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Supplementary Information - Standardized Measure of Discounted Future Net Cash Flows and Changes therein Relating to Proved Oil and Gas Reserves (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |
Sales of oil and gas produced, net of production costs | $ (22,065) | $ (36,003)
Net changes in prices and production costs | 24,826 | 12,432
Extensions, discoveries and improved recovery | 46,885 | 76,694
Revisions of previous quantity estimates | 1,942 | 19,808
Net change in development costs | 3,062 | (5,199)
Reserves sold | (274) | (21)
Reserves purchased | 971 | 1,372
Accretion of discount | 10,691 | 5,224
Net change in income taxes | (17,704) | (13,001)
Changes in production rates (timing) and other | (17,339) | (6,627)
Net change | 30,995 | 54,677
Standardized measure of discounted future net cash flow: |  |
Beginning of year | 106,914 | 52,237
End of year | $ 137,909 | $ 106,914

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- Definition Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of extensions, discoveries and improved recovery 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 35A -URI http://asc.fasb.org/extlink&oid=68064819&loc=SL6807758-109447 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=34713648&loc=d3e63123-109448 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 35 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62500-109447 + Details
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