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Insurance (Details 11) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012
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Maximum amount of deductibles for workers' compensation insurance per occurrence | $ 1
Maximum amount of deductibles for automobile liability and general liability insurance per accident or occurrence | $ 1

X
- Definition Maximum amount of risk retained by the entity before the automobile and general liability insurance arrangement begins to provide coverage. + References No definition available. + Details
Name: | mdu_MaximumAmountOfDeductiblesForAutomobileLiabilityAndGeneralLiabilityInsurancePerAccidentOrOccurrence
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Maximum amount of risk retained by the entity before the workers' compensation insurance arrangement begins to provide coverage. + References No definition available. + Details
Name: | mdu_MaximumAmountOfDeductiblesForWorkersCompensationInsurancePerOccurrence
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant