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Employee benefit plans Multiemployer Plan (Details 2) (Multiemployer plans, pension [Member], USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Multiemployer plans, pension [Member]
  
Multiemployer Plans [Line Items]  
Additional Multiemployer Plan Withdrawal Obligation $ 2.4mdu_AdditionalMultiemployerPlanWithdrawalObligation
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPensionMember
Additional Estimated Multiemployer Plan Withdrawal Obligation After Tax 1.5mdu_AdditionalEstimatedMultiemployerPlanWithdrawalObligationAfterTax
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPensionMember
Multiemployer plans, withdrawal obligation 16.4us-gaap_MultiemployerPlansWithdrawalObligation
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPensionMember
Estimated multiemployer plan, withdrawal obligation, after tax $ 9.8mdu_EstimatedMultiemployerPlanWithdrawalObligationAfterTax
/ us-gaap_MultiemployerPlanTypeAxis
= us-gaap_MultiemployerPlansPensionMember