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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
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