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Goodwill rollforward (Details) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Goodwill [Roll Forward]      
Balance as of beginning of period $ 635,204,000us-gaap_Goodwill [1] $ 636,039,000us-gaap_Goodwill [1] $ 636,039,000us-gaap_Goodwill [1]
Goodwill acquired during the year 0us-gaap_GoodwillAcquiredDuringPeriod 0us-gaap_GoodwillAcquiredDuringPeriod  
Goodwill acquired during the year/other     (835,000)us-gaap_GoodwillOtherChanges
Balance as of end of period 635,204,000us-gaap_Goodwill [1] 636,039,000us-gaap_Goodwill [1] 635,204,000us-gaap_Goodwill [1]
Natural gas distribution [Member]      
Goodwill [Roll Forward]      
Balance as of beginning of period 345,736,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember		 345,736,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember		 345,736,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
Goodwill acquired during the year 0us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember		 0us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember		  
Goodwill acquired during the year/other     0us-gaap_GoodwillOtherChanges
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
Balance as of end of period 345,736,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember		 345,736,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember		 345,736,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_NaturalGasDistributionMember
Pipeline and energy services [Member]      
Goodwill [Roll Forward]      
Balance as of beginning of period 9,737,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 [1] 9,737,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 [1] 9,737,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 [1]
Goodwill acquired during the year 0us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 0us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		  
Goodwill acquired during the year/other     0us-gaap_GoodwillOtherChanges
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
Balance as of end of period 9,737,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 [1] 9,737,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 [1] 9,737,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 [1]
Goodwill, impaired, accumulated impairment loss 12,300,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 12,300,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember		 12,300,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_PipelineAndEnergyServicesMember
Construction materials and contracting [Member]      
Goodwill [Roll Forward]      
Balance as of beginning of period 176,290,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember		 176,290,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember		 176,290,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
Goodwill acquired during the year 0us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember		 0us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember		  
Goodwill acquired during the year/other     0us-gaap_GoodwillOtherChanges
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
Balance as of end of period 176,290,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember		 176,290,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember		 176,290,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionMaterialsAndContractingMember
Construction services [Member]      
Goodwill [Roll Forward]      
Balance as of beginning of period 103,441,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember		 104,276,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember		 104,276,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
Goodwill acquired during the year 0us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember		 0us-gaap_GoodwillAcquiredDuringPeriod
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember		  
Goodwill acquired during the year/other     (835,000)us-gaap_GoodwillOtherChanges
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
Balance as of end of period $ 103,441,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember		 $ 104,276,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember		 $ 103,441,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= mdu_ConstructionServicesMember
[1] Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.