Toggle SGML Header (+)


Section 1: 10-K (MDU RESOURCES 2018 FORM 10-K)

Wdesk | Document
false--12-31FY20182018-12-310000067716YesfalseLarge Accelerated FilerMDU RESOURCES GROUP INCfalsefalse916028298880135695936061681886023284316134845316127268826010346316127280416187751526054620530181657946090764956052257956123404NoYes087600001.001.001.001.00500000000500000000500000000500000000195843297195843297196564907196564907001001001001001001001001001001001001001001YellowRedGreenGreenRedGreenGreenGreenGreenRedGreenRedYellowRedYellowRedGreenGreenYellowGreenGreenGreenGreenRedGreenYellowYellowRed2017-06-302017-12-312017-12-312017-12-312017-02-282017-06-302017-05-312017-04-302016-11-302017-12-312017-12-312017-12-312017-12-312017-12-312018-06-302018-12-312018-12-312018-12-312018-02-282018-06-302018-05-312018-04-302017-11-302018-12-312018-12-312018-12-312018-12-312018-12-312021-03-312018-01-312019-06-152024-07-312022-05-312020-06-30310003000140000075000836000116200014710007700065000350002260002240004290001425000645000721000980007500038000538921538921538921538921 0000067716 2018-01-01 2018-12-31 0000067716 2018-06-29 0000067716 2019-02-14 0000067716 2017-01-01 2017-12-31 0000067716 2016-01-01 2016-12-31 0000067716 us-gaap:UnregulatedOperationMember 2017-01-01 2017-12-31 0000067716 us-gaap:UnregulatedOperationMember 2016-01-01 2016-12-31 0000067716 us-gaap:RegulatedOperationMember 2017-01-01 2017-12-31 0000067716 us-gaap:UnregulatedOperationMember 2018-01-01 2018-12-31 0000067716 us-gaap:RegulatedOperationMember 2018-01-01 2018-12-31 0000067716 us-gaap:RegulatedOperationMember 2016-01-01 2016-12-31 0000067716 us-gaap:RetainedEarningsMember 2018-01-01 2018-12-31 0000067716 2018-12-31 0000067716 2017-12-31 0000067716 us-gaap:PreferredStockMember 2015-12-31 0000067716 us-gaap:RetainedEarningsMember 2016-12-31 0000067716 us-gaap:RetainedEarningsMember 2017-12-31 0000067716 us-gaap:TreasuryStockMember 2016-12-31 0000067716 us-gaap:RetainedEarningsMember 2017-01-01 2017-12-31 0000067716 us-gaap:RetainedEarningsMember 2018-12-31 0000067716 us-gaap:TreasuryStockMember 2018-12-31 0000067716 us-gaap:CommonStockMember 2017-12-31 0000067716 us-gaap:AdditionalPaidInCapitalMember 2017-12-31 0000067716 us-gaap:CommonStockMember 2015-12-31 0000067716 2016-12-31 0000067716 us-gaap:NoncontrollingInterestMember 2016-01-01 2016-12-31 0000067716 us-gaap:RetainedEarningsMember 2016-01-01 2016-12-31 0000067716 us-gaap:AdditionalPaidInCapitalMember 2016-01-01 2016-12-31 0000067716 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2017-01-01 2017-12-31 0000067716 us-gaap:TreasuryStockMember 2017-01-01 2017-12-31 0000067716 us-gaap:TreasuryStockMember 2018-01-01 2018-12-31 0000067716 us-gaap:AdditionalPaidInCapitalMember 2017-01-01 2017-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember us-gaap:AccumulatedOtherComprehensiveIncomeMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2017-12-31 0000067716 2015-12-31 0000067716 us-gaap:TreasuryStockMember 2017-12-31 0000067716 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2016-12-31 0000067716 us-gaap:AdditionalPaidInCapitalMember 2016-12-31 0000067716 us-gaap:CommonStockMember 2016-01-01 2016-12-31 0000067716 us-gaap:AdditionalPaidInCapitalMember 2018-01-01 2018-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember us-gaap:RetainedEarningsMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2015-12-31 0000067716 us-gaap:TreasuryStockMember 2015-12-31 0000067716 us-gaap:PreferredStockMember 2017-01-01 2017-12-31 0000067716 us-gaap:AdditionalPaidInCapitalMember 2018-12-31 0000067716 us-gaap:CommonStockMember 2016-12-31 0000067716 us-gaap:CommonStockMember 2018-12-31 0000067716 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2016-01-01 2016-12-31 0000067716 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2018-12-31 0000067716 us-gaap:NoncontrollingInterestMember 2015-12-31 0000067716 us-gaap:AccountingStandardsUpdate201409Member 2017-12-31 0000067716 us-gaap:AccountingStandardsUpdate201409Member us-gaap:RetainedEarningsMember 2017-12-31 0000067716 us-gaap:CommonStockMember 2018-01-01 2018-12-31 0000067716 us-gaap:RetainedEarningsMember 2015-12-31 0000067716 us-gaap:PreferredStockMember 2016-12-31 0000067716 us-gaap:NoncontrollingInterestMember 2018-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember 2018-01-01 2018-12-31 0000067716 us-gaap:AdditionalPaidInCapitalMember 2015-12-31 0000067716 us-gaap:AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2017-01-01 2017-12-31 0000067716 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-12-31 0000067716 us-gaap:AccumulatedTranslationAdjustmentMember 2017-01-01 2017-12-31 0000067716 us-gaap:AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember 2017-12-31 0000067716 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2016-12-31 0000067716 us-gaap:AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember 2017-01-01 2017-12-31 0000067716 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2017-12-31 0000067716 us-gaap:AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember 2018-12-31 0000067716 us-gaap:AccumulatedTranslationAdjustmentMember 2017-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember mdu:TaxCutsandJobsActMember us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2016-12-31 0000067716 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-01-01 2017-12-31 0000067716 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedTranslationAdjustmentMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedTranslationAdjustmentMember 2018-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember mdu:TaxCutsandJobsActMember us-gaap:AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember mdu:TaxCutsandJobsActMember us-gaap:AccumulatedTranslationAdjustmentMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedTranslationAdjustmentMember 2016-12-31 0000067716 us-gaap:AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember 2016-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember mdu:TaxCutsandJobsActMember us-gaap:AccumulatedOtherComprehensiveIncomeMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2018-12-31 0000067716 us-gaap:AccumulatedOtherComprehensiveIncomeMember 2018-01-01 2018-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember mdu:TaxCutsandJobsActMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-01-01 2018-12-31 0000067716 us-gaap:AccountingStandardsUpdate201409Member us-gaap:CalculatedUnderRevenueGuidanceInEffectBeforeTopic606Member 2017-12-31 0000067716 us-gaap:AccountingStandardsUpdate201409Member 2018-01-01 0000067716 us-gaap:AccountingStandardsUpdate201409Member us-gaap:DifferenceBetweenRevenueGuidanceInEffectBeforeAndAfterTopic606Member 2018-01-01 0000067716 srt:MinimumMember mdu:RiskAdjustedCostofCapitalMember 2018-10-01 2018-12-31 0000067716 srt:RestatementAdjustmentMember us-gaap:AccountingStandardsUpdate201707Member us-gaap:OtherNonoperatingIncomeExpenseMember 2017-01-01 2017-12-31 0000067716 srt:RestatementAdjustmentMember us-gaap:AccountingStandardsUpdate201707Member us-gaap:OtherNonoperatingIncomeExpenseMember 2016-01-01 2016-12-31 0000067716 srt:MaximumMember mdu:RiskAdjustedCostofCapitalMember 2018-10-01 2018-12-31 0000067716 us-gaap:AccountingStandardsUpdate201409Member 2018-01-01 2018-12-31 0000067716 us-gaap:AdjustmentsForNewAccountingPrincipleEarlyAdoptionMember us-gaap:OtherComprehensiveIncomeMember 2018-01-01 2018-03-31 0000067716 srt:MaximumMember us-gaap:AccountingStandardsUpdate201602Member us-gaap:LiabilityMember us-gaap:SubsequentEventMember 2019-01-01 2019-03-31 0000067716 mdu:LongtermGrowthRateProjectionMember 2018-10-01 2018-12-31 0000067716 srt:MinimumMember us-gaap:AccountingStandardsUpdate201602Member us-gaap:LiabilityMember us-gaap:SubsequentEventMember 2019-01-01 2019-03-31 0000067716 us-gaap:BuildingAndBuildingImprovementsMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 us-gaap:MachineryAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2017-12-31 0000067716 us-gaap:ElectricDistributionMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 us-gaap:LandMember us-gaap:UnregulatedOperationMember us-gaap:CorporateAndOtherMember 2018-12-31 0000067716 us-gaap:GasGatheringAndProcessingEquipmentMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2017-12-31 0000067716 us-gaap:BuildingAndBuildingImprovementsMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 us-gaap:ElectricDistributionMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-12-31 0000067716 us-gaap:MachineryAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2018-12-31 0000067716 us-gaap:LandMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2018-12-31 0000067716 us-gaap:GasTransmissionMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 us-gaap:BuildingAndBuildingImprovementsMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2017-12-31 0000067716 us-gaap:ElectricTransmissionMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2017-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember us-gaap:CorporateAndOtherMember 2018-01-01 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember us-gaap:CorporateAndOtherMember 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2017-12-31 0000067716 us-gaap:ElectricGenerationEquipmentMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2017-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2017-12-31 0000067716 us-gaap:BuildingAndBuildingImprovementsMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2018-12-31 0000067716 us-gaap:MachineryAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2018-12-31 0000067716 us-gaap:GasTransmissionMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2017-12-31 0000067716 us-gaap:MachineryAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 us-gaap:GasGatheringAndProcessingEquipmentMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2017-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:GasStorageMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2017-12-31 0000067716 us-gaap:ElectricTransmissionMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 us-gaap:LandMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2017-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 us-gaap:ElectricGenerationEquipmentMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-12-31 0000067716 mdu:AggregateReservesMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2018-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2017-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2017-12-31 0000067716 us-gaap:BuildingAndBuildingImprovementsMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-12-31 0000067716 us-gaap:GasGatheringAndProcessingEquipmentMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2017-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember us-gaap:CorporateAndOtherMember 2017-12-31 0000067716 us-gaap:MachineryAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2017-12-31 0000067716 us-gaap:ElectricTransmissionMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-12-31 0000067716 us-gaap:ElectricGenerationEquipmentMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 us-gaap:LandMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2018-12-31 0000067716 us-gaap:LandMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2017-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2018-12-31 0000067716 us-gaap:GasDistributionMember us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2018-12-31 0000067716 mdu:GasStorageMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-12-31 0000067716 us-gaap:GasDistributionMember us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2017-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2018-12-31 0000067716 us-gaap:GasDistributionMember us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:GatheringandprocessingMemberDomain us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2017-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2017-12-31 0000067716 us-gaap:LandMember us-gaap:UnregulatedOperationMember us-gaap:CorporateAndOtherMember 2017-12-31 0000067716 mdu:GatheringandprocessingMemberDomain us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2017-12-31 0000067716 mdu:GasStorageMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2018-12-31 0000067716 us-gaap:MachineryAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 us-gaap:GasTransmissionMember us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-12-31 0000067716 us-gaap:ElectricDistributionMember us-gaap:RegulatedOperationMember mdu:ElectricMember 2017-12-31 0000067716 mdu:AggregateReservesMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2017-12-31 0000067716 us-gaap:BuildingAndBuildingImprovementsMember us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2017-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2018-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 us-gaap:ConstructionInProgressMember us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2017-12-31 0000067716 us-gaap:OtherCapitalizedPropertyPlantAndEquipmentMember us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2017-12-31 0000067716 mdu:GatheringandprocessingMemberDomain us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedTranslationAdjustmentMember 2018-01-01 2018-12-31 0000067716 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2017-01-01 2017-12-31 0000067716 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2018-01-01 2018-12-31 0000067716 us-gaap:InterestRateContractMember us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember 2018-01-01 2018-12-31 0000067716 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedDefinedBenefitPlansAdjustmentMember 2017-01-01 2017-12-31 0000067716 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember 2018-01-01 2018-12-31 0000067716 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedTranslationAdjustmentMember 2017-01-01 2017-12-31 0000067716 us-gaap:InterestRateContractMember us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember 2017-01-01 2017-12-31 0000067716 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember us-gaap:AccumulatedNetUnrealizedInvestmentGainLossMember 2018-01-01 2018-12-31 0000067716 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember 2017-01-01 2017-12-31 0000067716 mdu:OtherUtilitySalesMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:OtherUtilitySalesMember 2018-01-01 2018-12-31 0000067716 mdu:NaturalGasGatheringMember 2018-01-01 2018-12-31 0000067716 mdu:IndustrialUtilitySalesMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:ConstructionMaterialsMember 2018-01-01 2018-12-31 0000067716 mdu:RevenuesOutOfScopeMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:RevenueFromContractsWithCustomersMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersegmentEliminationMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 mdu:RevenuesOutOfScopeMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 mdu:RevenuesOutOfScopeMember 2018-01-01 2018-12-31 0000067716 mdu:OtherRevenuesMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 mdu:ConstructionMaterialsAndContractingMember us-gaap:IntersubsegmentEliminationsMember 2018-01-01 2018-12-31 0000067716 us-gaap:NaturalGasStorageMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 mdu:OtherUtilitySalesMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 mdu:CommercialUtilitySalesMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersubsegmentEliminationsMember 2018-01-01 2018-12-31 0000067716 mdu:OtherRevenuesMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 mdu:OtherRevenuesMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersegmentEliminationMember us-gaap:CorporateAndOtherMember 2018-01-01 2018-12-31 0000067716 mdu:RevenuesOutOfScopeMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 mdu:OtherRevenuesMember 2018-01-01 2018-12-31 0000067716 us-gaap:CorporateAndOtherMember 2018-01-01 2018-12-31 0000067716 mdu:CommercialUtilitySalesMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:OtherRevenuesMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 mdu:RevenueFromContractsWithCustomersMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 mdu:OtherRevenuesMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 mdu:InsideSpecialtyContractingMember 2018-01-01 2018-12-31 0000067716 us-gaap:NaturalGasStorageMember 2018-01-01 2018-12-31 0000067716 mdu:IndustrialUtilitySalesMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 mdu:InsideSpecialtyContractingMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 mdu:RevenueFromContractsWithCustomersMember us-gaap:CorporateAndOtherMember 2018-01-01 2018-12-31 0000067716 mdu:ContractingServicesMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 mdu:RevenueFromContractsWithCustomersMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:RevenueFromContractsWithCustomersMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersegmentEliminationMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:RevenueFromContractsWithCustomersMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 mdu:ResidentialUtilitySalesMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:CommercialUtilitySalesMember 2018-01-01 2018-12-31 0000067716 mdu:NaturalGasTransportationMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:ResidentialUtilitySalesMember 2018-01-01 2018-12-31 0000067716 mdu:RevenueFromContractsWithCustomersMember 2018-01-01 2018-12-31 0000067716 mdu:OutsideSpecialtyContractingMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 mdu:RevenuesOutOfScopeMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 mdu:ResidentialUtilitySalesMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersegmentEliminationMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersegmentEliminationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 mdu:OtherRevenuesMember us-gaap:CorporateAndOtherMember 2018-01-01 2018-12-31 0000067716 mdu:RevenuesOutOfScopeMember us-gaap:CorporateAndOtherMember 2018-01-01 2018-12-31 0000067716 mdu:ConstructionMaterialsMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 mdu:NaturalGasTransportationMember 2018-01-01 2018-12-31 0000067716 mdu:RevenuesOutOfScopeMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 mdu:IndustrialUtilitySalesMember 2018-01-01 2018-12-31 0000067716 mdu:NaturalGasTransportationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 mdu:NaturalGasGatheringMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersegmentEliminationMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 mdu:OutsideSpecialtyContractingMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersegmentEliminationMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 mdu:ContractingServicesMember 2018-01-01 2018-12-31 0000067716 us-gaap:CommonStockMember 2018-12-31 0000067716 us-gaap:CommonStockMember 2018-01-01 2018-12-31 0000067716 mdu:A2018AcquisitionsMember 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:RefiningMember 2017-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:RefiningMember 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember 2018-01-01 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember 2017-01-01 2017-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember 2016-01-01 2016-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember 2017-12-31 0000067716 mdu:WBIEnergyMember mdu:DakotaPrairieRefiningLlcMember 2016-06-27 2016-06-27 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:RefiningMember 2018-01-01 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:RefiningMember 2016-04-01 2016-06-30 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember us-gaap:OtherRestructuringMember 2015-07-01 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember us-gaap:ContractTerminationMember 2016-01-01 2016-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:RefiningMember us-gaap:OtherRestructuringMember 2018-01-01 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:RefiningMember 2017-01-01 2017-12-31 0000067716 mdu:ExplorationAndProductionMember 2016-01-01 2016-03-31 0000067716 mdu:ExplorationAndProductionMember 2016-04-01 2016-06-30 0000067716 mdu:WBIEnergyMember mdu:DakotaPrairieRefiningLlcMember 2016-06-24 2016-06-24 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember us-gaap:DomesticCountryMember 2017-12-31 0000067716 mdu:RefiningMember mdu:OperatingexpenseaftertaxMember 2017-01-01 2017-03-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember us-gaap:StateAndLocalJurisdictionMember 2017-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:RefiningMember 2016-01-01 2016-12-31 0000067716 mdu:RefiningMember us-gaap:OperatingExpenseMember 2017-01-01 2017-03-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember mdu:FederalandStateTaxAuthoritiesMember 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember us-gaap:OtherRestructuringMember 2018-01-01 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldForSaleOrDisposedOfBySaleMember mdu:ExplorationAndProductionMember us-gaap:ContractTerminationMember 2016-04-01 2016-06-30 0000067716 us-gaap:SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember 2018-01-01 2018-12-31 0000067716 us-gaap:CustomerRelationshipsMember 2018-12-31 0000067716 us-gaap:OtherIntangibleAssetsMember 2017-12-31 0000067716 us-gaap:CustomerRelationshipsMember 2017-12-31 0000067716 us-gaap:OtherIntangibleAssetsMember 2018-12-31 0000067716 us-gaap:NoncompeteAgreementsMember 2017-12-31 0000067716 us-gaap:NoncompeteAgreementsMember 2018-12-31 0000067716 mdu:NaturalGasDistributionMember 2017-12-31 0000067716 mdu:ConstructionMaterialsAndContractingMember 2017-12-31 0000067716 mdu:ConstructionServicesMember 2016-12-31 0000067716 mdu:ConstructionServicesMember 2017-01-01 2017-12-31 0000067716 mdu:ConstructionServicesMember 2017-12-31 0000067716 mdu:NaturalGasDistributionMember 2016-12-31 0000067716 mdu:ConstructionMaterialsAndContractingMember 2016-12-31 0000067716 mdu:ConstructionMaterialsAndContractingMember 2017-01-01 2017-12-31 0000067716 mdu:NaturalGasDistributionMember 2017-01-01 2017-12-31 0000067716 mdu:NaturalGasDistributionMember 2018-12-31 0000067716 mdu:ConstructionMaterialsAndContractingMember 2018-12-31 0000067716 mdu:ConstructionServicesMember 2018-12-31 0000067716 mdu:LiabilitiesRefundableGasCostsMember 2018-12-31 0000067716 us-gaap:EnvironmentalRestorationCostsMember 2018-01-01 2018-12-31 0000067716 us-gaap:AssetRecoverableGasCostsMember 2018-12-31 0000067716 us-gaap:LossOnReacquiredDebtMember 2018-12-31 0000067716 us-gaap:LossOnReacquiredDebtMember 2017-12-31 0000067716 us-gaap:AssetRetirementObligationCostsMember 2017-12-31 0000067716 us-gaap:OtherRegulatoryAssetsLiabilitiesMember 2017-12-31 0000067716 us-gaap:LossOnReacquiredDebtMember 2018-01-01 2018-12-31 0000067716 us-gaap:PensionAndOtherPostretirementPlansCostsMember 2018-12-31 0000067716 us-gaap:DeferredIncomeTaxChargesMember 2018-12-31 0000067716 us-gaap:PensionAndOtherPostretirementPlansCostsMember 2017-12-31 0000067716 us-gaap:PensionAndOtherPostretirementPlansCostsMember 2018-12-31 0000067716 us-gaap:AssetRecoverableGasCostsMember 2018-01-01 2018-12-31 0000067716 us-gaap:OtherRegulatoryAssetsLiabilitiesMember 2018-12-31 0000067716 mdu:LiabilitiesRefundableGasCostsMember 2017-12-31 0000067716 us-gaap:RemovalCostsMember 2017-12-31 0000067716 us-gaap:EnvironmentalRestorationCostsMember 2018-12-31 0000067716 us-gaap:PensionAndOtherPostretirementPlansCostsMember 2017-12-31 0000067716 us-gaap:AssetRecoverableGasCostsMember 2017-12-31 0000067716 us-gaap:AssetRetirementObligationCostsMember 2018-12-31 0000067716 mdu:CostsRelatedToIdentifyingGenerationDevelopmentMember 2017-12-31 0000067716 us-gaap:DeferredIncomeTaxChargesMember 2018-01-01 2018-12-31 0000067716 us-gaap:DeferredIncomeTaxChargesMember 2017-12-31 0000067716 mdu:CostsRelatedToIdentifyingGenerationDevelopmentMember 2018-01-01 2018-12-31 0000067716 us-gaap:DeferredIncomeTaxChargesMember 2018-12-31 0000067716 us-gaap:OtherRegulatoryAssetsLiabilitiesMember 2017-12-31 0000067716 us-gaap:AssetRetirementObligationCostsMember 2018-01-01 2018-12-31 0000067716 us-gaap:PensionAndOtherPostretirementPlansCostsMember 2018-01-01 2018-12-31 0000067716 us-gaap:OtherRegulatoryAssetsLiabilitiesMember 2018-01-01 2018-12-31 0000067716 us-gaap:RemovalCostsMember 2018-12-31 0000067716 mdu:CostsRelatedToIdentifyingGenerationDevelopmentMember 2018-12-31 0000067716 us-gaap:OtherRegulatoryAssetsLiabilitiesMember 2018-12-31 0000067716 us-gaap:DeferredIncomeTaxChargesMember 2017-12-31 0000067716 us-gaap:EnvironmentalRestorationCostsMember 2017-12-31 0000067716 us-gaap:OtherNoncurrentAssetsMember us-gaap:StrandedCostsMember 2017-12-31 0000067716 us-gaap:OtherNoncurrentLiabilitiesMember us-gaap:StrandedCostsMember 2017-12-31 0000067716 us-gaap:CashSurrenderValueMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:MortgageBackedSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:MoneyMarketFundsMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:CashSurrenderValueMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:MoneyMarketFundsMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:MortgageBackedSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:FairValueMeasurementsRecurringMember 2018-12-31 0000067716 us-gaap:USTreasurySecuritiesMember 2017-12-31 0000067716 us-gaap:MortgageBackedSecuritiesMember 2017-12-31 0000067716 us-gaap:MoneyMarketFundsMember us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:MortgageBackedSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:MortgageBackedSecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:MoneyMarketFundsMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:CashSurrenderValueMember us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:CashSurrenderValueMember us-gaap:FairValueInputsLevel2Member us-gaap:FairValueMeasurementsRecurringMember 2017-12-31 0000067716 us-gaap:MortgageBackedSecuritiesMember 2018-12-31 0000067716 us-gaap:USTreasurySecuritiesMember 2018-12-31 0000067716 us-gaap:CarryingReportedAmountFairValueDisclosureMember 2017-12-31 0000067716 us-gaap:EstimateOfFairValueFairValueDisclosureMember 2017-12-31 0000067716 us-gaap:CarryingReportedAmountFairValueDisclosureMember 2018-12-31 0000067716 us-gaap:EstimateOfFairValueFairValueDisclosureMember 2018-12-31 0000067716 mdu:CascadeNaturalGasCorporationMember us-gaap:RevolvingCreditFacilityMember us-gaap:LongTermDebtMember 2017-12-31 0000067716 mdu:CentennialEnergyHoldingsIncMember mdu:CommercialPaperRevolvingCreditAgreementMember us-gaap:LongTermDebtMember 2018-12-31 0000067716 mdu:IntermountainGasCompanyMember us-gaap:RevolvingCreditFacilityMember us-gaap:LongTermDebtMember 2018-12-31 0000067716 mdu:CentennialEnergyHoldingsIncMember mdu:CommercialPaperRevolvingCreditAgreementMember us-gaap:LongTermDebtMember 2017-12-31 0000067716 mdu:CascadeNaturalGasCorporationMember us-gaap:RevolvingCreditFacilityMember us-gaap:LongTermDebtMember 2018-01-01 2018-12-31 0000067716 mdu:MduResourcesGroupIncMember mdu:CommercialPaperRevolvingCreditAgreementMember us-gaap:LongTermDebtMember 2018-12-31 0000067716 mdu:CentennialEnergyHoldingsIncMember mdu:CommercialPaperRevolvingCreditAgreementMember us-gaap:LongTermDebtMember 2018-01-01 2018-12-31 0000067716 mdu:IntermountainGasCompanyMember us-gaap:RevolvingCreditFacilityMember us-gaap:LongTermDebtMember 2017-12-31 0000067716 mdu:MduResourcesGroupIncMember mdu:CommercialPaperRevolvingCreditAgreementMember us-gaap:LongTermDebtMember 2017-12-31 0000067716 mdu:IntermountainGasCompanyMember us-gaap:RevolvingCreditFacilityMember us-gaap:LongTermDebtMember 2018-01-01 2018-12-31 0000067716 mdu:CascadeNaturalGasCorporationMember us-gaap:RevolvingCreditFacilityMember us-gaap:LongTermDebtMember 2018-12-31 0000067716 mdu:MduResourcesGroupIncMember mdu:CommercialPaperRevolvingCreditAgreementMember us-gaap:LongTermDebtMember 2018-01-01 2018-12-31 0000067716 us-gaap:LongTermDebtMember 2018-12-31 0000067716 us-gaap:CommercialPaperMember 2018-12-31 0000067716 mdu:CreditAgreementsMember 2017-12-31 0000067716 us-gaap:SeniorNotesMember 2017-12-31 0000067716 mdu:TermLoanAgreementsMember 2018-12-31 0000067716 us-gaap:SeniorNotesMember 2018-12-31 0000067716 mdu:TermLoanAgreementsMember 2017-12-31 0000067716 us-gaap:NotesPayableToBanksMember 2017-12-31 0000067716 us-gaap:LongTermDebtMember 2017-12-31 0000067716 us-gaap:MediumTermNotesMember 2017-12-31 0000067716 us-gaap:NotesPayableToBanksMember 2018-12-31 0000067716 mdu:CreditAgreementsMember 2018-12-31 0000067716 us-gaap:MediumTermNotesMember 2018-12-31 0000067716 us-gaap:CommercialPaperMember 2017-12-31 0000067716 mdu:MduResourcesGroupIncMember us-gaap:RevolvingCreditFacilityMember us-gaap:LongTermDebtMember 2018-12-31 0000067716 mdu:CentennialEnergyHoldingsIncMember us-gaap:RevolvingCreditFacilityMember us-gaap:LongTermDebtMember 2018-12-31 0000067716 mdu:WbiEnergyTransmissionIncMember mdu:UncommittedLongTermPrivateShelfAgreementMember us-gaap:LongTermDebtMember 2018-12-31 0000067716 us-gaap:AssetRetirementObligationCostsMember 2017-01-01 2017-12-31 0000067716 2017-04-01 2017-04-01 0000067716 mdu:PreferredStockSeries470Member 2017-04-01 2017-04-01 0000067716 mdu:PreferredStockSeries470Member 2016-01-01 2016-12-31 0000067716 us-gaap:PreferredStockMember 2018-12-31 0000067716 us-gaap:PreferredStockMember 2017-12-31 0000067716 mdu:PreferredStockSeries45Member 2017-04-01 2017-04-01 0000067716 us-gaap:PreferredClassAMember 2018-12-31 0000067716 mdu:PreferenceStockMember 2018-12-31 0000067716 mdu:PreferredStockSeries45Member 2016-01-01 2016-12-31 0000067716 mdu:PreferredStockWithParValueMember 2018-12-31 0000067716 mdu:PreferredStockSeries45Member 2017-04-01 0000067716 mdu:PreferredStockSeries470Member 2017-04-01 0000067716 mdu:CentennialMember 2018-01-01 2018-12-31 0000067716 srt:ParentCompanyMember 2018-12-31 0000067716 us-gaap:PerformanceSharesMember mdu:MarketConditionMember 2018-01-01 2018-12-31 0000067716 us-gaap:PerformanceSharesMember mdu:MarketConditionMember 2017-01-01 2017-12-31 0000067716 us-gaap:PerformanceSharesMember mdu:MarketConditionMember 2016-01-01 2016-12-31 0000067716 srt:MaximumMember us-gaap:PerformanceSharesMember mdu:PerformanceConditionMember 2018-01-01 2018-12-31 0000067716 us-gaap:PerformanceSharesMember 2018-01-01 2018-12-31 0000067716 us-gaap:RestrictedStockUnitsRSUMember 2018-01-01 2018-12-31 0000067716 us-gaap:PerformanceSharesMember 2016-01-01 2016-12-31 0000067716 srt:MaximumMember us-gaap:PerformanceSharesMember mdu:MarketConditionMember 2018-01-01 2018-12-31 0000067716 us-gaap:PerformanceSharesMember mdu:PerformanceConditionMember 2018-01-01 2018-12-31 0000067716 us-gaap:PerformanceSharesMember 2017-01-01 2017-12-31 0000067716 srt:MinimumMember us-gaap:PerformanceSharesMember mdu:PerformanceConditionMember 2018-01-01 2018-12-31 0000067716 srt:MinimumMember us-gaap:PerformanceSharesMember mdu:MarketConditionMember 2018-01-01 2018-12-31 0000067716 us-gaap:RestrictedStockUnitsRSUMember 2018-12-31 0000067716 us-gaap:PerformanceSharesMember 2018-12-31 0000067716 us-gaap:PerformanceSharesMember 2017-12-31 0000067716 mdu:GrantdateFebruary2018Member us-gaap:PerformanceSharesMember 2018-12-31 0000067716 mdu:GrantdateFebruary2017Member us-gaap:PerformanceSharesMember 2018-12-31 0000067716 mdu:GrantdateFebruary2016Member us-gaap:PerformanceSharesMember 2018-12-31 0000067716 mdu:GrantDateMarch2016Member us-gaap:PerformanceSharesMember 2018-12-31 0000067716 us-gaap:StateAndLocalJurisdictionMember 2017-12-31 0000067716 us-gaap:StrandedCostsMember 2017-10-01 2017-12-31 0000067716 us-gaap:StrandedCostsMember 2018-07-01 2018-09-30 0000067716 us-gaap:DiscontinuedOperationsHeldforsaleMember 2017-10-01 2017-12-31 0000067716 mdu:FederalandStateTaxingAuthorityMember 2018-12-31 0000067716 us-gaap:StateAndLocalJurisdictionMember 2018-12-31 0000067716 us-gaap:OtherComprehensiveIncomeMember 2017-10-01 2017-12-31 0000067716 2017-10-01 2017-12-31 0000067716 mdu:FederalandStateTaxingAuthorityMember 2017-12-31 0000067716 mdu:ContinuinganddiscontinuedoperationsMember 2017-01-01 2017-12-31 0000067716 mdu:ContinuinganddiscontinuedoperationsMember 2018-01-01 2018-12-31 0000067716 mdu:ContinuinganddiscontinuedoperationsMember 2016-01-01 2016-12-31 0000067716 mdu:PipelineandMidstreamMember 2016-01-01 2016-12-31 0000067716 us-gaap:CorporateAndOtherMember 2017-12-31 0000067716 us-gaap:IntersegmentEliminationMember 2017-01-01 2017-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldforsaleMember 2018-12-31 0000067716 mdu:PipelineandMidstreamMember 2017-01-01 2017-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:ElectricMember 2016-01-01 2016-12-31 0000067716 us-gaap:IntersegmentEliminationMember 2018-01-01 2018-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2017-01-01 2017-12-31 0000067716 mdu:ElectricMember 2016-01-01 2016-12-31 0000067716 mdu:PipelineandMidstreamMember 2017-12-31 0000067716 mdu:ElectricMember 2017-12-31 0000067716 mdu:NaturalGasDistributionMember 2016-01-01 2016-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2018-01-01 2018-12-31 0000067716 mdu:ElectricMember 2018-12-31 0000067716 mdu:ElectricMember 2016-12-31 0000067716 us-gaap:CorporateAndOtherMember 2016-01-01 2016-12-31 0000067716 mdu:PipelineandMidstreamMember 2018-12-31 0000067716 mdu:ConstructionServicesMember 2016-01-01 2016-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2017-01-01 2017-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 mdu:ConstructionMaterialsAndContractingMember 2016-01-01 2016-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2016-01-01 2016-12-31 0000067716 us-gaap:CorporateAndOtherMember 2017-01-01 2017-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:ElectricMember 2017-01-01 2017-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2017-01-01 2017-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2018-01-01 2018-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2017-01-01 2017-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2016-01-01 2016-12-31 0000067716 us-gaap:UnregulatedOperationMember us-gaap:CorporateAndOtherMember 2016-01-01 2016-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2016-01-01 2016-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:ElectricMember 2018-01-01 2018-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:PipelineandMidstreamMember 2017-01-01 2017-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:PipelineandMidstreamMember 2016-01-01 2016-12-31 0000067716 us-gaap:RegulatedOperationMember mdu:NaturalGasDistributionMember 2018-01-01 2018-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:ConstructionMaterialsAndContractingMember 2016-01-01 2016-12-31 0000067716 us-gaap:CorporateAndOtherMember 2018-12-31 0000067716 mdu:ElectricMember 2017-01-01 2017-12-31 0000067716 us-gaap:UnregulatedOperationMember us-gaap:CorporateAndOtherMember 2017-01-01 2017-12-31 0000067716 us-gaap:UnregulatedOperationMember mdu:ConstructionServicesMember 2018-01-01 2018-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldforsaleMember 2016-12-31 0000067716 us-gaap:CorporateAndOtherMember 2016-12-31 0000067716 us-gaap:UnregulatedOperationMember us-gaap:CorporateAndOtherMember 2018-01-01 2018-12-31 0000067716 us-gaap:IntersegmentEliminationMember 2016-01-01 2016-12-31 0000067716 mdu:PipelineandMidstreamMember 2016-12-31 0000067716 us-gaap:DiscontinuedOperationsHeldforsaleMember 2017-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:EquitiesSecuritiesOfInternationalCompaniesMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:CollectiveAndMutualFundsMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:CorporateDebtSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:EquitiesSecuritiesOfInternationalCompaniesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:EquitySecuritiesOfUSCompaniesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:CollectiveAndMutualFundsMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:CollectiveAndMutualFundsMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:CashAndCashEquivalentsMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:MunicipalBondsMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:MunicipalBondsMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:EquitySecuritiesOfUSCompaniesMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:CorporateDebtSecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:CashAndCashEquivalentsMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-12-31 0000067716 srt:MaximumMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 srt:MinimumMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 srt:MinimumMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-01-01 2018-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-01-01 2017-12-31 0000067716 srt:MaximumMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:EquitySecuritiesOfUSCompaniesMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:EquitySecuritiesOfUSCompaniesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:CashAndCashEquivalentsMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:InsuranceInvestmentContractMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:EquitiesSecuritiesOfInternationalCompaniesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:CashAndCashEquivalentsMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:InsuranceInvestmentContractMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:EquitiesSecuritiesOfInternationalCompaniesMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:InsuranceInvestmentContractMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2016-01-01 2016-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2016-01-01 2016-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2017-01-01 2017-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-01-01 2018-12-31 0000067716 us-gaap:CorporateDebtSecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:MunicipalBondsMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:EquitiesSecuritiesOfInternationalCompaniesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:CashAndCashEquivalentsMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:CashAndCashEquivalentsMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:CorporateDebtSecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:MunicipalBondsMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:CollectiveAndMutualFundsMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:CollectiveAndMutualFundsMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:CollectiveAndMutualFundsMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:EquitySecuritiesOfUSCompaniesMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:EquitySecuritiesOfUSCompaniesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:USTreasurySecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:EquitiesSecuritiesOfInternationalCompaniesMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2016-12-31 0000067716 us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2016-12-31 0000067716 srt:MinimumMember us-gaap:EquitySecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 srt:MaximumMember us-gaap:FixedIncomeSecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 srt:MinimumMember us-gaap:FixedIncomeSecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 srt:MaximumMember us-gaap:EquitySecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 srt:MaximumMember us-gaap:EquitySecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 srt:MinimumMember us-gaap:FixedIncomeSecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 srt:MinimumMember us-gaap:EquitySecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:PensionPlansDefinedBenefitMember 2018-12-31 0000067716 srt:MaximumMember us-gaap:FixedIncomeSecuritiesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2018-12-31 0000067716 mdu:InsuranceInvestmentContractMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:EquitiesSecuritiesOfInternationalCompaniesMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:EquitySecuritiesOfUSCompaniesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:CashAndCashEquivalentsMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:EquitySecuritiesOfUSCompaniesMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:FairValueInputsLevel1Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:InsuranceInvestmentContractMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:EquitiesSecuritiesOfInternationalCompaniesMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:InsuranceInvestmentContractMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 mdu:TotalAssetsMeasuredatFairValueMember us-gaap:FairValueInputsLevel2Member us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:CashAndCashEquivalentsMember us-gaap:UnderfundedPlanMember us-gaap:QualifiedPlanMember us-gaap:OtherPostretirementBenefitPlansDefinedBenefitMember 2017-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:NonqualifiedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2017-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:NonqualifiedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2018-01-01 2018-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:NonqualifiedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2017-01-01 2017-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:NonqualifiedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2018-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:NonqualifiedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2016-01-01 2016-12-31 0000067716 us-gaap:OtherInvestmentsMember us-gaap:UnfundedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2018-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2018-12-31 0000067716 mdu:LifeInsuranceCarriedOnPlanParticipantsMember us-gaap:UnfundedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2017-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2017-12-31 0000067716 us-gaap:OtherInvestmentsMember us-gaap:UnfundedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2017-12-31 0000067716 us-gaap:EquitySecuritiesMember us-gaap:UnfundedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2018-12-31 0000067716 us-gaap:EquitySecuritiesMember us-gaap:UnfundedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2017-12-31 0000067716 mdu:LifeInsuranceCarriedOnPlanParticipantsMember us-gaap:UnfundedPlanMember us-gaap:SupplementalEmployeeRetirementPlanDefinedBenefitMember 2018-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:NonqualifiedPlanMember 2018-01-01 2018-12-31 0000067716 mdu:DefinedContributionPlanMember 2017-01-01 2017-12-31 0000067716 mdu:DefinedContributionPlanMember 2018-01-01 2018-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:NonqualifiedPlanMember 2016-01-01 2016-12-31 0000067716 mdu:DefinedContributionPlanMember 2016-01-01 2016-12-31 0000067716 us-gaap:UnfundedPlanMember us-gaap:NonqualifiedPlanMember 2017-01-01 2017-12-31 0000067716 mdu:MinnesotaTeamstersConstructionDivisionPensionFundMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:IbewLocal648PensionPlanMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:EdisonPensionPlanMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:SheetMetalWorkersPensionPlanofSouthernCAAZandNVMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:IdahoPlumbersandPipefittersPensionPlanMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:SouthwestMarinePensionTrustMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:IbewLocal648PensionPlanMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:IBEWLocalNo.357PensionPlanAMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:ConstructionIndustryandLaborersJointPensionTrustforSoNevadaPlanAMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:NationalElectricalBenefitFundMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:IbewLocalNo82PensionPlanMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:NationalAutomaticSprinklerIndustryPensionFundMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:IBEWLocalNo.357PensionPlanAMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:SheetMetalWorkersPensionPlanofSouthernCAAZandNVMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:PensionTrustFundforOperatingEngineersMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:LocalUnion212IbewPensionTrustFundMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:MinnesotaTeamstersConstructionDivisionPensionFundMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:EdisonPensionPlanMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 us-gaap:MultiemployerPlanIndividuallyInsignificantMultiemployerPlansMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:ConstructionIndustryandLaborersJointPensionTrustforSoNevadaPlanAMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:LocalUnion212IbewPensionTrustFundMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:EdisonPensionPlanMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:IbewLocalNo82PensionPlanMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:AlaskaLaborersEmployersRetirementFundMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:SouthwestMarinePensionTrustMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 us-gaap:MultiemployerPlanIndividuallyInsignificantMultiemployerPlansMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:NationalAutomaticSprinklerIndustryPensionFundMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:LocalUnion212IbewPensionTrustFundMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:NationalElectricalBenefitFundMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:IBEWLocalNo.357PensionPlanAMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:IdahoPlumbersandPipefittersPensionPlanMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:ConstructionIndustryandLaborersJointPensionTrustforSoNevadaPlanAMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:SouthwestMarinePensionTrustMember us-gaap:MultiemployerPlansPensionMember 2018-01-01 2018-12-31 0000067716 mdu:PensionTrustFundforOperatingEngineersMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 us-gaap:MultiemployerPlanIndividuallyInsignificantMultiemployerPlansMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:IdahoPlumbersandPipefittersPensionPlanMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:AlaskaLaborersEmployersRetirementFundMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:PensionTrustFundforOperatingEngineersMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:NationalAutomaticSprinklerIndustryPensionFundMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:IbewLocalNo82PensionPlanMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:AlaskaLaborersEmployersRetirementFundMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:SheetMetalWorkersPensionPlanofSouthernCAAZandNVMember us-gaap:MultiemployerPlansPensionMember 2016-01-01 2016-12-31 0000067716 mdu:MinnesotaTeamstersConstructionDivisionPensionFundMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:IbewLocal648PensionPlanMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:NationalElectricalBenefitFundMember us-gaap:MultiemployerPlansPensionMember 2017-01-01 2017-12-31 0000067716 mdu:MultiemployerPlansDefinedContributionMember 2018-01-01 2018-12-31 0000067716 us-gaap:WithdrawalFromMultiemployerDefinedBenefitPlanMember mdu:OperatingEngineersLocal800AndWyContractorsAssociationPensionPlanForWyMember us-gaap:MultiemployerPlansPensionMember 2016-12-31 0000067716 mdu:MultiemployerPlansDefinedContributionMember 2017-01-01 2017-12-31 0000067716 srt:MinimumMember 2018-01-01 2018-12-31 0000067716 us-gaap:MultiemployerPlansPostretirementBenefitMember 2017-01-01 2017-12-31 0000067716 us-gaap:MultiemployerPlansPostretirementBenefitMember 2018-01-01 2018-12-31 0000067716 mdu:MultiemployerPlansDefinedContributionMember 2016-01-01 2016-12-31 0000067716 srt:MaximumMember 2018-01-01 2018-12-31 0000067716 us-gaap:MultiemployerPlansPostretirementBenefitMember 2016-01-01 2016-12-31 0000067716 us-gaap:JointlyOwnedElectricityGenerationPlantMember mdu:CoyoteStationMember 2018-12-31 0000067716 us-gaap:JointlyOwnedElectricityGenerationPlantMember mdu:WygenIiiMember 2017-12-31 0000067716 us-gaap:JointlyOwnedElectricityGenerationPlantMember mdu:CoyoteStationMember 2017-12-31 0000067716 us-gaap:JointlyOwnedElectricityGenerationPlantMember mdu:BigStoneStationMember 2018-12-31 0000067716 us-gaap:JointlyOwnedElectricityGenerationPlantMember mdu:BigStoneStationMember 2017-12-31 0000067716 us-gaap:JointlyOwnedElectricityGenerationPlantMember mdu:WygenIiiMember 2018-12-31 0000067716 mdu:TaxCutsandJobsActMember mdu:NaturalgasrateproceedingMember mdu:NDPSCMember us-gaap:SubsequentEventMember 2019-01-23 2019-01-23 0000067716 mdu:TaxCutsandJobsActMember mdu:NaturalgasrateproceedingMember mdu:SDPUCMember 2018-12-28 2018-12-28 0000067716 mdu:NaturalgasrateproceedingMember mdu:NDPSCMember 2017-09-06 2017-09-06 0000067716 mdu:TaxCutsandJobsActMember mdu:ElectricRateProceedingMember mdu:NDPSCMember 2018-11-20 2018-11-20 0000067716 mdu:TaxCutsandJobsActMember mdu:ElectricRateProceedingMember mdu:NDPSCMember 2018-09-26 2018-09-26 0000067716 mdu:TaxCutsandJobsActMember mdu:ElectricRateProceedingMember mdu:NDPSCMember 2018-03-09 2018-03-09 0000067716 mdu:ElectricRateProceedingMember mdu:MtpscMember 2018-09-28 2018-09-28 0000067716 mdu:TaxCutsandJobsActMember mdu:OPUCMember 2018-05-31 2018-05-31 0000067716 mdu:TaxCutsandJobsActMember mdu:NaturalgasrateproceedingMember mdu:OPUCMember us-gaap:SubsequentEventMember 2019-01-22 2019-01-22 0000067716 mdu:TaxCutsandJobsActMember mdu:NaturalgasrateproceedingMember mdu:NDPSCMember 2018-03-01 2018-03-01 0000067716 mdu:NaturalgasrateproceedingMember mdu:SDPUCMember 2018-10-23 2018-10-23 0000067716 mdu:PipelineReplacementCostRecoveryMember mdu:WUTCMember 2018-06-01 2018-06-01 0000067716 mdu:NaturalgasrateproceedingMember mdu:NDPSCMember 2017-07-21 2017-07-21 0000067716 mdu:TaxCutsandJobsActMember mdu:NaturalgasrateproceedingMember mdu:NDPSCMember 2018-09-26 2018-09-26 0000067716 mdu:TaxCutsandJobsActMember mdu:NaturalgasrateproceedingMember mdu:MNPUCMember 2018-12-05 2018-12-05 0000067716 mdu:TaxCutsandJobsActMember mdu:NaturalgasrateproceedingMember mdu:NDPSCMember 2018-02-14 2018-02-14 0000067716 mdu:PipelineReplacementCostRecoveryMember mdu:WUTCMember 2018-10-11 2018-10-11 0000067716 mdu:TaxCutsandJobsActMember mdu:ElectricRateProceedingMember mdu:MtpscMember 2018-12-21 2018-12-21 0000067716 mdu:ElectrictransmissionrateproceedingMember mdu:MISOMember 2018-09-04 2018-09-04 0000067716 mdu:ElectricRateProceedingMember mdu:SDPUCMember 2018-10-23 2018-10-23 0000067716 mdu:TaxCutsandJobsActMember mdu:ElectricRateProceedingMember mdu:WYPSCMember 2018-03-30 2018-03-30 0000067716 mdu:TaxCutsandJobsActMember mdu:ElectricRateProceedingMember mdu:SDPUCMember 2018-12-28 2018-12-28 0000067716 mdu:TaxCutsandJobsActMember mdu:NaturalgasrateproceedingMember mdu:NDPSCMember us-gaap:SubsequentEventMember 2019-01-09 2019-01-09 0000067716 mdu:BremertonWAManufacturedGasPlantSiteMember 2018-01-01 2018-12-31 0000067716 mdu:BellinghamWAManufacturedGasPlantSiteMember 2018-01-01 2018-12-31 0000067716 us-gaap:FinancialGuaranteeMember 2018-12-31 0000067716 mdu:EugeneORManufacturedGasPlantSiteMember 2018-12-31 0000067716 srt:MinimumMember mdu:BellinghamWAManufacturedGasPlantSiteMember 2018-01-01 2018-12-31 0000067716 mdu:BremertonWAManufacturedGasPlantSiteMember 2018-12-31 0000067716 mdu:EugeneORManufacturedGasPlantSiteMember 2018-01-01 2018-12-31 0000067716 srt:MaximumMember mdu:BremertonWAManufacturedGasPlantSiteMember 2018-01-01 2018-12-31 0000067716 srt:MaximumMember mdu:BellinghamWAManufacturedGasPlantSiteMember 2018-01-01 2018-12-31 0000067716 mdu:PortlandHarborSiteMember 2017-01-06 0000067716 srt:MinimumMember mdu:BremertonWAManufacturedGasPlantSiteMember 2018-01-01 2018-12-31 0000067716 mdu:PortlandHarborSiteMember 2018-12-31 0000067716 mdu:FuelContractMember 2018-12-31 0000067716 2018-04-01 2018-06-30 0000067716 2017-07-01 2017-09-30 0000067716 2017-04-01 2017-06-30 0000067716 2018-07-01 2018-09-30 0000067716 2018-10-01 2018-12-31 0000067716 2018-01-01 2018-03-31 0000067716 2017-01-01 2017-03-31 0000067716 srt:ParentCompanyMember 2016-01-01 2016-12-31 0000067716 srt:ParentCompanyMember 2018-01-01 2018-12-31 0000067716 srt:ParentCompanyMember 2017-01-01 2017-12-31 0000067716 srt:ParentCompanyMember 2017-12-31 0000067716 srt:ParentCompanyMember 2016-12-31 0000067716 srt:ParentCompanyMember 2015-12-31 0000067716 us-gaap:AllowanceForCreditLossMember 2017-01-01 2017-12-31 0000067716 us-gaap:AllowanceForCreditLossMember 2016-12-31 0000067716 us-gaap:AllowanceForCreditLossMember 2017-12-31 0000067716 us-gaap:AllowanceForCreditLossMember 2018-01-01 2018-12-31 0000067716 us-gaap:AllowanceForCreditLossMember 2015-12-31 0000067716 us-gaap:AllowanceForCreditLossMember 2016-01-01 2016-12-31 0000067716 us-gaap:AllowanceForCreditLossMember 2018-12-31 xbrli:pure xbrli:shares iso4217:USD xbrli:shares iso4217:USD
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549
FORM 10-K
ý ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
For the fiscal year ended December 31, 2018
OR
o TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
For the transition period from _____________ to ______________
Commission file number 1-03480
MDU RESOURCES GROUP, INC.
(Exact name of registrant as specified in its charter)
Delaware
 
30-1133956
(State or other jurisdiction of
incorporation or organization)
 
(I.R.S. Employer Identification No.)
1200 West Century Avenue
P.O. Box 5650
Bismarck, North Dakota 58506-5650
(Address of principal executive offices)
(Zip Code)
(701) 530-1000
(Registrant's telephone number, including area code)
Securities registered pursuant to Section 12(b) of the Act:
Title of each class
 
Name of each exchange on which registered
Common Stock, par value $1.00
 
New York Stock Exchange
Securities registered pursuant to Section 12(g) of the Act: None
Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes ý No o.
Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Exchange Act. Yes o No ý.
Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes ý No o.
Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit such files). Yes ý No o.
Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K (§ 229.405 of this chapter) is not contained herein, and will not be contained, to the best of the registrant's knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K. ý
Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of "large accelerated filer," "accelerated filer," "smaller reporting company," and "emerging growth company" in Rule 12b-2 of the Exchange Act.
Large accelerated filer ý
Accelerated filer o
Non-accelerated filer o 
Smaller reporting company o
 
Emerging growth company o

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. o
Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes o No ý.
State the aggregate market value of the voting common stock held by nonaffiliates of the registrant as of June 29, 2018: $5,621,805,532.
Indicate the number of shares outstanding of the registrant's common stock, as of February 14, 2019: 196,092,274 shares.
DOCUMENTS INCORPORATED BY REFERENCE
Relevant portions of the registrant's 2019 Proxy Statement, to be filed no later than 120 days from December 31, 2018, are incorporated by reference in Part III, Items 10, 11, 12, 13 and 14 of this Report.




Contents
 

Part I
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Item 1A
 
 
 
Item 1B
 
 
 
Item 3
 
 
 
Item 4
 
 
 
Part II
 
 
 
 
Item 5
 
 
 
Item 6
 
 
 
Item 7
 
 
 
Item 7A
 
 
 
Item 8
 
 
 
Item 9
 
 
 
Item 9A
 
 
 
Item 9B
 
 
 
Part III
 
 
 
 
Item 10
 
 
 
Item 11
 
 
 
Item 12
 
 
 
Item 13
 
 
 
Item 14
 
 
 
Part IV
 
 
 
 
Item 15
 
 
 
Item 16
 
 
 
 
 
 
 
 
 

 
2 MDU Resources Group, Inc. Form 10-K



Definitions
 

The following abbreviations and acronyms used in this Form 10-K are defined below:
Abbreviation or Acronym
 
AFUDC
Allowance for funds used during construction
Army Corps
U.S. Army Corps of Engineers
ASC
FASB Accounting Standards Codification
ASU
FASB Accounting Standards Update
Audit Committee
Audit Committee of the board of directors of the Company
Bcf
Billion cubic feet
Big Stone Station
475-MW coal-fired electric generating facility near Big Stone City, South Dakota (22.7 percent ownership)
Brazilian Transmission Lines
Company's former investment in companies owning three electric transmission lines in Brazil
BSSE
345-kilovolt transmission line from Ellendale, North Dakota, to Big Stone City, South Dakota
Btu
British thermal unit
Calumet
Calumet Specialty Products Partners, L.P.
Cascade
Cascade Natural Gas Corporation, an indirect wholly owned subsidiary of MDU Energy Capital
Centennial
Centennial Energy Holdings, Inc., a direct wholly owned subsidiary of the Company
Centennial Capital
Centennial Holdings Capital LLC, a direct wholly owned subsidiary of Centennial
Centennial's Consolidated EBITDA
Centennial's consolidated net income from continuing operations plus the related interest expense, taxes, depreciation, depletion, amortization of intangibles and any non-cash charge relating to asset impairment for the preceding 12-month period
Centennial Resources
Centennial Energy Resources LLC, a direct wholly owned subsidiary of Centennial
CERCLA
Comprehensive Environmental Response, Compensation and Liability Act
Clean Air Act
Federal Clean Air Act
Clean Water Act
Federal Clean Water Act
Company
MDU Resources Group, Inc. (formerly known as MDUR Newco), which, as the context requires, refers to the previous MDU Resources Group, Inc. prior to January 1, 2019, and the new holding company of the same name after January 1, 2019
Coyote Creek
Coyote Creek Mining Company, LLC, a subsidiary of The North American Coal Corporation
Coyote Station
427-MW coal-fired electric generating facility near Beulah, North Dakota (25 percent ownership)
CyROC
Cyber Risk Oversight Committee
Dakota Prairie Refinery
20,000-barrel-per-day diesel topping plant built by Dakota Prairie Refining in southwestern North Dakota
Dakota Prairie Refining
Dakota Prairie Refining, LLC, a limited liability company previously owned by WBI Energy and Calumet (previously included in the Company's refining segment)
D.C. Circuit Court
United States Court of Appeals for the District of Columbia Circuit
dk
Decatherm
Dodd-Frank Act
Dodd-Frank Wall Street Reform and Consumer Protection Act
EBITDA
Earnings before interest, taxes, depreciation, depletion and amortization
EIN
Employer Identification Number
EPA
United States Environmental Protection Agency
ERISA
Employee Retirement Income Security Act of 1974
ESA
Endangered Species Act
Exchange Act
Securities Exchange Act of 1934, as amended
FASB
Financial Accounting Standards Board
FERC
Federal Energy Regulatory Commission
Fidelity
Fidelity Exploration & Production Company, a direct wholly owned subsidiary of WBI Holdings (previously referred to as the Company's exploration and production segment)
FIP
Funding improvement plan
GAAP
Accounting principles generally accepted in the United States of America
GHG
Greenhouse gas
Great Plains
Great Plains Natural Gas Co., a public utility division of the Company prior to the closing of the Holding Company Reorganization and a public utility division of Montana-Dakota as of January 1, 2019
GVTC
Generation Verification Test Capacity

 
MDU Resources Group, Inc. Form 10-K 3



Definitions
 

Holding Company Reorganization
The internal holding company reorganization completed on January 1, 2019, pursuant to the agreement and plan of merger, dated as of December 31, 2018, by and among Montana-Dakota, the Company and MDUR Newco Sub, which resulted in the Company becoming a holding company and owning all of the outstanding capital stock of Montana-Dakota.
IBEW
International Brotherhood of Electrical Workers
ICWU
International Chemical Workers Union
Intermountain
Intermountain Gas Company, an indirect wholly owned subsidiary of MDU Energy Capital
IPUC
Idaho Public Utilities Commission
Item 8
Financial Statements and Supplementary Data
Knife River
Knife River Corporation, a direct wholly owned subsidiary of Centennial
Knife River - Northwest
Knife River Corporation - Northwest, an indirect wholly owned subsidiary of Knife River
K-Plan
Company's 401(k) Retirement Plan
kW
Kilowatts
kWh
Kilowatt-hour
LWG
Lower Willamette Group
MD&A
Management's Discussion and Analysis of Financial Condition and Results of Operations
Mdk
Thousand dk
MDU Construction Services
MDU Construction Services Group, Inc., a direct wholly owned subsidiary of Centennial
MDU Energy Capital
MDU Energy Capital, LLC, a direct wholly owned subsidiary of the Company
MDUR Newco
MDUR Newco, Inc., a public holding company created by implementing the Holding Company Reorganization, now known as the Company
MDUR Newco Sub
MDUR Newco Sub, Inc., a direct, wholly owned subsidiary of MDUR Newco, which was merged with and into Montana–Dakota in the Holding Company Reorganization
MEPP
Multiemployer pension plan
MISO
Midcontinent Independent System Operator, Inc.
MMBtu
Million Btu
MMcf
Million cubic feet
MMdk
Million dk
MNPUC
Minnesota Public Utilities Commission
Montana-Dakota
Montana-Dakota Utilities Co. (formerly known as MDU Resources Group, Inc.), a public utility division of the Company prior to the closing of the Holding Company Reorganization and a direct wholly owned subsidiary of MDU Energy Capital as of January 1, 2019
Montana DEQ
Montana Department of Environmental Quality
MPPAA
Multiemployer Pension Plan Amendments Act of 1980
MTPSC
Montana Public Service Commission
MW
Megawatt
NDPSC
North Dakota Public Service Commission
NERC
North American Electric Reliability Corporation
NGL
Natural gas liquids
Non-GAAP
Not in accordance with GAAP
Oil
Includes crude oil and condensate
OPUC
Oregon Public Utility Commission
Oregon DEQ
Oregon State Department of Environmental Quality
PCBs
Polychlorinated biphenyls
Pronghorn
Natural gas processing plant located near Belfield, North Dakota (WBI Energy Midstream's 50 percent ownership interests were sold effective January 1, 2017)
Proxy Statement
Company's 2019 Proxy Statement to be filed no later than April 30, 2019
PRP
Potentially Responsible Party
RCRA
Resource Conservation and Recovery Act
ROD
Record of Decision
RP
Rehabilitation plan
SDPUC
South Dakota Public Utilities Commission
SEC
United States Securities and Exchange Commission

 
4 MDU Resources Group, Inc. Form 10-K



Definitions
 

Securities Act
Securities Act of 1933, as amended
Securities Act Industry Guide 7
Description of Property by Issuers Engaged or to be Engaged in Significant Mining Operations
Sheridan System
A separate electric system owned by Montana-Dakota
SSIP
System Safety and Integrity Program
Stock Purchase Plan
Company's Dividend Reinvestment and Direct Stock Purchase Plan which was terminated effective December 5, 2016
TCJA
Tax Cuts and Jobs Act
Tesoro
Tesoro Refining & Marketing Company LLC
Thurston County Superior Court
State of Washington Thurston County Superior Court
UA
United Association of Journeyman and Apprentices of the Plumbing and Pipefitting Industry of the United States and Canada
United States Supreme Court
Supreme Court of the United States
VIE
Variable interest entity
Washington DOE
Washington State Department of Ecology
WBI Energy
WBI Energy, Inc., a direct wholly owned subsidiary of WBI Holdings
WBI Energy Midstream
WBI Energy Midstream, LLC, an indirect wholly owned subsidiary of WBI Holdings
WBI Energy Transmission
WBI Energy Transmission, Inc., an indirect wholly owned subsidiary of WBI Holdings
WBI Holdings
WBI Holdings, Inc., a direct wholly owned subsidiary of Centennial
WUTC
Washington Utilities and Transportation Commission
Wygen III
100-MW coal-fired electric generating facility near Gillette, Wyoming (25 percent ownership)
WYPSC
Wyoming Public Service Commission
ZRCs
Zonal resource credits - a MW of demand equivalent assigned to generators by MISO for meeting system reliability requirements

 
MDU Resources Group, Inc. Form 10-K 5



Part I
 


Forward-Looking Statements
This Form 10-K contains forward-looking statements within the meaning of Section 21E of the Exchange Act. Forward-looking statements are all statements other than statements of historical fact, including without limitation those statements that are identified by the words "anticipates," "estimates," "expects," "intends," "plans," "predicts" and similar expressions, and include statements concerning plans, objectives, goals, strategies, future events or performance, and underlying assumptions (many of which are based, in turn, upon further assumptions) and other statements that are other than statements of historical facts. From time to time, the Company may publish or otherwise make available forward-looking statements of this nature, including statements contained within Item 7 - MD&A - Business Segment Financial and Operating Data.
Forward-looking statements involve risks and uncertainties, which could cause actual results or outcomes to differ materially from those expressed. The Company's expectations, beliefs and projections are expressed in good faith and are believed by the Company to have a reasonable basis, including without limitation, management's examination of historical operating trends, data contained in the Company's records and other data available from third parties. Nonetheless, the Company's expectations, beliefs or projections may not be achieved or accomplished.
Any forward-looking statement contained in this document speaks only as of the date on which the statement is made, and the Company undertakes no obligation to update any forward-looking statement or statements to reflect events or circumstances that occur after the date on which the statement is made or to reflect the occurrence of unanticipated events. New factors emerge from time to time, and it is not possible for management to predict all of the factors, nor can it assess the effect of each factor on the Company's business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statement. All forward-looking statements, whether written or oral and whether made by or on behalf of the Company, are expressly qualified by the risk factors and cautionary statements in this Form 10-K, including statements contained within Item 1A - Risk Factors.
Items 1 and 2. Business and Properties
General
The Company is a regulated energy delivery and construction materials and services business. Montana-Dakota was incorporated under the laws of the state of Delaware in 1924. The Company was incorporated under the laws of the state of Delaware in 2018. Its principal executive offices are at 1200 West Century Avenue, P.O. Box 5650, Bismarck, North Dakota 58506-5650, telephone (701) 530-1000.
On November 21, 2017, the Company announced that its board of directors had directed senior management to explore reorganization to a holding company structure. The purpose of the reorganization was to make the public utility divisions into a subsidiary of the holding company, just as the other operating companies are wholly owned subsidiaries. On November 15, 2018, the board of directors approved the Holding Company Reorganization and authorized senior management to take the necessary and appropriate actions to effectuate the Holding Company Reorganization. On January 2, 2019, the Company announced the completion of the Holding Company Reorganization, which resulted in Montana-Dakota and Great Plains becoming a subsidiary of the Company. The merger was conducted pursuant to Section 251(g) of the General Corporation Law of the State of Delaware, which provides for the formation of a holding company without a vote of the stockholders of the constituent corporation. Immediately after consummation of the Holding Company Reorganization, the Company had, on a consolidated basis, the same assets, businesses and operations as Montana-Dakota had immediately prior to the consummation of the Holding Company Reorganization. As a result of the Holding Company Reorganization, the Company became the successor issuer to Montana-Dakota pursuant to Rule 12g-3(a) of the Exchange Act, and as a result, the Company's common stock was deemed registered under Section 12(b) of the Exchange Act.
The Company operates with a two-platform business model. Its regulated energy delivery platform and its construction materials and services platform are each comprised of different operating segments. Some of these segments experience seasonality related to the industries in which they operate. The two-platform approach helps balance this seasonality and the risk associated with each type of industry. Through its regulated energy delivery platform, the Company provides electric and natural gas services to customers, generates, transmits and distributes electricity, and provides natural gas transportation, storage and gathering services. These businesses are regulated by state public service commissions and/or the FERC. The construction materials and services platform provides construction services to a variety of industries, including commercial, industrial and governmental, and provides construction materials through aggregate mining and marketing of related products, such as ready-mixed concrete and asphalt.
The Company is organized into five reportable business segments. These business segments include: electric, natural gas distribution, pipeline and midstream, construction materials and contracting, and construction services. The Company's business segments are determined based on the Company's method of internal reporting, which generally segregates the strategic business units due to differences

 
6 MDU Resources Group, Inc. Form 10-K



Part I
 

in products, services and regulation. The internal reporting of these segments is defined based on the reporting and review process used by the Company's chief executive officer.
The Company, through its wholly owned subsidiary, MDU Energy Capital, owns Montana-Dakota, Cascade and Intermountain. Montana-Dakota, Cascade and Intermountain are the natural gas distribution segment. Montana-Dakota also comprises the electric segment.
The Company, through its wholly owned subsidiary, Centennial, owns WBI Holdings, Knife River, MDU Construction Services, Centennial Resources and Centennial Capital. WBI Holdings is the pipeline and midstream segment, Knife River is the construction materials and contracting segment, MDU Construction Services is the construction services segment, and Centennial Resources and Centennial Capital are both reflected in the Other category.
For more information on the Company's business segments, see Item 8 - Note 15.
As of December 31, 2018, the Company had 11,797 employees with 218 employed at MDU Resources Group, Inc., 1,004 at Montana-Dakota, 338 at Cascade, 242 at Intermountain, 317 at WBI Holdings, 3,967 at Knife River and 5,711 at MDU Construction Services. The number of employees at certain Company operations fluctuates during the year depending upon the number and size of construction projects. The Company considers its relations with employees to be satisfactory.
The following information regarding the number of employees represented by labor contracts is as of December 31, 2018.
At Montana-Dakota and WBI Energy Transmission, 349 and 70 employees, respectively, are represented by the IBEW. Labor contracts with such employees are in effect through April 30, 2021, and March 31, 2022, respectively.
At Cascade, 197 employees are represented by the ICWU. The labor contract with the field operations group is effective through March 31, 2021.
At Intermountain, 130 employees are represented by the UA. Labor contracts with such employees are in effect through September 30, 2019.
Knife River operates under 43 labor contracts that represent 673 of its construction materials and contracting employees. Knife River is in negotiations on five of its labor contracts.
MDU Construction Services has 126 labor contracts representing the majority of its employees. MDU Construction Services is not currently in negotiations on any of its labor contracts.
The majority of the labor contracts contain provisions that prohibit work stoppages or strikes and provide for binding arbitration dispute resolution in the event of an extended disagreement.
The Company's principal properties, which are of varying ages and are of different construction types, are generally in good condition, are well maintained and are generally suitable and adequate for the purposes for which they are used.
The financial results and data applicable to each of the Company's business segments, as well as their financing requirements, are set forth in Item 7 - MD&A and Item 8 - Note 15 and Supplementary Financial Information.
The operations of the Company and certain of its subsidiaries are subject to federal, state and local laws and regulations providing for air, water and solid waste pollution control; state facility-siting regulations; zoning and planning regulations of certain state and local authorities; federal health and safety regulations; and state hazard communication standards. The Company believes that it is in substantial compliance with these regulations, except as to what may be ultimately determined with regard to items discussed in Environmental matters in Item 8 - Note 19. There are no pending CERCLA actions for any of the Company's properties, other than the Portland, Oregon, Harbor Superfund Site and the Bremerton Gasworks Superfund Site.
The Company produces GHG emissions primarily from its fossil fuel electric generating facilities, as well as from natural gas pipeline and storage systems, and operations of equipment and fleet vehicles. GHG emissions also result from customer use of natural gas for heating and other uses. As interest in reductions in GHG emissions has grown, the Company has developed renewable generation with lower or no GHG emissions. Governmental legislative and regulatory initiatives regarding environmental and energy policy are continuously evolving and could negatively impact the Company's operations and financial results. Until legislation and regulation are finalized, the impact of these measures cannot be accurately predicted. The Company will continue to monitor legislative and regulatory activity related to environmental and energy policy initiatives. Disclosure regarding specific environmental matters applicable to each of the Company's businesses is set

 
MDU Resources Group, Inc. Form 10-K 7



Part I
 

forth under each business description later. In addition, for a discussion of the Company's risks related to environmental laws and regulations, see Item 1A - Risk Factors.
The Company uses technology in substantially all aspects of its business operations and requires uninterrupted operation of information technology systems and network infrastructure. These systems may be vulnerable to failures or unauthorized access. The Company has policies, procedures and processes in place designed to strengthen and protect these systems, which includes the Company’s enterprise information technology and operation technology groups continually evaluating new tools and techniques that can be implemented to reduce the risk of a cyber breach.
The Company created CyROC to oversee the Company’s approach to cybersecurity. CyROC is responsible for supplying management at all levels and the Audit Committee with analyses, appraisals, recommendations and pertinent information concerning cyber defense of the Company’s electronic information and information technology systems. CyROC provides a quarterly cybersecurity report to the Audit Committee. For a discussion of the Company's risks related to cybersecurity, see Item 1A - Risk Factors.
This annual report on Form 10-K, the Company's quarterly reports on Form 10-Q and current reports on Form 8-K, and any amendments to those reports filed or furnished pursuant to Section 13(a) or 15(d) of the Exchange Act are available free of charge through the Company's Web site as soon as reasonably practicable after the Company has electronically filed such reports with, or furnished such reports to, the SEC. The Company's Web site address is www.mdu.com. The information available on the Company's Web site is not part of this annual report on Form 10-K.
Electric
General Montana-Dakota provides electric service at retail, serving 143,022 residential, commercial, industrial and municipal customers in 184 communities and adjacent rural areas in Montana, North Dakota, South Dakota and Wyoming as of December 31, 2018. For more information on the retail customer classes served, see the table below. The principal properties owned by Montana-Dakota for use in its electric operations include interests in 16 electric generating units at 11 facilities and two small portable diesel generators, as further described under System Supply, System Demand and Competition, approximately 3,200 and 4,900 miles of transmission and distribution lines, respectively, and 75 transmission and 297 distribution substations. Montana-Dakota has obtained and holds, or is in the process of renewing, valid and existing franchises authorizing it to conduct its electric operations in all of the municipalities it serves where such franchises are required. Montana-Dakota intends to protect its service area and seek renewal of all expiring franchises. At December 31, 2018, Montana-Dakota's net electric plant investment was $1.5 billion and rate base was $1.2 billion.
The retail customers served and respective revenues by class for the electric business were as follows:
 
2018
2017
2016
 
Customers
Served

Revenues

Customers
Served

Revenues

Customers
Served

Revenues

 
(Dollars in thousands)
Residential
118,426

$
126,173

118,379

$
121,171

118,483

$
117,014

Commercial
22,756

141,961

22,764

140,856

22,693

135,390

Industrial
236

36,081

242

34,417

244

31,913

Other
1,604

7,882

1,516

8,275

1,528

7,580

 
143,022

$
312,097

142,901

$
304,719

142,948

$
291,897

Other electric revenues for Montana-Dakota were $23.0 million, $38.1 million and $30.4 million for the years ended December 31, 2018, 2017 and 2016, respectively.
The percentage of electric retail revenues by jurisdiction was as follows:
 
2018

2017

2016

North Dakota
66
%
66
%
68
%
Montana
20
%
20
%
19
%
Wyoming
9
%
9
%
8
%
South Dakota
5
%
5
%
5
%
Retail electric rates, service, accounting and certain security issuances are subject to regulation by the MTPSC, NDPSC, SDPUC and WYPSC. The interstate transmission and wholesale electric power operations of Montana-Dakota also are subject to regulation by the FERC under provisions of the Federal Power Act, as are interconnections with other utilities and power generators, the issuance of certain securities, accounting and other matters.

 
8 MDU Resources Group, Inc. Form 10-K



Part I
 

Through MISO, Montana-Dakota has access to wholesale energy, ancillary services and capacity markets for its interconnected system. MISO is a regional transmission organization responsible for operational control of the transmission systems of its members. MISO provides security center operations, tariff administration and operates day-ahead and real-time energy markets, ancillary services and capacity markets. As a member of MISO, Montana-Dakota's generation is sold into the MISO energy market and its energy needs are purchased from that market.
System Supply, System Demand and Competition Through an interconnected electric system, Montana-Dakota serves markets in portions of western North Dakota, eastern Montana and northern South Dakota. The interconnected system consists of 15 electric generating units at 10 facilities and two small portable diesel generators, which have an aggregate nameplate rating attributable to Montana-Dakota's interest of 750,318 kW and total net ZRCs of 532.3 in 2018. ZRCs are a MW of demand equivalent measure and are allocated to individual generators to meet planning reserve margin requirements within MISO. For 2018, Montana-Dakota's total ZRCs, including its firm purchase power contracts, were 574.5. Montana-Dakota's planning reserve margin requirement within MISO was 537.2 for 2018. The maximum electric peak demand experienced to date attributable to Montana-Dakota's sales to retail customers on the interconnected system was 611,542 kW in August 2015. Montana-Dakota's latest forecast for its interconnected system indicates that its annual peak will continue to occur during the summer and the sales growth rate through 2023 will approximate two percent annually. Montana-Dakota's interconnected system electric generating capability includes five steam-turbine generating units at four facilities using coal for fuel, four combustion turbine units at three facilities, three wind electric generating facilities, two reciprocating internal combustion engines at one facility, a heat recovery electric generating facility and two small portable diesel generators.
In June 2016, Montana-Dakota and a partner began construction on the BSSE project within the footprint of MISO. Montana-Dakota began bringing the project on-line on February 5, 2019. On October 31, 2018, the Company finalized the purchase and placed into service the Thunder Spirit Wind farm expansion in southwest North Dakota, which includes 16 turbines. With the addition of the expansion, the total Thunder Spirit Wind farm generation capacity is approximately 155 MW. The original 107.5-MW wind farm includes 43 turbines; it was purchased by Montana-Dakota in December 2015. For more information on these projects, see Item 7 - MD&A - Electric and Natural Gas Distribution.
Additional energy is purchased as needed, or in lieu of generation if more economical, from the MISO market, and in 2018, Montana-Dakota purchased approximately 22 percent of its net kWh needs for its interconnected system through the MISO market.
Approximately 21 percent of the electricity delivered to customers from Montana-Dakota's owned generation in 2018 was from renewable resources. Although Montana-Dakota's generation resource capacity has increased to serve the needs of customers, the carbon dioxide emission intensity of the electric generation resource fleet has been reduced by approximately 24 percent since 2003 and is expected to continue to decline.
Through the Sheridan System, Montana-Dakota serves Sheridan, Wyoming, and neighboring communities. The maximum peak demand experienced to date attributable to Montana-Dakota sales to retail customers on that system was approximately 63,686 kW in July 2018. Montana-Dakota has a power supply contract with Black Hills Power, Inc. to purchase up to 49,000 kW of capacity annually through December 31, 2023. Wygen III serves a portion of the needs of its Sheridan-area customers.

 
MDU Resources Group, Inc. Form 10-K 9



Part I
 

The following table sets forth details applicable to the Company's electric generating stations:
Generating Station
Type
Nameplate Rating (kW)

2018 ZRCs

(a) 
2018 Net Generation (kWh in thousands)

Interconnected System:
 
 
 
 
 
North Dakota:
 
 
 
 
 
Coyote (b)
Steam
103,647

80.8

 
765,233

Heskett
Steam
86,000

87.4

 
504,357

Heskett
Combustion Turbine
89,038

61.6

 
3,981

Glen Ullin
Heat Recovery
7,500

4.8

 
44,940

Cedar Hills
Wind
19,500

4.4

 
49,933

Diesel Units
Oil
3,650

3.6

 
6

Thunder Spirit
Wind
155,500

21.1

 
407,947

South Dakota:
 
 
 
 
 
Big Stone (b)
Steam
94,111

103.8

 
521,187

Montana:
 
 
 
 
 
Lewis & Clark
Steam
44,000

50.3

 
235,882

Lewis & Clark
Reciprocating Internal Combustion Engine
18,700

16.7

 
8,497

Glendive
Combustion Turbine
75,522

70.6

 
2,734

Miles City
Combustion Turbine
23,150

21.6

 
273

Diamond Willow
Wind
30,000

5.6

 
86,103

 
 
750,318

532.3

 
2,631,073

Sheridan System:
 
 

 

 
 

Wyoming:
 
 
 
 
 

Wygen III (b)
Steam
28,000

N/A

 
209,280

 
 
778,318

532.3

 
2,840,353

(a)
Interconnected system only. MISO requires generators to obtain their summer capability through the GVTC. The GVTC is then converted to ZRCs by applying each generator's forced outage factor against its GVTC. Wind generator's ZRCs are calculated based on a wind capacity study performed annually by MISO. ZRCs are used to meet supply obligations within MISO.
(b)
Reflects Montana-Dakota's ownership interest.
 
