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Employee Benefit Plans Investments in nonqualified plans (Details 13) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Nonqualified Defined Benefit and Contribution Plan Investments [Line Items] |  |
Investments in nonqualified benefit plans | $ 101.4 mdu_InvestmentsInNonqualifiedBenefitPlans | $ 98.1 mdu_InvestmentsInNonqualifiedBenefitPlans
Life insurance carried on plan participants [Member] |  |
Nonqualified Defined Benefit and Contribution Plan Investments [Line Items] |  |
Investments in nonqualified benefit plans | 32.8 mdu_InvestmentsInNonqualifiedBenefitPlans / mdu_NonqualifiedDefinedBenefitandContributionPlansInvestmentsAxis = mdu_LifeInsuranceCarriedOnPlanParticipantsMember | 31.4 mdu_InvestmentsInNonqualifiedBenefitPlans / mdu_NonqualifiedDefinedBenefitandContributionPlansInvestmentsAxis = mdu_LifeInsuranceCarriedOnPlanParticipantsMember
Equity securities [Member] |  |
Nonqualified Defined Benefit and Contribution Plan Investments [Line Items] |  |
Investments in nonqualified benefit plans | 54.9 mdu_InvestmentsInNonqualifiedBenefitPlans / mdu_NonqualifiedDefinedBenefitandContributionPlansInvestmentsAxis = us-gaap_EquitySecuritiesMember | 53.5 mdu_InvestmentsInNonqualifiedBenefitPlans / mdu_NonqualifiedDefinedBenefitandContributionPlansInvestmentsAxis = us-gaap_EquitySecuritiesMember
Other investments [Member] |  |
Nonqualified Defined Benefit and Contribution Plan Investments [Line Items] |  |
Investments in nonqualified benefit plans | $ 13.7 mdu_InvestmentsInNonqualifiedBenefitPlans / mdu_NonqualifiedDefinedBenefitandContributionPlansInvestmentsAxis = us-gaap_OtherInvestmentsMember | $ 13.2 mdu_InvestmentsInNonqualifiedBenefitPlans / mdu_NonqualifiedDefinedBenefitandContributionPlansInvestmentsAxis = us-gaap_OtherInvestmentsMember

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