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Supplementary Oil And Gas Information (Unaudited) - Proved and Proved Developed Natural Gas Reserve Quantities (Details) - Natural Gas - Bcf	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Proved Developed And Undeveloped Reserves [Roll Forward]
Proved reserves, net, beginning balance	4,782	4,354	5,240
Extensions and discoveries (in Bcf)	173	106	90
Improved recovery (in Bcf)	159	202	414
Purchases of reserves in place (in Bcf)	2,615	34	67
Sales of reserves in place (in Bcf)	(4)	0	(3)
Production (in Bcf)	(524)	(528)	(542)
Economic revisions due to prices (in Bcf)	100	248	(748)
Revisions of prior estimates (in Bcf)	399	367	(164)
Proved reserves, net, ending balance	7,701	4,782	4,354
Net proved developed reserves (in Bcf)	3,144	2,381	2,417	3,112
North America
Proved Developed And Undeveloped Reserves [Roll Forward]
Proved reserves, net, beginning balance	4,728	4,306	5,199
Extensions and discoveries (in Bcf)	173	106	90
Improved recovery (in Bcf)	159	202	414
Purchases of reserves in place (in Bcf)	2,614	34	67
Sales of reserves in place (in Bcf)	(4)	0	(3)
Production (in Bcf)	(515)	(511)	(523)
Economic revisions due to prices (in Bcf)	97	246	(746)
Revisions of prior estimates (in Bcf)	402	346	(192)
Proved reserves, net, ending balance	7,655	4,728	4,306
Net proved developed reserves (in Bcf)	3,116	2,342	2,382	3,081
North Sea
Proved Developed And Undeveloped Reserves [Roll Forward]
Proved reserves, net, beginning balance	16	27	25
Extensions and discoveries (in Bcf)	0	0	0
Improved recovery (in Bcf)	0	0	0
Purchases of reserves in place (in Bcf)	0	0	0
Sales of reserves in place (in Bcf)	0	0	0
Production (in Bcf)	(4)	(9)	(11)
Economic revisions due to prices (in Bcf)	0	0	0
Revisions of prior estimates (in Bcf)	0	(2)	13
Proved reserves, net, ending balance	12	16	27
Net proved developed reserves (in Bcf)	6	11	23	22
Offshore Africa
Proved Developed And Undeveloped Reserves [Roll Forward]
Proved reserves, net, beginning balance	38	21	16
Extensions and discoveries (in Bcf)	0	0	0
Improved recovery (in Bcf)	0	0	0
Purchases of reserves in place (in Bcf)	0	0	0
Sales of reserves in place (in Bcf)	0	0	0
Production (in Bcf)	(5)	(8)	(8)
Economic revisions due to prices (in Bcf)	4	2	(2)
Revisions of prior estimates (in Bcf)	(3)	23	15
Proved reserves, net, ending balance	34	38	21
Net proved developed reserves (in Bcf)	22	28	12	9

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- Definition

Proved Developed And Undeveloped Reserves1 [Roll Forward]

+ References

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Proved Developed And Undeveloped Reserves, Economic Revisions Due To Prices

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- Definition

Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

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- Definition

Changes in proved reserve estimates resulting from application of improved recovery techniques.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

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- Definition

The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

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Production of proved reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

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Purchase of minerals in place.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

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Revisions represent changes in previous estimates of proved reserves, either upward or downward, resulting from new information (except for an increase in proved acreage) normally obtained from development drilling and production history or resulting from change in economic factors.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
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-Section Item 18
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Sales of minerals in place.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
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Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Form 20-F
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