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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
Dec. 31, 2020
Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]    
Sales of oil and gas produced, net of production costs $ (13,945) $ (15,938)
Net changes in prices and production costs (16,578) (42,409)
Extensions, discoveries and improved recovery 314 31,276
Revisions of previous quantity estimates (36,919) (3,319)
Net change in development costs 20,724 (53,143)
Reserves sold (874) (113)
Reserves purchased 218 174
Accretion of discount 8,161 13,791
Net change in income taxes 5,386 5,572
Changes in production rates (timing) and other (6,480) 7,812
Net change (39,993) (56,297)
Standardized measure of discounted future net cash flow:    
Beginning of year 81,612 137,909
End of year $ 41,619 $ 81,612