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v2.4.1.9

Reclassifications out of accumulated other comprehensive (loss) (Details 15) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 1,163,204 us-gaap_Revenues | $ 1,370,455 us-gaap_Revenues | $ 1,094,046 us-gaap_Revenues | $ 1,042,853 us-gaap_Revenues | $ 1,184,423 us-gaap_Revenues | $ 1,285,782 us-gaap_Revenues | $ 1,060,595 us-gaap_Revenues | $ 931,604 us-gaap_Revenues | $ 4,670,558 us-gaap_Revenues | $ 4,462,404 us-gaap_Revenues | $ 4,075,431 us-gaap_Revenues
Interest expense |  |  |  |  |  |  |  |  | (87,016)us-gaap_InterestExpense | (83,917)us-gaap_InterestExpense | (76,699)us-gaap_InterestExpense
Income (loss) before income taxes |  |  |  |  |  |  |  |  | 411,130 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 415,618 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | (45,467)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Income taxes |  |  |  |  |  |  |  |  | (119,969)us-gaap_IncomeTaxExpenseBenefit | (136,736)us-gaap_IncomeTaxExpenseBenefit | 31,146 us-gaap_IncomeTaxExpenseBenefit
Net income (loss) |  |  |  |  |  |  |  |  | 294,338 us-gaap_ProfitLoss | 278,570 us-gaap_ProfitLoss | (754)us-gaap_ProfitLoss
Earnings (loss) from equity method investments |  |  |  |  |  |  |  |  | (41)us-gaap_IncomeLossFromEquityMethodInvestments | (132)us-gaap_IncomeLossFromEquityMethodInvestments | 5,383 us-gaap_IncomeLossFromEquityMethodInvestments
Reclassification adjustment for gain (loss) on derivative instruments included in net income |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Income (loss) before income taxes |  |  |  |  |  |  |  |  | (1,107)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember | 6,737 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
Income taxes |  |  |  |  |  |  |  |  | 413 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember | (2,548)us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
Net income (loss) |  |  |  |  |  |  |  |  | (694)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember | 4,189 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
Amortization of postretirement liability losses included in net periodic benefit cost |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Income taxes |  |  |  |  |  |  |  |  | 492 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember | 1,276 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
Net income (loss) |  |  |  |  |  |  |  |  | (796)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember | (2,001)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
Net periodic benefit cost |  |  |  |  |  |  |  |  | (1,288)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember | (3,277)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
Reclassification adjustment for loss on foreign currency translation adjustment included in net income |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  |  |  |  |  |  | 0 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember | (143)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember |
Earnings (loss) from equity method investments |  |  |  |  |  |  |  |  | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember | (213)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember |
Earnings (loss) from equity method investments, tax |  |  |  |  |  |  |  |  | 0 mdu_EarningsLossFromEquityMethodInvestmentsTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember | 70 mdu_EarningsLossFromEquityMethodInvestmentsTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember |
Reclassification adjustment for loss on available-for-sale investments included in net income [Member] |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Income taxes |  |  |  |  |  |  |  |  | 73 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember | 59 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
Net income (loss) |  |  |  |  |  |  |  |  | (135)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember | (109)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
Other income |  |  |  |  |  |  |  |  | (208)us-gaap_OtherIncome / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember | (168)us-gaap_OtherIncome / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
Total reclassifications |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  |  |  |  |  |  | (1,625)us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember | 1,936 us-gaap_ProfitLoss / us-gaap_StatementEquityComponentsAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember |
Commodity derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | (468)us-gaap_Revenues / us-gaap_DerivativeByNatureAxis = mdu_CommodityDerivativeInstrumentsMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember | 7,803 us-gaap_Revenues / us-gaap_DerivativeByNatureAxis = mdu_CommodityDerivativeInstrumentsMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
Interest rate derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income |  |  |  |  |  |  |  |  |  |  |
Reclassification adjustment out of accumulated other comprehensive income |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  |  | $ (639)us-gaap_InterestExpense / us-gaap_DerivativeByNatureAxis = mdu_InterestRateDerivativeInstrumentsMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember | $ (1,066)us-gaap_InterestExpense / us-gaap_DerivativeByNatureAxis = mdu_InterestRateDerivativeInstrumentsMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |

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