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Income resulting from oil and natural gas producing activites (Details 3) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Revenues:      
Sales to affiliates $ 47,045,000us-gaap_ResultsOfOperationsSalesOrTransfersToEntitysOtherOperations $ 45,099,000us-gaap_ResultsOfOperationsSalesOrTransfersToEntitysOtherOperations $ 35,966,000us-gaap_ResultsOfOperationsSalesOrTransfersToEntitysOtherOperations
Sales to external customers 468,668,000us-gaap_ResultsOfOperationsSalesRevenueToUnaffiliatedEnterprises 497,018,000us-gaap_ResultsOfOperationsSalesRevenueToUnaffiliatedEnterprises 379,647,000us-gaap_ResultsOfOperationsSalesRevenueToUnaffiliatedEnterprises
Realized gain on commodity derivatives 8,458,000us-gaap_ResultsOfOperationsRevenueOther 173,000us-gaap_ResultsOfOperationsRevenueOther 33,628,000us-gaap_ResultsOfOperationsRevenueOther
Unrealized (gain) loss on commodity derivatives 23,400,000us-gaap_UnrealizedGainLossOnCommodityContracts (6,267,000)us-gaap_UnrealizedGainLossOnCommodityContracts (624,000)us-gaap_UnrealizedGainLossOnCommodityContracts
Production costs 146,793,000us-gaap_ResultsOfOperationsProductionOrLiftingCosts 144,136,000us-gaap_ResultsOfOperationsProductionOrLiftingCosts 134,795,000us-gaap_ResultsOfOperationsProductionOrLiftingCosts
Depreciation, depletion and amortization 193,944,000us-gaap_ResultsOfOperationsDepreciationDepletionAndAmortizationAndValuationProvisions [1] 182,352,000us-gaap_ResultsOfOperationsDepreciationDepletionAndAmortizationAndValuationProvisions [1] 157,078,000us-gaap_ResultsOfOperationsDepreciationDepletionAndAmortizationAndValuationProvisions [1]
Write-downs of oil and natural gas properties 0us-gaap_ResultsOfOperationsImpairmentOfOilAndGasProperties 0us-gaap_ResultsOfOperationsImpairmentOfOilAndGasProperties 391,800,000us-gaap_ResultsOfOperationsImpairmentOfOilAndGasProperties
Pretax income (loss) 206,834,000us-gaap_ResultsOfOperationsIncomeBeforeIncomeTaxes 209,535,000us-gaap_ResultsOfOperationsIncomeBeforeIncomeTaxes (235,056,000)us-gaap_ResultsOfOperationsIncomeBeforeIncomeTaxes
Income tax expense (benefit) 75,483,000us-gaap_ResultsOfOperationsIncomeTaxExpense 75,836,000us-gaap_ResultsOfOperationsIncomeTaxExpense (88,612,000)us-gaap_ResultsOfOperationsIncomeTaxExpense
Results of operations for producing activities 131,351,000us-gaap_ResultsOfOperationsOilAndGasProducingActivitiesNetIncomeExcludingCorporateOverheadAndInterestCosts 133,699,000us-gaap_ResultsOfOperationsOilAndGasProducingActivitiesNetIncomeExcludingCorporateOverheadAndInterestCosts (146,444,000)us-gaap_ResultsOfOperationsOilAndGasProducingActivitiesNetIncomeExcludingCorporateOverheadAndInterestCosts
Accretion of discount for asset retirement obligations oil and gas $ 3,500,000mdu_AccretionOfDiscountForAssetRetirementObligationsOilAndGas $ 3,600,000mdu_AccretionOfDiscountForAssetRetirementObligationsOilAndGas $ 3,300,000mdu_AccretionOfDiscountForAssetRetirementObligationsOilAndGas
[1] Includes accretion of discount for asset retirement obligations of $3.5 million, $3.6 million and $3.3 million for the years ended December 31, 2014, 2013 and 2012, respectively, as discussed in Note 10.