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Supplemental Information on Oil and Gas Exploration and Production Activities (Unaudited) - Oil and Gas Reserves (Details) bbl in Thousands, Bcf in Thousands | 12 Months Ended
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Dec. 31, 2020 bbl Bcf | Dec. 31, 2019 bbl Bcf | Dec. 31, 2018 bbl Bcf
Proved Developed and Undeveloped Reserves [Roll Forward] |  |  |
Beginning of the period | Bcf | 4,528,000 | 4,743,000 | 5,395,000
Purchases of reserves | Bcf | 0 | 0 | 0
Extensions and discoveries | Bcf | 240,000 | 1,097,000 | 711,000
Sales of reserves | Bcf | (75,000) | (77,000) | (45,000)
Revisions of prior reserve estimates | Bcf | (1,725,000) | (734,000) | (821,000)
Current production | Bcf | (380,000) | (502,000) | (497,000)
End of period | Bcf | 2,588,000 | 4,528,000 | 4,743,000
Proved developed reserves (Volume) | Bcf | 1,527,000 | 1,984,000 | 2,115,000
Proved undeveloped reserves (Volume) | Bcf | 1,061,000 | 2,544,000 | 2,628,000
Oil |  |  |
Proved Developed and Undeveloped Reserves [Roll Forward] |  |  |
Beginning of the period | bbl | 18,000 | 21,000 | 19,000
Purchases of reserves | bbl | 0 | 0 | 0
Extensions and discoveries | bbl | 1,000 | 4,000 | 5,000
Sales of reserves | bbl | 0 | (2,000) | 0
Revisions of prior reserve estimates | bbl | (4,000) | (2,000) | 0
Current production | bbl | (2,000) | (2,000) | (3,000)
End of period | bbl | 13,000 | 18,000 | 21,000
Proved developed reserves (Volume) | bbl | 7,000 | 8,000 | 10,000
Proved undeveloped reserves (Volume) | bbl | 7,000 | 10,000 | 11,000
Natural Gas |  |  |
Proved Developed and Undeveloped Reserves [Roll Forward] |  |  |
Beginning of the period | Bcf | 4,048,000 | 4,134,000 | 4,825,000
Purchases of reserves | Bcf | 0 | 0 | 0
Extensions and discoveries | Bcf | 216,000 | 997,000 | 622,000
Sales of reserves | Bcf | (74,000) | (63,000) | (43,000)
Revisions of prior reserve estimates | Bcf | (1,564,000) | (562,000) | (827,000)
Current production | Bcf | (345,000) | (458,000) | (444,000)
End of period | Bcf | 2,281,000 | 4,048,000 | 4,134,000
Proved developed reserves (Volume) | Bcf | 1,358,000 | 1,757,000 | 1,813,000
Proved undeveloped reserves (Volume) | Bcf | 923,000 | 2,291,000 | 2,321,000
Natural Gas Liquids |  |  |
Proved Developed and Undeveloped Reserves [Roll Forward] |  |  |
Beginning of the period | bbl | 62,000 | 81,000 | 76,000
Purchases of reserves | bbl | 0 | 0 | 0
Extensions and discoveries | bbl | 3,000 | 13,000 | 10,000
Sales of reserves | bbl | 0 | 0 | 0
Revisions of prior reserve estimates | bbl | (23,000) | (27,000) | 1,000
Current production | bbl | (4,000) | (5,000) | (6,000)
End of period | bbl | 38,000 | 62,000 | 81,000
Proved developed reserves (Volume) | bbl | 22,000 | 30,000 | 41,000
Proved undeveloped reserves (Volume) | bbl | 16,000 | 32,000 | 40,000

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- Definition Proved Developed and Undeveloped Reserves [Roll Forward] + References No definition available. + Details
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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 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 + Details
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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 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61797-109447 + Details
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- Definition 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 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447 + Details
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- Definition 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 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447 + Details
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- Definition 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 -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 (a) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447 + Details
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- Definition Sales 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 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61831-109447 + Details
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- Definition 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 -Topic 932 -SubTopic 235 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61797-109447 + Details
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- Definition Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable 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 -Topic 932 -SubTopic 235 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61797-109447 + Details
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