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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, 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