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Regulatory matters and revenues subject to refund (Details) (USD $) | 0 Months Ended
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Aug. 11, 2014 | Oct. 03, 2014 | Feb. 03, 2015 | Jan. 07, 2015 | Feb. 06, 2015
MTPSC [Member] |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |
Total gas rate increase requested | $ 3,000,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_MtpscMember |  |  |  |
Percent above current rates requested | 3.60%us-gaap_PublicUtilitiesRequestedRateIncreaseDecreasePercentage / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_MtpscMember |  |  |  |
WYPSC [Member] |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |
Total gas rate increase requested |  | 788,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_WYPSCMember |  |  |
Percent above current rates requested |  | 4.10%us-gaap_PublicUtilitiesRequestedRateIncreaseDecreasePercentage / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_WYPSCMember |  |  |
Subsequent Event [Member] | MTPSC [Member] |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |
Interim rate increase (decrease), amount |  |  | 2,000,000 us-gaap_PublicUtilitiesInterimRateIncreaseDecreaseAmount / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_MtpscMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |
Interim rate increase (decrease), percent |  |  | 2.30%us-gaap_PublicUtilitiesInterimRateIncreaseDecreasePercentage / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_MtpscMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |
Subsequent Event [Member] | NDPSC-Generation [Member] |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |
Approved rate adjustment increase (decrease), amount |  |  |  | 5,300,000 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_NDPSCGenerationMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Subsequent Event [Member] | NDPSC [Member] |  |  |  |  |
Regulatory matters and revenues subject to refund [Line Items] |  |  |  |  |
Total gas rate increase requested |  |  |  |  | 4,300,000 us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_NDPSCMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Percent above current rates requested |  |  |  |  | 3.40%us-gaap_PublicUtilitiesRequestedRateIncreaseDecreasePercentage / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_NDPSCMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Amount of requested interim increase in annual gas rates |  |  |  |  | $ 4,300,000 mdu_AmountOfRequestedInterimIncreaseInAnnualGasRates / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_NDPSCMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Requested percentage of interim gas rate increase |  |  |  |  | 3.40%mdu_RequestedPercentageOfInterimGasRateIncrease / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = mdu_NDPSCMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition The percentage of interim gas rate increase requested. + References No definition available. + Details
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- Definition Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency. + References No definition available. + Details
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