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Derivative Instruments Derivative instruments (Details) (USD $) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Mar. 31, 2013
Derivative instruments [Line Items] |  |  |
Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into AOCI |  |  | $ 1,800,000 mdu_AmountofunrealizedgainlossonhedgeatdedesignationdatetobereclassifiedintoAOCI
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value |  | 7,500,000 us-gaap_DerivativeNetLiabilityPositionAggregateFairValue |
Assets needed for immediate settlement aggregate fair value for cash flow heding instruments |  | 7,500,000 us-gaap_AssetsNeededForImmediateSettlementAggregateFairValue |
Cash flow hedge gain (loss) to be reclassified within twelve months | $ (400,000)us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths |  |
Oil swap and or collar [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative, nonmonetary notional amount | 270,000 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mdu_OilSwapAndOrCollarMember | 2,900,000 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mdu_OilSwapAndOrCollarMember |
Natural gas swap or and collar [Member] |  |  |
Derivative instruments [Line Items] |  |  |
Derivative, nonmonetary notional amount | 5,000,000 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mdu_NaturalGasSwapAndorCollarMember | 18,300,000 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mdu_NaturalGasSwapAndorCollarMember |

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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Namespace Prefix: | invest_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Definition Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into accumulated other comprehensive income over time. + References No definition available. + Details
Name: | mdu_AmountofunrealizedgainlossonhedgeatdedesignationdatetobereclassifiedintoAOCI
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The aggregate fair value of assets needed to immediately settle the derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994 + Details
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Period Type: | duration

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- Details
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- Definition The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959 + Details
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Namespace Prefix: | us-gaap_
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- Details
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Data Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |