Disclosures about oil and natural gas activities (unaudited) (Standardized Measure of Discounted Future Net Cash Flows) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash flows	$ 2,424,620	$ 3,255,771	$ 2,331,940
Future production costs	(1,040,314)	(1,187,071)	(899,380)
Future development and abandonment costs	(304,229)	(450,220)	(336,828)
Future income tax provisions	0	0	0
Net future cash flows	1,080,077	1,618,480	1,095,732
Less effect of 10% discount factor	(565,874)	(932,114)	(597,859)
Standardized measure of discounted future net cash flows	$ 514,203	$ 686,366	$ 497,873	$ 528,781

Amount of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for costs to be incurred in developing proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for income tax, based on the future income less future expense derived from production and sale of proved oil and gas reserves.

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Amount, after future accumulated cash outflow for development and production costs and future income tax, of estimated future accumulated cash inflow from the sale of proved oil and 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 33
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Amount of estimated future accumulated cash outflow for costs to be incurred in producing proved oil and gas reserves.

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Amount of discount on future accumulated cash inflow, after deducting future accumulated cash outflow for development and production costs and future income tax, from sale of proved oil and gas reserves, using a discount rate of ten percent a year.

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Amount of discounted future accumulated cash inflow relating to proved oil and gas reserves.

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 31
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62455-109447

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