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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, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]    
Sales of oil and gas produced, net of production costs $ (47,074)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts $ (48,976)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts
Net changes in prices and production costs (18,510)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts (10,149)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts
Extensions, discoveries and improved recovery 198,235us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts 218,648us-gaap_ExtensionsDiscoveriesAdditionsAndImprovedRecoveryLessRelatedCosts
Revisions of previous quantity estimates 67,245us-gaap_RevisionsOfPreviousQuantityEstimates (733)us-gaap_RevisionsOfPreviousQuantityEstimates
Net change in development costs (69,923)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts (129,672)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts
Reserves sold (69)us-gaap_DecreaseDueToSalesOfMineralsInPlace  
Reserves purchased 0us-gaap_IncreaseDueToPurchasesOfMineralsInPlace 0us-gaap_IncreaseDueToPurchasesOfMineralsInPlace
Accretion of discount 23,823us-gaap_AccretionOfDiscount 21,327us-gaap_AccretionOfDiscount
Net change in income taxes (56,744)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves (23,007)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves
Changes in production rates (timing) and other 8,077us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther (2,483)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther
Net change 105,060us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease 24,955us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease
Standardized measure of discounted future net cash flow:    
Beginning of year 238,228us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves 213,273us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves
End of year $ 343,288us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves $ 238,228us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves