XML 73 R59.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Regulatory matters (Details) - USD ($) | Apr. 28, 2017 | Apr. 07, 2017 | Apr. 06, 2017 | Jan. 17, 2017 | Jan. 10, 2017 | Dec. 21, 2016 | Dec. 12, 2016 | Dec. 02, 2016 | Nov. 21, 2016 | Oct. 14, 2016 | Sep. 06, 2016 | Aug. 12, 2016 | Jun. 10, 2016 | Apr. 29, 2016 | Mar. 25, 2016 | Mar. 09, 2016 | Feb. 10, 2016 | Jan. 05, 2016 | Jan. 01, 2016 | Sep. 30, 2015
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
MNPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,600,000
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.40%
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,500,000 |
Interim rate increase (decrease), percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.40% |
Rate increase approved |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 |  |  |  |  |  |  |  |  |
Percent above current rates approved |  |  |  |  |  |  |  |  |  |  | 5.20% |  |  |  |  |  |  |  |  |
OPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,900,000 |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.80% |  |  |  |  |  |
Rate increase approved |  |  |  |  |  |  | $ 754,000 |  |  |  |  |  |  |  |  |  |  |  |  |
WYPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,200,000 |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  |  | 13.10% |  |  |  |  |  |  |
IPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  |  |  | $ 10,200,000 |  |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  |  |  | 4.10% |  |  |  |  |  |  |  |
Public utilities, requested rate increase (decrease), amended, amount |  |  |  | $ 9,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MISO [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue requirement |  |  |  |  | $ 11,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MTPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities gas and electric tax tracking adjustment |  |  |  |  |  |  |  | $ 814,000 |  |  |  |  |  |  |  |  |  |  |  |
Public utilities gas tax tracking adjustment, approved |  |  |  | 474,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities electric tax tracking adjustment, approved |  |  |  | $ 251,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MNPUC-Gas Cost Tariff [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, gas utility infrastructure cost tariff, requested |  |  |  |  |  | $ 456,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
MTPSC-Phase 2 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,700,000 |  |  |  |  |
NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase requested |  |  |  |  |  |  |  |  |  | $ 13,400,000 |  |  |  |  |  |  |  |  |  |
Percent above current rates requested |  |  |  |  |  |  |  |  |  | 6.60% |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  | $ 11,700,000 | $ 13,000,000 |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), percent |  |  |  |  |  |  |  |  | 5.80% | 6.50% |  |  |  |  |  |  |  |  |  |
NDPSC-Riders [Member] | NDPSC-Transmission Adjustment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 6,800,000 |  |  |
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |
NDPSC-Riders [Member] | NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,100,000 |  |
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |
NDPSC-Riders [Member] | NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interim rate increase (decrease), amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9,700,000 |  |  |  |
Return on equity, percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |  |
Subsequent event [Member] | WYPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  | $ 2,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent above current rates approved |  |  | 11.10% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | IPUC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved | $ 4,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent above current rates approved | 1.60% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved return on equity, percentage | 9.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | NDPSC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement |  | $ 7,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement, percentage |  | 3.70% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities refund from current rates, percentage |  | 19.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | NDPSC-Renewable Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement return on equity, percentage |  | 9.65% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | NDPSC-Electric Rider [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement return on equity, percentage |  | 9.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent event [Member] | NDPSC-Riders [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement return on equity, percentage |  | 9.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

X
- Definition Public Utilities Electric Tax Tracking Adjustment, Approved + References No definition available. + Details
Name: | mdu_PublicUtilitiesElectricTaxTrackingAdjustmentApproved
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Public Utilities Gas Tax Tracking Adjustment, Approved + References No definition available. + Details
Name: | mdu_PublicUtilitiesGasTaxTrackingAdjustmentApproved
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Public Utilities, Gas Utility Infrastructure Cost Tariff, Requested + References No definition available. + Details
Name: | mdu_PublicUtilitiesGasUtilityInfrastructureCostTariffRequested
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Public Utilities Gas and Electric Tax Tracking Adjustment, Requested + References No definition available. + Details
Name: | mdu_PublicUtilitiesGasandElectricTaxTrackingAdjustment
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Public utilities refund from current rates, percentage. + References No definition available. + Details
Name: | mdu_PublicUtilitiesRefundFromCurrentRatesPercentage
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Revenue requirement for a multivalue project. + References No definition available. + Details
Name: | mdu_RevenueRequirement
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Settlement agreement. + References No definition available. + Details
Name: | mdu_SettlementAgreement
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Settlement agreement, percentage. + References No definition available. + Details
Name: | mdu_SettlementAgreementPercentage
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Settlement agreement return on equity, percentage. + References No definition available. + Details
Name: | mdu_SettlementAgreementReturnonEquityPercentage
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of public utility's approved return on equity. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesGeneralDisclosuresLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesInterimRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesInterimRateIncreaseDecreasePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of public utility's amended requested rate increase (decrease) with regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmendedAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of public utility's requested rate increase (decrease) with regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of public utility's requested rate increase (decrease) with regulatory agency. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesRequestedRateIncreaseDecreasePercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of public utility's requested return on equity. + References No definition available. + