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Derivative instruments (Details 9) | 12 Months Ended
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Dec. 31, 2012
Derivative [Line Items] |
Maximum length of time allowed to hedge forecasted sales | 36 months
Maximum length of time allowed to hedge interest rate | 24 months
Maximum length of time allowed to hedge foreign currency | 12-month period
Maximum length of time allowed to hedge forecasted purchases | three years
Maximum period for settlement of all interest rate derivative transactions | 90 days
Maximum period for settlement of foreign currency derivative transactions | 12-month period

X
- Definition Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted sales transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments. + References No definition available. + Details
Name: | mdu_MaximumLengthOfTimeAllowedToHedgeCashFlowHedgeSales
Namespace Prefix: | mdu_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted purchase transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments. + References No definition available. + Details
Name: | mdu_MaximumLengthOfTimeAllowedToHedgeInCashFlowHedgePurchase
Namespace Prefix: | mdu_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum length of time over which the entity is hedging its exposure to foreign currency. + References No definition available. + Details
Name: | mdu_MaximumLengthOfTimeAllowedToHedgeInForeignCurrencyCashFlowHedge
Namespace Prefix: | mdu_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments. + References No definition available. + Details
Name: | mdu_MaximumLengthOfTimeAllowedToHedgeInInterestRateCashFlowHedge
Namespace Prefix: | mdu_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum length of time for the settlement of all interest rate derivative transactions. + References No definition available. + Details
Name: | mdu_MaximumPeriodForSettlementOfAllInterestRateDerivativeTransactions
Namespace Prefix: | mdu_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum length of time for the settlement of foreign currency derivative transactions. + References No definition available. + Details
Name: | mdu_MaximumPeriodForSettlementOfForeignCurrencyDerivativeTransactions
Namespace Prefix: | mdu_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DerivativeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration