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Regulatory matters and revenues subject to refund (Details) (USD $) In Millions, unless otherwise specified | 6 Months Ended
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Jun. 30, 2011
NDPSC [Member] |
Regulated Operations, Schedule of Regulatory Matters [Line Items] |
Total electric rate increase requested | $ 15.4
Percent above current rates requested (in hundredths) | 14.00%
Amount of approved interim increase in annual electric rates | 7.6
Revised total annual electric rate increase requested | 8.8
Revised percentage of increase requested (in hundredths) | 7.70%
Reductions in rate increase sought by regulatory agency | 8.3
Amount of total rate increase included in settlement agreement | 7.6
The number of MW of a simple cycle natural gas turbine | 88
Estimated project cost of the natural gas turbine and associated facilities | 85.6
MTPSC [Member] |
Regulated Operations, Schedule of Regulatory Matters [Line Items] |
Total electric rate increase requested | 5.5
Percent above current rates requested (in hundredths) | 13.00%
Total amount of requested interim electric rate increase | 3.1
Percentage of interim electric rate increase requested (in hundredths) | 7.40%
Amount of approved interim increase in annual electric rates | $ 2.6
Approved percentage of interim electric rate increase (in hundredths) | 6.28%

X
- Definition The amount of the approved interim increase in annual electric rates + Details
Name: | mdu_AmountOfApprovedInterimIncreaseInAnnualElectricRates
Namespace Prefix: | mdu
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of the total rate increase included in the settlement agreement filed by the NDPSC Advocacy Staff and the Missouri Valley Resource Council on March 14, 2011. + Details
Name: | mdu_AmountOfTotalRateIncreaseIncludedInSettlementAgreement
Namespace Prefix: | mdu
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The approved percentage of interim electric rate increase. + Details
Name: | mdu_ApprovedPercentageOfInterimElectricRateIncrease
Namespace Prefix: | mdu
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Approved percentage of rate increase over current rates to cover costs + Details
Name: | mdu_ApprovedPercentageOfInterimIncrease
Namespace Prefix: | mdu
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Estimated project cost of the natural gas turbine and associated facilities. + Details
Name: | mdu_EstimatedProjectCostOfNaturalGasTurbineAndAssociatedFacilities
Namespace Prefix: | mdu
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of MW of simple cycle natural gas turbine for which an advance determination of prudence was filed. + Details
Name: | mdu_NumberOfMwOfSimpleCycleNaturalGasTurbine
Namespace Prefix: | mdu
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of the annual increase in electric rate over current rates requested by Montana-Dakota. + Details
Name: | mdu_PercentAboveCurrentRatesRequested
Namespace Prefix: | mdu
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The percentage of requested increase in interim electric rates over the current rates. + Details
Name: | mdu_PercentageOfInterimElectricRateIncreaseRequested
Namespace Prefix: | mdu
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The reductions in the amount of the rate increase sought by NDPSC Advocacy Staff. + Details
Name: | mdu_ReductionsInRateIncreaseSoughtByRegulatoryAgency
Namespace Prefix: | mdu
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Details
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Namespace Prefix: | mdu
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The total amount of the revised annual increase in electric rate requested by Montana-Dakota. + Details
Name: | mdu_RevisedTotalAnnualElectricRateIncreaseRequested
Namespace Prefix: | mdu
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The total amount of the requested interim electric rate increase. + Details
Name: | mdu_TotalAmountOfRequestedInterimElectricRateIncrease
Namespace Prefix: | mdu
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The total amount of the annual increase in electric rate requested by Montana-Dakota. + Details
Name: | mdu_TotalElectricRateIncreaseRequested
Namespace Prefix: | mdu
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration