XML 155 R101.htm IDEA: XBRL DOCUMENT v2.4.1.9
Fair value - pension and postretirement (Details 9) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year $ 354,363,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 $ 334,844,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 $ 309,184,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_PensionPlansDefinedBenefitMember
Percentage investment in common stock of large-cap U.S. companies 13.00%mdu_PercentageInvestmentInCommonStockOfLargeCapUSCompanies
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= us-gaap_PensionPlansDefinedBenefitMember		 19.00%mdu_PercentageInvestmentInCommonStockOfLargeCapUSCompanies
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= us-gaap_PensionPlansDefinedBenefitMember		  
Percentage investment in US Government securities 13.00%mdu_PercentageInvestmentinUSGovernmentSecurities
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= us-gaap_PensionPlansDefinedBenefitMember		 12.00%mdu_PercentageInvestmentinUSGovernmentSecurities
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= us-gaap_PensionPlansDefinedBenefitMember		  
Percentage investment in corporate bonds 23.00%mdu_PercentageInvestmentInCorporateBonds
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 27.00%mdu_PercentageInvestmentInCorporateBonds
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= us-gaap_PensionPlansDefinedBenefitMember		  
Percentage investment in common stock of international companies 33.00%mdu_PercentageInvestmentInCommonStockOfInternationalCompanies
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= us-gaap_PensionPlansDefinedBenefitMember		 13.00%mdu_PercentageInvestmentInCommonStockOfInternationalCompanies
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= us-gaap_PensionPlansDefinedBenefitMember		  
Percentage investment in other investments 18.00%mdu_PercentageInvestmentInOtherInvestments
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= us-gaap_PensionPlansDefinedBenefitMember		 18.00%mdu_PercentageInvestmentInOtherInvestments
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= us-gaap_PensionPlansDefinedBenefitMember		  
Percentage investment in common stock of mid-cap U.S. companies   11.00%mdu_PercentageInvestmentInCommonStockOfMidCapUSCompanies
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Estimated future employer contributions in next fiscal year 3,900,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
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= us-gaap_PensionPlansDefinedBenefitMember		    
Pension Benefits | Cash equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 5,631,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashAndCashEquivalentsMember
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= us-gaap_PensionPlansDefinedBenefitMember		 9,406,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_CashAndCashEquivalentsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Pension Benefits | Equity securities of U.S. companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 39,077,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_EquitySecuritiesOfUSCompaniesMember
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= us-gaap_PensionPlansDefinedBenefitMember		 62,599,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= mdu_EquitySecuritiesOfUSCompaniesMember
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= us-gaap_PensionPlansDefinedBenefitMember		  
Pension Benefits | Equities Securities of International companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 5,189,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_EquitiesSecuritiesOfInternationalCompaniesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 39,437,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= mdu_EquitiesSecuritiesOfInternationalCompaniesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Pension Benefits | Collective and Mutual funds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 209,852,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_CollectiveAndMutualFundsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 [1] 158,748,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= mdu_CollectiveAndMutualFundsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		 [2]  
Pension Benefits | Corporate debt securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 59,471,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CorporateDebtSecuritiesMember
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= us-gaap_PensionPlansDefinedBenefitMember		 42,721,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_CorporateDebtSecuritiesMember
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= us-gaap_PensionPlansDefinedBenefitMember		  
Pension Benefits | Municipal bonds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 10,462,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_MunicipalBondsMember
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= us-gaap_PensionPlansDefinedBenefitMember		 7,561,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_MunicipalBondsMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Pension Benefits | U.S. Government securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 21,850,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasurySecuritiesMember
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= us-gaap_PensionPlansDefinedBenefitMember		 11,822,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_USTreasurySecuritiesMember
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Pension Benefits | Total assets measured at fair value [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 351,532,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_TotalAssetsMeasuredatFairValueMember
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= us-gaap_PensionPlansDefinedBenefitMember		 332,294,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= mdu_TotalAssetsMeasuredatFairValueMember
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= us-gaap_PensionPlansDefinedBenefitMember		  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_FairValueInputsLevel1Member		  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 39,077,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_EquitySecuritiesOfUSCompaniesMember
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= us-gaap_FairValueInputsLevel1Member		  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equities Securities of International companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 5,189,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_EquitiesSecuritiesOfInternationalCompaniesMember
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Collective and Mutual funds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 132,403,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_CollectiveAndMutualFundsMember
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= us-gaap_FairValueInputsLevel1Member		 [2]  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_FairValueInputsLevel1Member		  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 15,001,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_USTreasurySecuritiesMember
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= us-gaap_FairValueInputsLevel1Member		  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Total assets measured at fair value [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 191,670,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_TotalAssetsMeasuredatFairValueMember
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= us-gaap_FairValueInputsLevel1Member		  
Pension Benefits | Significant Other Observable Inputs (Level 2) | Cash equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 5,631,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_CashAndCashEquivalentsMember
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= us-gaap_FairValueInputsLevel2Member		  
Pension Benefits | Significant Other Observable Inputs (Level 2) | Equity securities of U.S. companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Pension Benefits | Significant Other Observable Inputs (Level 2) | Equities Securities of International companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= mdu_EquitiesSecuritiesOfInternationalCompaniesMember
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= us-gaap_FairValueInputsLevel2Member		  
Pension Benefits | Significant Other Observable Inputs (Level 2) | Collective and Mutual funds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 77,449,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_CollectiveAndMutualFundsMember
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Pension Benefits | Significant Other Observable Inputs (Level 2) | Corporate debt securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 59,471,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Pension Benefits | Significant Other Observable Inputs (Level 2) | Municipal bonds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 10,462,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
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= us-gaap_FairValueInputsLevel2Member		  
Pension Benefits | Significant Other Observable Inputs (Level 2) | U.S. Government securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 6,849,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_FairValueInputsLevel2Member		  
Pension Benefits | Significant Other Observable Inputs (Level 2) | Total assets measured at fair value [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 159,862,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Pension Benefits | Significant Unobservable Inputs (Level 3) | Corporate debt securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Pension Benefits | Significant Unobservable Inputs (Level 3) | U.S. Government securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
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Pension Benefits | Significant Unobservable Inputs (Level 3) | Total assets measured at fair value [Member]      
Defined Benefit Plan Disclosure [Line Items]      
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Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 87,586,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Percentage investment in common stock of large-cap U.S. companies 54.00%mdu_PercentageInvestmentInCommonStockOfLargeCapUSCompanies
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Percentage investment in US Government securities 11.00%mdu_PercentageInvestmentinUSGovernmentSecurities
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Percentage investment in corporate bonds 10.00%mdu_PercentageInvestmentInCorporateBonds
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Percentage investment in other investments 15.00%mdu_PercentageInvestmentInOtherInvestments
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Percentage investment in mortgage backed securities 10.00%mdu_PercentageInvestmentInMortgageBackedSecurities
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Percentage investment in common stock of mid-cap U.S. companies   8.00%mdu_PercentageInvestmentInCommonStockOfMidCapUSCompanies
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Estimated future employer contributions in next fiscal year 400,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
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Other Postretirement Benefits | Cash equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 2,097,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Equity securities of U.S. companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 2,614,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Equities Securities of International companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 25,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Insurance investment contract      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 82,846,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Total assets measured at fair value [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 87,582,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 2,639,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities of U.S. companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 2,614,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equities Securities of International companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 25,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_FairValueInputsLevel1Member		  
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Insurance investment contract      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
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Other Postretirement Benefits | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 84,943,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Cash equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 2,097,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		  
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Equity securities of U.S. companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= mdu_EquitySecuritiesOfUSCompaniesMember
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Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Equities Securities of International companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
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Other Postretirement Benefits | Significant Other Observable Inputs (Level 2) | Insurance investment contract      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 82,846,000us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Significant Unobservable Inputs (Level 3)      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Cash equivalents [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Equity securities of U.S. companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Equities Securities of International companies [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Other Postretirement Benefits | Significant Unobservable Inputs (Level 3) | Insurance investment contract      
Defined Benefit Plan Disclosure [Line Items]      
Fair value of net plan assets at end of year $ 0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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[1] Collective and mutual funds invest approximately 13 percent in common stock of large-cap U.S. companies, 13 percent in U.S. Government securities, 23 percent in corporate bonds, 33 percent in common stock of international companies and 18 percent in other investments.
[2] Collective and mutual funds invest approximately 11 percent in common stock of mid-cap U.S. companies, 19 percent in common stock of large-cap U.S. companies, 12 percent in U.S. Government securities, 27 percent in corporate bonds, 13 percent in common stock of international companies and 18 percent in other investments.
[3] The insurance contract invests approximately 54 percent in common stock of large-cap U.S. companies, 11 percent in U.S. Government securities, 10 percent in mortgage-backed securities, 10 percent in corporate bonds and 15 percent in other investments.
[4] The insurance contract invests approximately 55 percent in common stock of large-cap U.S. companies, 12 percent in U.S. Government securities, 8 percent in mortgage-backed securities, 8 percent in common stock of mid-cap U.S. companies, 9 percent in corporate bonds and 8 percent in other investments.