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Property, plant and equipment (Details 3) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Property, plant and equipment [Line Items] |  |  |  |  |  |
Interest capitalized | $ 8,586 us-gaap_InterestCostsCapitalized |  | $ 6,033 us-gaap_InterestCostsCapitalized |  | $ 8,659 us-gaap_InterestCostsCapitalized |
AFUDC - borrowed | 3,022 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest |  | 2,767 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest |  | 2,483 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest |
AFUDC - equity | 5,803 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity |  | 3,322 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity |  | 4,530 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity |
Property, plant and equipment | 9,693,171 us-gaap_PropertyPlantAndEquipmentGross |  | 8,803,866 us-gaap_PropertyPlantAndEquipmentGross |  |  |
Less accumulated depreciation, depletion and amortization | 4,166,407 us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment |  | 3,872,487 us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment |  | 3,608,912 us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment |
Net property, plant and equipment | 5,526,764 us-gaap_PropertyPlantAndEquipmentNet |  | 4,931,379 us-gaap_PropertyPlantAndEquipmentNet |  | 4,498,839 us-gaap_PropertyPlantAndEquipmentNet |
BOE average rate for amortization under units-of-production method (in dollars per BOE) | 21.17 us-gaap_AmortizationExpensePerPhysicalUnitOfProduction |  | 17.41 us-gaap_AmortizationExpensePerPhysicalUnitOfProduction |  | 15.28 us-gaap_AmortizationExpensePerPhysicalUnitOfProduction |
Oil and natural gas properties not subject to amortization | 132,141 us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative |  | 124,869 us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative |  | 191,794 us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative |
Electric: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,457,101 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember | [1] | 1,315,822 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember | [1] | 1,150,584 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember | [1]
Natural gas distribution: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,904,759 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember | [1] | 1,776,901 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember | [1] | 1,689,950 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember | [1]
Pipeline and energy services: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,220,233 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 962,172 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 816,533 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |
Exploration and production: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 3,402,879 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ExplorationAndProductionMember |  | 3,060,848 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ExplorationAndProductionMember |  | 2,764,560 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ExplorationAndProductionMember |
Construction materials and contracting: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,529,942 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  | 1,510,355 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  | 1,504,981 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |
Construction services: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 144,395 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  | 134,948 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  | 130,624 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |
Other: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 50,937 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 49,997 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 50,519 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |
Regulated: | Electric: | Generation |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 627,952 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ElectricGenerationEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  | 570,394 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ElectricGenerationEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  |  |
Weighted average depreciable life in years | 42 years |  |  |  |  |
Regulated: | Electric: | Distribution |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 343,692 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ElectricDistributionMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  | 308,202 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ElectricDistributionMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  |  |
Weighted average depreciable life in years | 39 years |  |  |  |  |
Regulated: | Electric: | Transmission |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 229,997 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ElectricTransmissionMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  | 196,824 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ElectricTransmissionMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  |  |
Weighted average depreciable life in years | 48 years |  |  |  |  |
Regulated: | Electric: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 150,445 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  | 141,365 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  |  |
Regulated: | Electric: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 105,015 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  | 99,037 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ElectricMember |  |  |
Weighted average depreciable life in years | 15 years |  |  |  |  |
Regulated: | Natural gas distribution: | Distribution |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,481,390 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasDistributionMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  | 1,384,587 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasDistributionMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  |  |
Weighted average depreciable life in years | 40 years |  |  |  |  |
Regulated: | Natural gas distribution: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 59,310 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  | 46,763 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  |  |
Regulated: | Natural gas distribution: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 364,059 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  | 345,551 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  |  |
Weighted average depreciable life in years | 27 years |  |  |  |  |
Regulated: | Pipeline and energy services: | Transmission |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 449,276 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasTransmissionMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 418,594 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasTransmissionMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Weighted average depreciable life in years | 53 years |  |  |  |  |
Regulated: | Pipeline and energy services: | Gathering |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 39,595 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasGatheringAndProcessingEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 39,597 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_GasGatheringAndProcessingEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Weighted average depreciable life in years | 20 years |  |  |  |  |
Regulated: | Pipeline and energy services: | Storage |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 43,994 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_GasStorageMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 42,939 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_GasStorageMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Weighted average depreciable life in years | 60 years |  |  |  |  |
Regulated: | Pipeline and energy services: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 5,386 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 6,937 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Regulated: | Pipeline and energy services: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 39,910 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 39,504 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_RegulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Weighted average depreciable life in years | 33 years |  |  |  |  |
Nonregulated: | Eliminations |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | (17,075)us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConsolidationEliminationsMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  | (7,177)us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConsolidationEliminationsMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  |  |
Nonregulated: | Pipeline and energy services: | Midstream |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 227,598 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_MidstreamMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 213,063 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_MidstreamMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Weighted average depreciable life in years | 16 years |  |  |  |  |
Nonregulated: | Pipeline and energy services: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 314,304 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 188,641 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Nonregulated: | Pipeline and energy services: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 100,170 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 12,897 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Weighted average depreciable life in years | 18 years |  |  |  |  |
Nonregulated: | Exploration and production: | Oil and natural gas properties |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 3,337,177 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_OilAndNaturalGasPropertiesMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ExplorationAndProductionMember | [2] | 3,017,879 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_OilAndNaturalGasPropertiesMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ExplorationAndProductionMember | [2] |  |
Nonregulated: | Exploration and production: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 65,702 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ExplorationAndProductionMember |  | 42,969 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ExplorationAndProductionMember |  |  |
Weighted average depreciable life in years | 8 years |  |  |  |  |
Nonregulated: | Construction materials and contracting: | Land |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 125,372 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  | 125,551 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  |  |
Nonregulated: | Construction materials and contracting: | Buildings and improvements |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 70,566 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_BuildingAndBuildingImprovementsMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  | 70,000 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_BuildingAndBuildingImprovementsMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  |  |
Weighted average depreciable life in years | 19 years |  |  |  |  |
Nonregulated: | Construction materials and contracting: | Machinery, vehicles and equipment |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 921,564 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_MachineryVehiclesAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  | 906,774 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_MachineryVehiclesAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  |  |
Weighted average depreciable life in years | 12 years |  |  |  |  |
Nonregulated: | Construction materials and contracting: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 8,709 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  | 13,315 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  |  |
Nonregulated: | Construction materials and contracting: | Aggregate reserves |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 403,731 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_AggregateReservesMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember | [3] | 394,715 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_AggregateReservesMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember | [3] |  |
Nonregulated: | Construction services: | Land |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 5,265 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  | 4,821 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  |  |
Nonregulated: | Construction services: | Buildings and improvements |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 17,936 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_BuildingAndBuildingImprovementsMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  | 16,628 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_BuildingAndBuildingImprovementsMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  |  |
Weighted average depreciable life in years | 20 years |  |  |  |  |
Nonregulated: | Construction services: | Machinery, vehicles and equipment |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 112,973 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_MachineryVehiclesAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  | 105,991 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mdu_MachineryVehiclesAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  |  |
Weighted average depreciable life in years | 6 years |  |  |  |  |
Nonregulated: | Construction services: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 8,221 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  | 7,508 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  |  |
Weighted average depreciable life in years | 4 years |  |  |  |  |
Nonregulated: | Other: | Land |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 2,837 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 2,837 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |  |
Nonregulated: | Other: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | $ 48,100 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | $ 47,160 us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |  |
Weighted average depreciable life in years | 23 years |  |  |  |  |
[1] | Includes allocations of common utility property.
[2] | Amortized on the units-of-production method based on total proved reserves at a BOE average rate of $21.17, $17.41 and $15.28 for the years ended December 31, 2014, 2013 and 2012, respectively. Includes oil and natural gas properties accounted for under the full-cost method, of which $132.1 million and $124.9 million were excluded from amortization at December 31, 2014 and 2013, respectively.
[3] | Depleted on the units-of-production method.

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