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Consolidated Statements of Comprehensive Income (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Net income (loss) $ 294,338us-gaap_ProfitLoss $ 278,570us-gaap_ProfitLoss $ (754)us-gaap_ProfitLoss
Net unrealized gain (loss) on derivative instruments qualifying as hedges:      
Net unrealized gain (loss) on derivative instruments arising during the period, net of tax of $0, $(3,116) and $4,829 in 2014, 2013 and 2012, respectively 0us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax (5,594)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax 8,497us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax
Reclassification adjustment for (gain) loss on derivative instruments included in net income, net of tax of $413, $(2,548) and $(5,141) in 2014, 2013 and 2012, respectively 694us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax (4,189)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax (8,754)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesNetOfTax
Net unrealized gain (loss) on derivative instruments qualifying as hedges 694us-gaap_OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax (9,783)us-gaap_OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax (257)us-gaap_OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax
Postretirement liability adjustment:      
Postretirement liability gains (losses) arising during the period, net of tax of $(7,665), $11,818 and $(2,060) in 2014, 2013 and 2012, respectively (12,409)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax 18,539us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax (3,106)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $492, $1,276 and $1,379 in 2014, 2013 and 2012, respectively 796us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansNetOfTax 2,001us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansNetOfTax 2,079us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansNetOfTax
Reclassification of postretirement liability adjustment to regulatory asset, net of tax of $4,509, $0 and $0 in 2014, 2013 and 2012, respectively 7,202mdu_OtherComprehensiveIncomeLossReclassificationAdjustmentfromAOCIPostretirementLiabilityAdjustmentToRegulatoryAssetBeforeTax 0mdu_OtherComprehensiveIncomeLossReclassificationAdjustmentfromAOCIPostretirementLiabilityAdjustmentToRegulatoryAssetBeforeTax 0mdu_OtherComprehensiveIncomeLossReclassificationAdjustmentfromAOCIPostretirementLiabilityAdjustmentToRegulatoryAssetBeforeTax
Postretirement liability adjustment (4,411)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax 20,540us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax (1,027)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
Foreign currency translation adjustment:      
Foreign currency translation adjustment recognized during the period, net of tax of $(99), $(177) and $(296) in 2014, 2013 and 2012, respectively (162)us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossArisingDuringPeriodNetOfTax (299)us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossArisingDuringPeriodNetOfTax (476)us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossArisingDuringPeriodNetOfTax
Reclassification adjustment for (gain) loss on foreign currency translation adjustment included in net income, net of tax of $0, $70 and $2 in 2014, 2013 and 2012, respectively 0us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationReclassificationAdjustmentFromAOCIRealizedUponSaleOrLiquidationBeforeTax 143us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationReclassificationAdjustmentFromAOCIRealizedUponSaleOrLiquidationBeforeTax 3us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationReclassificationAdjustmentFromAOCIRealizedUponSaleOrLiquidationBeforeTax
Foreign currency translation adjustment (162)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax (156)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax (473)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax
Net unrealized gain (loss) on available-for-sale investments:      
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(83), $(105) and $(52) in 2014, 2013 and 2012, respectively (154)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax (194)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax (97)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $73, $59 and $72 in 2014, 2013 and 2012, respectively 135us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax 109us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax 134us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Net unrealized gain (loss) on available-for-sale investments (19)us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax (85)us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax 37us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax
Other comprehensive income (loss) (3,898)us-gaap_OtherComprehensiveIncomeLossNetOfTax 10,516us-gaap_OtherComprehensiveIncomeLossNetOfTax (1,720)us-gaap_OtherComprehensiveIncomeLossNetOfTax
Comprehensive income (loss) 290,440us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest 289,086us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest (2,474)us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterest
Comprehensive loss attributable to noncontrolling interest (3,895)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest (363)us-gaap_NetIncomeLossAttributableToNoncontrollingInterest 0us-gaap_NetIncomeLossAttributableToNoncontrollingInterest
Comprehensive income (loss) attributable to common stockholders 294,335us-gaap_ComprehensiveIncomeNetOfTax 289,449us-gaap_ComprehensiveIncomeNetOfTax (2,474)us-gaap_ComprehensiveIncomeNetOfTax
Other Comprehensive Income (Loss), Tax, Parenthetical Disclosures [Abstract]      
Net unrealized gain (loss) on derivative instruments arising during the period, tax 0us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodTax (3,116)us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodTax 4,829us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodTax
Reclassification adjustment for (gain) loss on derivative instruments included in net income, tax 413us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax (2,548)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax (5,141)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIOnDerivativesTax
Postretirement liability gains (losses) arising during the period, tax (7,665)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansBeforeReclassificationAdjustmentsTax 11,818us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansBeforeReclassificationAdjustmentsTax (2,060)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansBeforeReclassificationAdjustmentsTax
Amortization of postretirement liability losses included in net periodic benefit cost, tax 492us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax 1,276us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax 1,379us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax
Reclassification of postretirement liability adjustment to regulatory asset, tax 4,509mdu_OtherComprehensiveIncomeLossReclassificationAdjustmentfromAOCIPostretirementLiabilityAdjustmentToRegulatoryAssetTax 0mdu_OtherComprehensiveIncomeLossReclassificationAdjustmentfromAOCIPostretirementLiabilityAdjustmentToRegulatoryAssetTax 0mdu_OtherComprehensiveIncomeLossReclassificationAdjustmentfromAOCIPostretirementLiabilityAdjustmentToRegulatoryAssetTax
Foreign currency translation adjustment recognized during the period, tax (99)us-gaap_OtherComprehensiveIncomeForeignCurrencyTranslationGainLossArisingDuringPeriodTax (177)us-gaap_OtherComprehensiveIncomeForeignCurrencyTranslationGainLossArisingDuringPeriodTax (296)us-gaap_OtherComprehensiveIncomeForeignCurrencyTranslationGainLossArisingDuringPeriodTax
Reclassification adjustment for (gain) loss on foreign currency translation adjustment included in net income, tax 0us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationReclassificationAdjustmentFromAOCIRealizedUponSaleOrLiquidationTax 70us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationReclassificationAdjustmentFromAOCIRealizedUponSaleOrLiquidationTax 2us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationReclassificationAdjustmentFromAOCIRealizedUponSaleOrLiquidationTax
Net unrealized gains (losses) on available-for-sale investments arising during the period, tax (83)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax (105)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax (52)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax
Reclassification adjustments for (gain) loss on available-for-sale investments included in net income, tax $ 73us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax $ 59us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax $ 72us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesTax