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Supplemental Oil and Gas Reserve Information - Unaudited - Changes in Standardized Measure of Discounted Future Net Cash Flows Applicable to Proved Oil and Natural Gas Reserves (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Standardized Measure of Discounted Future Net Cash Flows |  |  |
January 1 | $ 2,844,369 | $ 4,050,306 | $ 3,300,695
Net changes in prices and production costs | (1,088,936) | (1,070,192) | 1,003,008
Net changes in future development costs | 4,640 | 131,003 | (89,304)
Sales of crude oil and natural gas, net | (407,417) | (943,989) | (1,155,223)
Purchases of reserves in place | 0 | 0 | 385,763
Sales of reserves in place | 0 | (36,907) | (197,867)
Revisions of previous quantity estimates | (694,320) | (732,253) | (115,015)
Previously estimated development costs incurred | 87,640 | 246,311 | 303,364
Accretion of discount | 293,445 | 467,426 | 368,374
Net change in income taxes | (76,066) | 533,872 | (240,908)
Changes in timing and other | (62,171) | (238,908) | 26,223
December 31 | 948,877 | 2,844,369 | 4,050,306
Extensions |  |  |
Standardized Measure of Discounted Future Net Cash Flows |  |  |
Extensions | $ 47,693 | $ 437,700 | $ 461,196

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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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- Definition Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of change in estimated future development costs. + 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 35 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62500-109447 Reference 4: 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 + Details
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