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Regulatory matters and revenues subject to refund (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended | 0 Months Ended | 1 Months Ended
--- | --- | --- | ---
Sep. 30, 2011 NDPSC [Member] | Feb. 08, 2011 MTPSC [Member] | Aug. 31, 2010 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% |
Estimated project cost of the natural gas turbine and associated facilities | $ 85.6 |  |

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

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- Definition The approved percentage of interim electric rate increase. + Details
Name: | mdu_ApprovedPercentageOfInterimElectricRateIncrease
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Data Type: | num:percentItemType
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- Definition Estimated project cost of the natural gas turbine and associated facilities. + Details
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Balance Type: | debit
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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
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- Definition The percentage of requested increase in interim electric rates over the current rates. + Details
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Namespace Prefix: | mdu
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- Details
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- Definition The total amount of the requested interim electric rate increase. + Details
Name: | mdu_TotalAmountOfRequestedInterimElectricRateIncrease
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- 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