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Supplementary Information - Capitalized Costs Relating to Oil and Gas Producing Activities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]    
Proved Developed oil and gas properties $ 476,570 $ 417,824
Proved Undeveloped oil and gas properties 0 0
Total Capitalized Costs 476,570 417,824
Accumulated depreciation, depletion and valuation allowance 263,569 230,333
Net Capitalized Costs $ 213,001 $ 187,491