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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, 2016
Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]    
Sales of oil and gas produced, net of production costs $ (10,762) $ (10,426)
Net changes in prices and production costs (6,895) (511,116)
Extensions, discoveries and improved recovery 27,706 24,967
Revisions of previous quantity estimates (5,214) (180,048)
Net change in development costs (26,953) 235,285
Reserves sold   (1,160)
Reserves purchased   737
Accretion of discount 5,880 34,329
Net change in income taxes (2,600) 146,339
Changes in production rates (timing) and other 12,273 (23,393)
Net change (6,565) (284,486)
Standardized measure of discounted future net cash flow:    
Beginning of year 58,802 343,288
End of year $ 52,237 $ 58,802