Supplemental Oil and Gas Disclosures (Summary of Changes in Reserves - United States) (Details) ft³ in Billions	12 Months Ended
Dec. 31, 2018 MBbls ft³	Dec. 31, 2017 MBbls ft³	Dec. 31, 2016 MBbls ft³
Reserve Quantities [Line Items]
Proved reserves, beginning of year	14,775	5,253	6,215
Revisions of previous estimates	526	2,332	(354)
Extensions, discoveries and other additions	1,009	8,087	282
Production	(946)	(897)	(875)
Disposition of reserves in place	(3,443)		(15)
Proved reserves, end of year	11,921	14,775	5,253
United States [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	14,775	5,253	6,215
Extensions, discoveries and other additions	1,009	8,087	282
Production	(946)	(897)	(875)
Disposition of reserves in place	(3,443)		(15)
Proved reserves, end of year	11,921	14,775	5,253
Proved developed reserves:
Proved developed reserves	5,557	7,920	5,176
Proved undeveloped reserves:
Proved undeveloped reserves	6,364	6,855	77
United States [Member] \| Natural Gas [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year	11,126	4,866	5,917
Extensions, discoveries and other additions	753	5,159	198
Production	(807)	(797)	(788)
Disposition of reserves in place	(3,440)		(15)
Proved reserves, end of year	8,044	11,126	4,866
Proved developed reserves:
Proved developed reserves	4,395	6,979	4,789
Proved undeveloped reserves:
Proved undeveloped reserves	3,649	4,147	77
United States [Member] \| Oil [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year \| MBbls	65,636	10,523	8,753
Extensions, discoveries and other additions \| MBbls	5,830	55,772	2,417
Production \| MBbls	(3,407)	(2,327)	(2,192)
Disposition of reserves in place \| MBbls	(250)		(19)
Proved reserves, end of year \| MBbls	69,007	65,636	10,523
Proved developed reserves:
Proved developed reserves \| MBbls	18,037	14,513	10,523
Proved undeveloped reserves:
Proved undeveloped reserves \| MBbls	50,970	51,123
United States [Member] \| NGL [Member]
Reserve Quantities [Line Items]
Proved reserves, beginning of year \| MBbls	542,455	53,931	40,947
Extensions, discoveries and other additions \| MBbls	36,823	432,220	11,576
Production \| MBbls	(19,706)	(14,245)	(12,372)
Disposition of reserves in place \| MBbls	(276)		(14)
Proved reserves, end of year \| MBbls	577,063	542,455	53,931
Proved developed reserves:
Proved developed reserves \| MBbls	175,480	142,213	53,931
Proved undeveloped reserves:
Proved undeveloped reserves \| MBbls	401,583	400,242
Price Revision [Member] \| United States [Member]
Reserve Quantities [Line Items]
Revisions of previous estimates	154	1,691	(1,037)
Price Revision [Member] \| United States [Member] \| Natural Gas [Member]
Reserve Quantities [Line Items]
Revisions of previous estimates	96	1,327	(983)
Price Revision [Member] \| United States [Member] \| Oil [Member]
Reserve Quantities [Line Items]
Revisions of previous estimates \| MBbls	788	3,197	(582)
Price Revision [Member] \| United States [Member] \| NGL [Member]
Reserve Quantities [Line Items]
Revisions of previous estimates \| MBbls	8,912	57,447	(8,337)
Other than Price [Member] \| United States [Member]
Reserve Quantities [Line Items]
Revisions of previous estimates	372	641	683
Other than Price [Member] \| United States [Member] \| Natural Gas [Member]
Reserve Quantities [Line Items]
Revisions of previous estimates	316	571	537
Other than Price [Member] \| United States [Member] \| Oil [Member]
Reserve Quantities [Line Items]
Revisions of previous estimates \| MBbls	410	(1,529)	2,146
Other than Price [Member] \| United States [Member] \| NGL [Member]
Reserve Quantities [Line Items]
Revisions of previous estimates \| MBbls	8,855	13,102	22,131

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.

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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447

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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.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
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-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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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.

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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
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-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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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
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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+ References

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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
-Topic 932
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