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Regulatory matters and revenues subject to refund (Details) (USD $) In Millions, unless otherwise specified | Dec. 21, 2012 | Sep. 26, 2012
--- | --- | ---
Regulatory Assets [Line Items] |  |
Total gas rate increase requested | $ 1.5 | $ 3.5
Percent above current rates requested | 3.30% | 5.90%
Amount of requested interim increase in annual gas rates |  | $ 1.7
Requested percentage of interim gas rate increase |  | 2.90%

X
- Definition The amount of requested interim increase in annual gas rates. + References No definition available. + Details
Name: | mdu_AmountOfRequestedInterimIncreaseInAnnualGasRates
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The percentage of the annual increase in rates over current rates requested. + References No definition available. + Details
Name: | mdu_PercentAboveCurrentRatesRequested
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The percentage of interim gas rate increase requested. + References No definition available. + Details
Name: | mdu_RequestedPercentageOfInterimGasRateIncrease
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The total amount of the annual increase in gas rates requested. + References No definition available. + Details
Name: | mdu_TotalGasRateIncreaseRequested
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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