Virtually all of the current fuel requirements of the Heskett and Lewis & Clark stations are met with coal supplied by subsidiaries of Westmoreland Coal Company under contracts that expire in December 2021 and December 2020, respectively. The Heskett and Lewis & Clark coal supply agreements provide for the purchase of coal necessary to supply the coal requirements of these stations at contracted pricing. Montana-Dakota estimates the Heskett and Lewis & Clark coal requirement to be in the range of 425,000 to 460,000 tons and 250,000 to 350,000 tons per contract year, respectively.
The owners of Coyote Station, including Montana-Dakota, have a contract with Coyote Creek for coal supply to the Coyote Station that expires December 2040. Montana-Dakota estimates the Coyote Station coal supply agreement to be approximately 2.5 million tons per contract year. For more information, see Item 8 - Note 19.
The owners of Big Stone Station, including Montana-Dakota, have a coal supply agreement with Peabody COALSALES, LLC to meet all of the Big Stone Station's fuel requirements for 2019 and 2020, with the exception of 250,000 tons in 2019, which was previously committed to be purchased from Contura Coal Sales, LLC, all at contracted pricing.
Montana-Dakota has a coal supply agreement with Wyodak Resources Development Corp., to supply the coal requirements of Wygen III at contracted pricing through June 1, 2060. Montana-Dakota estimates the maximum annual coal consumption of the facility to be 585,000 tons.
The average cost of coal purchased, including freight, at Montana-Dakota's electric generating stations (including the Big Stone, Coyote and Wygen III stations) was as follows:
Years ended December 31,
2018

2017

2016

Average cost of coal per MMBtu
$
2.00

$
2.07

$
1.89

Average cost of coal per ton
$
29.08

$
30.04

$
27.45


 
10 MDU Resources Group, Inc. Form 10-K



Part I
 

Montana-Dakota expects that it has secured adequate capacity available through existing baseload generating stations, renewable generation, turbine peaking stations, demand reduction programs and firm contracts to meet the peak customer demand requirements of its customers through 2028. Future capacity that is needed to replace contracts, generation retirements and meet system growth requirements is expected to be met by constructing new generation resources, or acquiring additional capacity through power purchase contracts or the MISO capacity auction.
Montana-Dakota has major interconnections with its neighboring utilities and considers these interconnections adequate for coordinated planning, emergency assistance, exchange of capacity and energy and power supply reliability.
Montana-Dakota is subject to competition in varying degrees, in certain areas, from rural electric cooperatives, on-site generators, co-generators and municipally owned systems. In addition, competition in varying degrees exists between electricity and alternative forms of energy such as natural gas.
Regulatory Matters and Revenues Subject to Refund In North Dakota, Montana-Dakota's results of operations reflect monthly increases or decreases in electric fuel and purchased power costs (including demand charges) and Montana-Dakota is deferring those electric fuel and purchased power costs that are greater or less than amounts presently being recovered through its existing rate schedules. In Montana, a monthly Fuel and Purchased Power Tracking Adjustment mechanism allows Montana-Dakota's results of operations to reflect 90 percent of the increases or decreases in electric fuel and purchased power costs (including demand charges) and Montana-Dakota is deferring 90 percent of costs that are greater or less than amounts presently being recovered through its existing rate schedules. A fuel adjustment clause contained in South Dakota's jurisdictional electric rate schedules allows Montana-Dakota's results of operations to reflect monthly increases or decreases in electric fuel and purchased power costs. In Wyoming, an annual Electric Power Supply Cost Adjustment mechanism allows Montana-Dakota's results of operations to reflect increases or decreases in purchased power costs (including demand charges) related to power supply and Montana-Dakota is deferring costs that are greater or less than amounts presently being recovered through its existing rate schedules. Such orders generally provide that these amounts are recoverable or refundable through rate adjustments which are filed annually. Montana-Dakota's results of operations reflect 95 percent of the increases or decreases from the base purchased power costs and in addition also reflects 85 percent of the increases or decreases from the base coal price, which is also recovered through the Electric Power Supply Cost Adjustment. For more information on regulatory assets and liabilities, see Item 8 - Note 6.
For the Thunder Spirit Wind project, Montana-Dakota implemented a renewable resource cost adjustment rider, and all of Montana-Dakota's wind resources pertaining to North Dakota electric operations were placed in this rider upon a final order of the most recent North Dakota electric general rate case. Montana-Dakota also has in place in North Dakota a transmission tracker to recover transmission costs associated with MISO and the Southwest Power Pool, regional transmission organizations serving parts of Montana-Dakota's system, along with certain of the transmission investments not recovered through retail rates. The tracking mechanism has an annual true-up.
In South Dakota, Montana-Dakota recovers the South Dakota investment in the Thunder Spirit Wind project through an Infrastructure Rider tracking mechanism that is subject to an annual true-up. Montana-Dakota also has in place in South Dakota a transmission tracker to recover transmission costs associated with MISO and the Southwest Power Pool, regional transmission organizations serving parts of Montana-Dakota's system, along with certain of the transmission investments not recovered through retail rates. This tracking mechanism also has an annual true-up.
In Montana, Montana-Dakota recovers in rates through a tracking mechanism the increases associated with its allocated share of Montana state and local taxes assessed to electric operations on an after tax basis.
For more information on regulatory matters, see Item 8 - Note 18.
Environmental Matters Montana-Dakota's electric operations are subject to federal, state and local laws and regulations providing for air, water and solid waste pollution control; state facility-siting regulations; zoning and planning regulations of certain state and local authorities; federal health and safety regulations; and state hazard communication standards. Montana-Dakota believes it is in substantial compliance with these regulations.
Montana-Dakota's electric generating facilities have Title V Operating Permits, under the Clean Air Act, issued by the states in which they operate. Each of these permits has a five-year life. Near the expiration of these permits, renewal applications are submitted. Permits continue in force beyond the expiration date, provided the application for renewal is submitted by the required date, usually six months prior to expiration. The Title V Operating Permit renewal application for Coyote Station was submitted timely to the North Dakota Department of Health in September 2017, with the permit expected to be issued in 2019. Wygen III is allowed to operate under the facility's construction permit until the Title V Operating Permit is issued by the Wyoming Department of Environmental Quality. The Title V Operating Permit application for Wygen III was submitted timely in January 2011, with the permit expected to be issued in 2019.

 
MDU Resources Group, Inc. Form 10-K 11



Part I
 

State water discharge permits issued under the requirements of the Clean Water Act are maintained for power production facilities on the Yellowstone and Missouri rivers. These permits also have five-year lives. Montana-Dakota renews these permits as necessary prior to expiration. Other permits held by these facilities may include an initial siting permit, which is typically a one-time, preconstruction permit issued by the state; state permits to dispose of combustion by-products; state authorizations to withdraw water for operations; and Army Corps permits to construct water intake structures. Montana-Dakota's Army Corps permits grant one-time permission to construct and do not require renewal. Other permit terms vary and the permits are renewed as necessary.
Montana-Dakota's electric operations are very small-quantity generators of hazardous waste and subject only to minimum regulation under the RCRA. Montana-Dakota routinely handles PCBs from its electric operations in accordance with federal requirements. PCB storage areas are registered with the EPA as required.
Montana-Dakota incurred $9.2 million of environmental capital expenditures in 2018, mainly for coal ash management projects at Lewis & Clark Station, Big Stone Station and Coyote Station. Environmental capital expenditures are estimated to be $6.8 million, $2.7 million and $1.8 million in 2019, 2020 and 2021, respectively, for various environmental projects, including coal ash management at power plants. Montana-Dakota's capital and operational expenditures could also be affected by future air emission regulations, including a future GHG regulation that may replace the Clean Power Plan rule published by the EPA in October 2015. The Clean Power Plan requires existing fossil fuel-fired electric generating facilities to reduce carbon dioxide emissions. For more information, see Item 1A - Risk Factors.
Natural Gas Distribution
General The Company's natural gas distribution operations consist of Montana-Dakota, Cascade and Intermountain, which sell natural gas at retail, serving 957,727 residential, commercial and industrial customers in 339 communities and adjacent rural areas across eight states as of December 31, 2018, and provide natural gas transportation services to certain customers on the Company's systems. For more information on the retail customer classes served, see the table below. These services are provided through distribution systems aggregating approximately 19,400 miles. The natural gas distribution operations have obtained and hold, or are in the process of renewing, valid and existing franchises authorizing them to conduct their natural gas operations in all of the municipalities they serve where such franchises are required. These operations intend to protect their service areas and seek renewal of all expiring franchises. At December 31, 2018, the natural gas distribution operations' net natural gas distribution plant investment was $1.7 billion and rate base was $1.1 billion.
The retail customers served and respective revenues by class for the natural gas distribution operations were as follows:
 
2018
2017
2016
 
Customers
Served

Revenues

Customers
Served

Revenues

Customers
Served

Revenues

 
(Dollars in thousands)
Residential
850,595

$
464,697

833,255

$
477,699

818,163

$
429,828

Commercial
106,297

279,566

104,795

283,899

103,438

253,333

Industrial
835

24,555

817

24,030

807

23,337

 
957,727

$
768,818

938,867

$
785,628

922,408

$
706,498

Transportation and other revenues for the natural gas distribution operations were $54.4 million, $62.8 million and $59.6 million for the years ended December 31, 2018, 2017 and 2016, respectively.
The percentage of the natural gas distribution operations' retail sales revenues by jurisdiction was as follows:
 
2018

2017

2016

Idaho
30
%
33
%
34
%
Washington
26
%
26
%
26
%
North Dakota
15
%
13
%
13
%
Montana
9
%
9
%
8
%
Oregon
8
%
8
%
8
%
South Dakota
7
%
6
%
6
%
Minnesota
3
%
3
%
3
%
Wyoming
2
%
2
%
2
%
The natural gas distribution operations are subject to regulation by the IPUC, MNPUC, MTPSC, NDPSC, OPUC, SDPUC, WUTC and WYPSC regarding retail rates, service, accounting and certain security issuances.

 
12 MDU Resources Group, Inc. Form 10-K



Part I
 

System Supply, System Demand and Competition The natural gas distribution operations serve retail natural gas markets, consisting principally of residential and firm commercial space and water heating users, in portions of Idaho; western Minnesota; eastern Montana; North Dakota; central and eastern Oregon; western and north-central South Dakota; western, southeastern and south-central Washington; and northern Wyoming. These markets are highly seasonal and sales volumes depend largely on the weather, the effects of which are mitigated in certain jurisdictions by a weather normalization mechanism discussed in Regulatory Matters. In addition to the residential and commercial sales, the utilities transport natural gas for larger commercial and industrial customers who purchase their own supply of natural gas.
Competition in varying degrees exists between natural gas and other fuels and forms of energy. The natural gas distribution operations have established various natural gas transportation service rates for their distribution businesses to retain interruptible commercial and industrial loads. These services have enhanced the natural gas distribution operations' competitive posture with alternative fuels, although certain customers have bypassed the distribution systems by directly accessing transmission pipelines within close proximity. These bypasses did not have a material effect on results of operations.
The natural gas distribution operations and various distribution transportation customers obtain their system requirements directly from producers, processors and marketers. The Company's purchased natural gas is supplied by a portfolio of contracts specifying market-based pricing and is transported under transportation agreements with WBI Energy Transmission, Northern Border Pipeline Company, Northwest Pipeline LLC, South Dakota Intrastate Pipeline, TransCanada Corporation, Northern Natural Gas, Gas Transmission Northwest LLC, Northwestern Energy, Viking Gas Transmission Company, Westcoast Energy Inc., Ruby Pipeline LLC, Foothills Pipe Lines Ltd. and NOVA Gas Transmission Ltd. The natural gas distribution operations have contracts for storage services to provide gas supply during the winter heating season and to meet peak day demand with various storage providers, including WBI Energy Transmission, Dominion Energy Questar Pipeline, LLC, Northwest Pipeline LLC and Northern Natural Gas. In addition, certain of the operations have entered into natural gas supply management agreements with various parties. Demand for natural gas, which is a widely traded commodity, has historically been sensitive to seasonal heating and industrial load requirements as well as changes in market price. The natural gas distribution operations believe that, based on current and projected domestic and regional supplies of natural gas and the pipeline transmission network currently available through their suppliers and pipeline service providers, supplies are adequate to meet their system natural gas requirements for the next decade.
Regulatory Matters The natural gas distribution operations' retail natural gas rate schedules contain clauses permitting adjustments in rates based upon changes in natural gas commodity, transportation and storage costs. Current tariffs allow for recovery or refunds of under- or over-recovered gas costs through rate adjustments which are filed annually.
Montana-Dakota's North Dakota and South Dakota natural gas tariffs contain weather normalization mechanisms applicable to certain firm customers that adjust the distribution delivery charge revenues to reflect weather fluctuations during the November 1 through May 1 billing periods.
In Montana, Montana-Dakota recovers in rates through a tracking mechanism the increases associated with Montana state and local taxes assessed to natural gas operations on an after tax basis.
In Minnesota and Washington, Great Plains and Cascade recover in rates through a cost recovery tracking mechanism, qualifying capital investments related to the safety and integrity of its pipeline system.
On December 28, 2015, the OPUC approved an extension of Cascade's decoupling mechanism until January 1, 2020, with an agreement that Cascade would initiate a review of the mechanism by September 30, 2019. Cascade also has an earnings sharing mechanism with respect to its Oregon jurisdictional operations as required by the OPUC.
On July 7, 2016, the WUTC approved a full decoupling mechanism where Cascade is allowed recovery of an average revenue per customer regardless of actual consumption. The mechanism also includes an earnings sharing component if Cascade earns beyond its authorized return. The decoupling mechanism will be reviewed by Cascade following the end of 2019.
On December 22, 2016, the MNPUC approved a request by Great Plains to implement a full revenue decoupling mechanism pilot project for three years. The decoupling mechanism will reflect the period January 1 through December 31. Great Plains intends to seek continuation of the decoupling mechanism effective upon expiration of the pilot project.
For more information on regulatory matters, see Item 8 - Note 18.

 
MDU Resources Group, Inc. Form 10-K 13



Part I
 

Environmental Matters The natural gas distribution operations are subject to federal, state and local environmental, facility-siting, zoning and planning laws and regulations. The Company believes its natural gas distribution operations are in substantial compliance with those regulations.
The Company's natural gas distribution operations are very small-quantity generators of hazardous waste, and subject only to minimum regulation under the RCRA. Washington state rule defines Cascade as a small-quantity generator, but regulation under the rule is similar to RCRA. Certain locations of the natural gas distribution operations routinely handle PCBs from their natural gas operations in accordance with federal requirements. PCB storage areas are registered with the EPA as required. Capital and operational expenditures for natural gas distribution operations could be affected in a variety of ways by potential new GHG legislation or regulation. In particular, such legislation or regulation would likely increase capital expenditures for energy efficiency and conservation programs and operational costs associated with GHG emissions compliance. Natural gas distribution operations expect to recover the operational and capital expenditures for GHG regulatory compliance in rates consistent with the recovery of other reasonable costs of complying with environmental laws and regulations.
The natural gas distribution operations did not incur any material environmental expenditures in 2018. Except as to what may be ultimately determined with regard to the issues described in the following paragraph, the natural gas distribution operations do not expect to incur any material capital expenditures related to environmental compliance with current laws and regulations through 2021.
Montana-Dakota has ties to six historic manufactured gas plants as a successor corporation or through direct ownership of the plant. Montana-Dakota is investigating one of these former manufactured gas plant sites and providing input on another site investigation conducted by a third party. To the extent not covered by insurance, Montana-Dakota may seek recovery in its natural gas rates charged to customers for certain investigation and remediation costs incurred for these sites. Cascade has ties to nine historic manufactured gas plants as a successor corporation or through direct ownership of the plant. Cascade is involved in the investigation and remediation of three of these manufactured gas plants in Washington and Oregon. See Item 8 - Note 19 for a further discussion of these three manufactured gas plants. To the extent not covered by insurance, Cascade will seek recovery of investigation and remediation costs through its natural gas rates charged to customers.
Pipeline and Midstream
General WBI Energy owns and operates both regulated and nonregulated businesses. The regulated business of this segment, WBI Energy Transmission, owns and operates approximately 4,000 miles of natural gas transmission, gathering and storage lines in Minnesota, Montana, North Dakota, South Dakota and Wyoming. WBI Energy Transmission's underground storage fields in Montana and Wyoming provide storage services to local distribution companies, industrial customers, natural gas marketers and others, and serve to enhance system reliability. Its system is strategically located near five natural gas producing basins, making natural gas supplies available to its transportation and storage customers. The system has 14 interconnecting points with other pipeline facilities allowing for the receipt and/or delivery of natural gas to and from other regions of the country and from Canada. Under the Natural Gas Act, as amended, WBI Energy Transmission is subject to the jurisdiction of the FERC regarding certificate, rate, service and accounting matters, and at December 31, 2018, its net plant investment was $458.1 million.
The nonregulated business of this segment owns and operates gathering facilities in Montana and Wyoming. In total, facilities include approximately 800 miles of operated field gathering lines, some of which interconnect with WBI Energy's regulated pipeline system. The nonregulated business provides natural gas gathering services and a variety of other energy-related services, including cathodic protection and energy efficiency product sales and installation services to large end-users.
A majority of its pipeline and midstream business is transacted in the northern Great Plains and Rocky Mountain regions of the United States.
System Supply, System Demand and Competition Natural gas supplies emanate from traditional and nontraditional production activities in the region from both on-system and off-system supply sources. New incremental supply from nontraditional sources have developed, such as the Bakken area in Montana and North Dakota, which has helped offset declines in traditional regional supply sources and supports WBI Energy Transmission's transportation and storage services. In addition, off-system supply sources are available through the Company's interconnections with other pipeline systems. WBI Energy Transmission continues to look for opportunities to increase transportation, gathering and storage services through system expansion and/or other pipeline interconnections or enhancements that could provide substantial future benefits.
WBI Energy Transmission's underground natural gas storage facilities have a certificated storage capacity of approximately 353 Bcf, including 193 Bcf of working gas capacity, 85 Bcf of cushion gas and 75 Bcf of native gas. These storage facilities enable customers to purchase natural gas throughout the year and meet winter peak requirements.

 
14 MDU Resources Group, Inc. Form 10-K



Part I
 

WBI Energy Transmission competes with several pipelines for its customers' transportation, storage and gathering business and at times may discount rates in an effort to retain market share. However, the strategic location of its system near five natural gas producing basins and the availability of underground storage and gathering services, along with interconnections with other pipelines, serve to enhance its competitive position.
Although certain of WBI Energy Transmission's firm customers, including its largest firm customer Montana-Dakota, serve relatively secure residential, commercial and industrial end-users, they generally all have some price-sensitive end-users that could switch to alternate fuels.
WBI Energy Transmission transports substantially all of Montana-Dakota's natural gas, primarily utilizing firm transportation agreements, which for 2018 represented 32 percent of WBI Energy Transmission's subscribed firm transportation contract demand. The majority of the firm transportation agreements with Montana-Dakota expire in June 2022. In addition, Montana-Dakota has contracts with WBI Energy Transmission to provide firm storage services to facilitate meeting Montana-Dakota's winter peak requirements expiring in July 2035.
The nonregulated business competes for existing customers in the fields in which it operates. Its focus on customer service and the variety of services it offers serve to enhance its competitive position.
Environmental Matters The pipeline and midstream operations are generally subject to federal, state and local environmental, facility-siting, zoning and planning laws and regulations. The Company believes it is in substantial compliance with those regulations.
Ongoing operations are subject to the Clean Air Act, the Clean Water Act, the RCRA and other state and federal regulations. Administration of many provisions of these laws has been delegated to the states where WBI Energy and its subsidiaries operate. Permit terms vary and all permits carry operational compliance conditions. Some permits require annual renewal, some have terms ranging from one to five years and others have no expiration date. Permits are renewed and modified, as necessary, based on defined permit expiration dates, operational demand and/or regulatory changes.
Detailed environmental assessments and/or environmental impact statements as required by the National Environmental Policy Act are included in the FERC's environmental review process for both the construction and abandonment of WBI Energy Transmission's natural gas transmission pipelines, compressor stations and storage facilities.
The pipeline and midstream operations did not incur any material environmental expenditures in 2018 and do not expect to incur any material capital expenditures related to environmental compliance with current laws and regulations through 2021.
Construction Materials and Contracting
General Knife River operates construction materials and contracting businesses headquartered in Alaska, California, Hawaii, Idaho, Iowa, Minnesota, Montana, North Dakota, Oregon, South Dakota, Texas, Washington and Wyoming. These operations mine, process and sell construction aggregates (crushed stone, sand and gravel); produce and sell asphalt mix; and supply ready-mixed concrete. These products are used in most types of construction, performed by Knife River and other companies, including roads, freeways and bridges, as well as homes, schools, shopping centers, office buildings and industrial parks. Knife River focuses on vertical integration of its contracting services with its construction materials to support the aggregate based product lines including aggregate placement, asphalt and concrete paving, and site development and grading. Although not common to all locations, other products include the sale of cement, liquid asphalt for various commercial and roadway applications, various finished concrete products and other building materials and related contracting services.
During 2018, Knife River acquired construction materials and contracting businesses with operations in Oregon, Minnesota and South Dakota. For more information on acquisitions, see Item 8 - Note 3.
Knife River's backlog was approximately $706 million, $486 million and $538 million at December 31, 2018, 2017 and 2016, respectively. The increase in backlog at December 31, 2018, compared to backlog at December 31, 2017, was primarily attributable to higher backlog of federal and state agency work, as well as higher backlog of private construction projects. Backlog increases with awards of new contracts and decreases as work is performed on existing contracts. Knife River expects to complete a significant amount of the backlog at December 31, 2018, during the next 12 months.
Knife River's backlog is comprised of the anticipated revenues from the uncompleted portion of services to be performed under job-specific contracts. A project is included in backlog when a contract is awarded and agreement on contract terms has been reached. However, backlog does not contain contracts for time and material projects that a fixed amount cannot be determined. Backlog is comprised of: (a) original contract amounts, (b) change orders for which customers have approved and (c) claim amounts that have been made against customers for which are determined to have a legal basis under existing contractual arrangements and as to which recovery is considered to be probable. Such claim amounts were immaterial for all periods presented. Backlog may be subject to delay, default or cancellation at the

 
MDU Resources Group, Inc. Form 10-K 15



Part I
 

election of the customers. Historically, cancellations have not had a materially adverse effect on backlog. Due to the nature of its contractual arrangements, in many instances Knife River's customers are not committed to the specific volumes of services to be purchased under a contract, but rather Knife River is committed to perform these services if and to the extent requested by the customer. Therefore, there can be no assurance as to the customers' requirements during a particular period or that such estimates, or backlog estimates in general, at any point in time are predictive of future revenues.
Competition Knife River's construction materials products and contracting services are marketed under highly competitive conditions. Price is the principal competitive force to which these products and services are subject, with service, quality, delivery time and proximity to the customer also being significant factors. Knife River focuses on markets located near aggregate sites to reduce transportation costs which allows Knife River to remain competitive with the pricing of aggregate products. The number and size of competitors varies in each of Knife River's principal market areas and product lines.
The demand for construction materials products and contracting services is significantly influenced by the cyclical nature of the construction industry in general. In addition, construction materials and contracting services activity in certain locations may be seasonal in nature due to the effects of weather. The key economic factors affecting product demand are changes in the level of local, state and federal governmental spending on roads and infrastructure projects, general economic conditions within the market area that influence both the commercial and residential sectors, and prevailing interest rates.
Knife River's customers are a diverse group which includes federal, state and municipal government agencies, commercial and residential developers, and private parties. The mix of sales by customer will vary each year depending on the work available. Knife River is not dependent on any single customer or group of customers for sales of its products and services, the loss of which would have a material adverse effect on its construction materials businesses.
Reserve Information Aggregate reserve estimates are calculated based on the best available data. This data is collected from drill holes and other subsurface investigations, as well as investigations of surface features such as mine high walls and other exposures of the aggregate reserves. Mine plans, production history and geologic data also are utilized to estimate reserve quantities.
Estimates are based on analyses of the data described above by experienced internal mining engineers, operating personnel and geologists. Property setbacks and other regulatory restrictions and limitations are identified to determine the total area available for mining. Data described previously are used to calculate the thickness of aggregate materials to be recovered. Topography associated with alluvial sand and gravel deposits is typically flat and volumes of these materials are calculated by applying the thickness of the resource over the areas available for mining. Volumes are then converted to tons by using an appropriate conversion factor. Typically, 1.5 tons per cubic yard in the ground is used for sand and gravel deposits.
Topography associated with the hard rock reserves is typically much more diverse. Therefore, using available data, a final topography map is created and computer software is utilized to compute the volumes between the existing and final topographies. Volumes are then converted to tons by using an appropriate conversion factor. Typically, 2 tons per cubic yard in the ground is used for hard rock quarries.
Estimated reserves are probable reserves as defined in Securities Act Industry Guide 7. Remaining reserves are based on estimates of volumes that can be economically extracted and sold to meet current market and product applications. The reserve estimates include only salable tonnage and thus exclude waste materials that are generated in the crushing and processing phases of the operation. Approximately 938 million tons of the 1.0 billion tons of aggregate reserves are permitted reserves. The remaining reserves are on properties that are expected to be permitted for mining under current regulatory requirements. The data used to calculate the remaining reserves may require revisions in the future to account for changes in customer requirements and unknown geological occurrences. The years remaining were calculated by dividing remaining reserves by the three-year average sales, including estimated sales from acquired reserves prior to acquisition, from 2016 through 2018. Actual useful lives of these reserves will be subject to, among other things, fluctuations in customer demand, customer specifications, geological conditions and changes in mining plans.

 
16 MDU Resources Group, Inc. Form 10-K



Part I
 

The following table sets forth details applicable to the Company's aggregate reserves under ownership or lease as of December 31, 2018, and sales for the years ended December 31, 2018, 2017 and 2016:
 
Number of Sites
(Crushed Stone)
 
Number of Sites
(Sand & Gravel)
 
Tons Sold (000's)
Estimated Reserves
(000's tons)

Lease Expiration
Reserve
Life
(years)

 
Production Area
owned

leased

 
owned

leased

 
2018

2017

2016

 
Anchorage, AK


 
1


 
725

1,425

1,343

13,823

N/A
12

 
Hawaii

5

 


 
1,734

1,614

1,901

49,159

2023-2064
28

 
Northern CA


 
9

1

 
1,798

1,785

1,604

42,720

2028
25

 
Southern CA

2

 


 
356

55

224

91,211

2035
Over 100

 
Portland, OR
2

4

 
5

3

 
5,402

4,694

4,044

212,822

2028-2057
45

 
Eugene, OR
3

4

 
5


 
743

633

662

153,301

2021-2049
Over 100

 
Central OR/WA/ID

1

 
6

2

 
2,362

2,160

1,685

85,396

2020-2087
41

 
Southwest OR
5

5

 
10

6

 
2,395

2,367

2,689

108,998

2021-2053
44

 
Central MT


 
3

1

 
1,081

1,065

1,135

15,238

2023
14

 
Northwest MT


 
9

1

 
1,965

1,745

1,514

63,182

2020
36

 
Wyoming


 
1

2

 
626

613

742

9,466

2019-2020
14

 
Central MN
1

1

 
43

7

 
2,890

2,773

2,831

65,225

2019-2028
18

*
Northern MN
2


 
14

2