Disclosures about oil and natural gas activities (unaudited) (Summary of Changes in Quantities of Proved Reserves) (Details)	12 Months Ended
Dec. 31, 2019 MBoe MBbls MMcf	Dec. 31, 2018 MBoe MBbls MMcf	Dec. 31, 2017 MBoe MBbls MMcf	Dec. 31, 2016 MBoe MBbls MMcf
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]
Balance at beginning of period \| MBoe	94,807	76,287	131,301
Sales of minerals in place \| MBoe	0	(6,160)	(75,241)
Extensions and discoveries \| MBoe	21,271	27,406	21,040
Revisions \| MBoe	(9,927)	4,764	7,586
Production \| MBoe	(9,593)	(7,490)	(8,399)
Balance at end of the period \| MBoe	96,558	94,807	76,287
Proved developed reserves \| MBoe	64,761	55,468	50,734	56,076
Proved undeveloped reserves \| MBoe	31,797	39,339	25,553	75,225
Oil
Proved Developed and Undeveloped Reserves [Roll Forward]
Balance at beginning of period	32,297	29,604	96,621
Sales of minerals in place	0	(2,422)	(74,918)
Extensions and discoveries	4,766	6,545	8,957
Revisions	(6,703)	1,254	3,515
Production	(3,111)	(2,684)	(4,571)
Balance at end of period	27,249	32,297	29,604
Proved developed reserves	18,447	18,051	18,301	28,590
Proved undeveloped reserves	8,802	14,246	11,303	68,031
Natural gas
Proved Developed and Undeveloped Reserves [Roll Forward]
Balance at beginning of period \| MMcf	220,218	170,166	135,449
Sales of minerals in place \| MMcf	0	(14,184)	(1,663)
Extensions and discoveries \| MMcf	48,967	69,189	39,843
Revisions \| MMcf	(26,340)	12,596	11,135
Production \| MMcf	(22,095)	(17,549)	(14,598)
Balance at end of period \| MMcf	220,750	220,218	170,166
Proved developed reserves \| MMcf	152,187	135,425	123,451	108,800
Proved undeveloped reserves \| MMcf	68,563	84,793	46,715	26,649
Natural gas liquids
Proved Developed and Undeveloped Reserves [Roll Forward]
Balance at beginning of period	25,807	18,322	12,105
Sales of minerals in place	0	(1,374)	(46)
Extensions and discoveries	8,343	9,329	5,442
Revisions	1,166	1,411	2,216
Production	(2,799)	(1,881)	(1,395)
Balance at end of period	32,517	25,807	18,322
Proved developed reserves	20,949	14,846	11,858	9,352
Proved undeveloped reserves	11,568	10,961	6,464	2,753

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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Energy measure of increase in quantity of proved reserve of oil and gas resulting from the extension of the proved acreage of previously discovered reservoir or discovery of new field with proved reserve or of new reservoir of proved reserve in old field, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-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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Energy measure of proved reserve of oil and gas, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ 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

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Energy measure of decrease in quantity of proved reserve of oil and gas resulting from production, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 5
-Subparagraph (e)
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Energy measure of increase (decrease) in quantity of proved reserve of oil and gas related to revision of a previous estimate resulting from new information, except for an increase in proved acreage, or change in economic factors, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
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-Paragraph 5
-Subparagraph (a)
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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
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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.

+ References

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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

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.

+ 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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-SubTopic 235
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Energy measure of decrease in quantity of proved reserve of oil and gas resulting from sale of mineral in place, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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-SubTopic 235
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-Paragraph 5
-Subparagraph (f)
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Sales of minerals in place.

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-Name Form 20-F
-Section Item 18
-Subsection Instruction 2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Energy measure of interests in proved developed reserves of 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.

+ References

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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

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Energy measure of proved undeveloped reserves of 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.

+ References

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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

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