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Change in discounted future net cash flows (Details 7) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Change in the standardized measure of discounted future net cash flows [Roll Forward] |  |  |
Beginning of year | $ 1,014,000 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 883,400 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 978,800 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves
Net revenues from production | (368,900)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts | (398,000)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts | (280,800)us-gaap_SalesAndTransfersOfOilAndGasProducedNetOfProductionCosts
Net change in sales prices and production costs related to future production | 86,300 us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts | 162,200 us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts | (406,300)us-gaap_NetIncreaseDecreaseInSalesAndTransferPricesAndProductionCosts
Extensions and discoveries, net of future production-related costs | 231,900 mdu_ExtensionsDiscoveriesLessRelatedCosts | 366,500 mdu_ExtensionsDiscoveriesLessRelatedCosts | 355,300 mdu_ExtensionsDiscoveriesLessRelatedCosts
Improved recovery, net of future production-related costs | 0 mdu_ImprovedRecoveryNetOfProductionCosts | 0 mdu_ImprovedRecoveryNetOfProductionCosts | 0 mdu_ImprovedRecoveryNetOfProductionCosts
Purchases of proved reserves, net of future production-related costs | 103,800 us-gaap_IncreaseDueToPurchasesOfMineralsInPlace | 0 us-gaap_IncreaseDueToPurchasesOfMineralsInPlace | 0 us-gaap_IncreaseDueToPurchasesOfMineralsInPlace
Sales of proved reserves | (219,300)us-gaap_DecreaseDueToSalesOfMineralsInPlace | (37,800)us-gaap_DecreaseDueToSalesOfMineralsInPlace | (2,600)us-gaap_DecreaseDueToSalesOfMineralsInPlace
Changes in estimated future development costs | 65,100 us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts | 6,700 us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts | 37,600 us-gaap_IncreaseDecreaseInEstimatedFutureDevelopmentCosts
Development costs incurred during the current year | 104,600 us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod | 141,500 us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod | 77,700 us-gaap_PreviouslyEstimatedDevelopmentCostsIncurredDuringPeriod
Accretion of discount | 109,400 us-gaap_AccretionOfDiscount | 94,600 us-gaap_AccretionOfDiscount | 121,400 us-gaap_AccretionOfDiscount
Net change in income taxes | (33,400)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves | (141,400)us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves | 110,000 us-gaap_IncreaseDecreaseInFutureIncomeTaxExpenseEstimatesOnFutureCashFlowsRelatedToProvedOilAndGasReserves
Revisions of previous estimates | (16,300)us-gaap_RevisionsOfPreviousQuantityEstimates | (55,800)us-gaap_RevisionsOfPreviousQuantityEstimates | (100,700)us-gaap_RevisionsOfPreviousQuantityEstimates
Other | (2,300)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther | (7,900)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther | (7,000)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesOther
Net change | 60,900 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease | 130,600 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease | (95,400)us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowOfProvedOilAndGasReservesPeriodIncreaseDecrease
End of year | $ 1,074,900 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 1,014,000 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 883,400 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves

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- Definition Amount of increase (decrease) in standardized measure of discounted future net cash flow as a result of passage of time. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 35 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62500-109447 + Details
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