Supplementary Information - Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Future cash inflows	$ 518,783	$ 384,198
Future production costs	(253,896)	(175,099)
Future development costs	(24,584)	(34,798)
Future income tax expenses	(38,589)	(20,884)
Future Net Cash Flows	201,714	153,417
10% annual discount for estimated timing of cash flows	(63,805)	(46,503)
Standardized Measure of Discounted Future Net Cash Flows	$ 137,909	$ 106,914	$ 52,237

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

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- Definition

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

+ References

+ Details

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- Definition

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.

+ References

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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
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62479-109447

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

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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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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- Definition

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.

+ References

+ Details

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- Definition

Amount of discounted future accumulated cash inflow relating to 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 31
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62455-109447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 30
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62424-109447

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-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62500-109447

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62479-109447

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