Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Gas Producing Activities - Changes in Standardized Measure of Discounted Future Net Cash Flows (Detail)

v2.4.1.9
Supplemental Information on Oil and Gas Producing Activities - Changes in Standardized Measure of Discounted Future Net Cash Flows (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Changes in Standardized Measure of Discounted Future Net Cash Flows:      
Balance at Beginning of Period $ 137,436us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves $ 152,902us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves $ 166,187us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves
Sales of oil and gas, net of production costs (95,973)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts (132,662)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts (168,563)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts
Net changes in prices and production costs (28,098)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts (52,056)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts (11,223)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts
Revisions of previous quantity estimates 74,497us-gaap_RevisionsOfPreviousQuantityEstimates 43,815us-gaap_RevisionsOfPreviousQuantityEstimates 155,111us-gaap_RevisionsOfPreviousQuantityEstimates
Additions 2,188us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease 29,620us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease 69,092us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease
Changes in estimated future development costs 31,686us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts (5,345)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts (67,834)us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts
Development costs incurred during the period   44,389us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod 34,944us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod
Accretion of discount 24,163us-gaap_AccretionOfDiscount 15,290us-gaap_AccretionOfDiscount 16,619us-gaap_AccretionOfDiscount
Net change of income taxes (15,609)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves 26,120us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves 7,445us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves
Change in production rates (timing) and other 19,097us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther 15,363us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther (48,876)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther
Balance at End of Period $ 149,387us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves $ 137,436us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves $ 152,902us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves