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

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