customers									917	1,297	1,222
Exploration and Production
Segment Reporting Information [Line Items]
Revenues from external customers									1,391	1,740	2,551
Depreciation, depletion and amortization expense											514
Montage merger-related expenses									41
Marketing
Segment Reporting Information [Line Items]
Revenues from external customers									917	1,298	1,311
Depreciation, depletion and amortization expense											46
Intersegment Revenues
Segment Reporting Information [Line Items]
Revenues from external customers									(1,185)	(1,515)	(2,408)
Intersegment Revenues | Marketing
Segment Reporting Information [Line Items]
Revenues from external customers									(1,228)	(1,552)	(2,275)
Intersegment Revenues | Exploration and Production
Segment Reporting Information [Line Items]
Revenues from external customers									43	37	26
Intersegment Revenues | Marketing
Segment Reporting Information [Line Items]
Revenues from external customers									(1,228)	(1,552)	(2,434)
Intersegment Revenues | Marketing | Marketing
Segment Reporting Information [Line Items]
Revenues from external customers									(1,200)	(1,600)	(2,300)
Operating Segments | Exploration and Production
Segment Reporting Information [Line Items]
Revenues from external customers									1,348	1,703	2,525
Depreciation, depletion and amortization expense									348	462
Impairments									2,830	13	15
Operating loss									(2,864)	283	794
Interest expense									94	65	124
Gain (loss) on derivatives									224	274	(118)
Gain (Loss) on Early Extinguishment of Debt									0	0	0
Other income, net									0	(9)	2
Provision (benefit) for income taxes									407	(411)	1
Assets	4,654				6,235				4,654	6,235	4,872
Capital investments									899	1,138	1,231
Restructuring charges									16	11	37
Operating Segments | Exploration and Production | Marketing
Segment Reporting Information [Line Items]
Revenues from external customers									0	0	0
Operating Segments | Marketing
Segment Reporting Information [Line Items]
Revenues from external customers									2,145	2,850	3,745
Depreciation, depletion and amortization expense									9	9
Impairments									0	3	155
Operating loss									(7)	(13)	4
Interest expense									0	0	0
Gain (loss) on derivatives									0	0	0
Gain (Loss) on Early Extinguishment of Debt									0	0	0
Other income, net									0	0	(2)
Provision (benefit) for income taxes									0	0	0
Assets	381				314				381	314	539
Capital investments									0	0	9
Restructuring charges											2
Operating Segments | Marketing | Non Core Gathering Assets
Segment Reporting Information [Line Items]
Impairments											10
Operating Segments | Marketing | Marketing
Segment Reporting Information [Line Items]
Revenues from external customers									2,145	2,849	3,497
Other
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization expense									0	0	0
Impairments									0	0	1
Operating loss									0	0	(1)
Interest expense									0	0	0
Gain (loss) on derivatives									0	0	0
Gain (Loss) on Early Extinguishment of Debt									35	8	(17)
Other income, net									1	2	0
Provision (benefit) for income taxes									0	0	0
Assets	$ 125				$ 168				125	168	386
Capital investments									$ 0	$ 2	$ 8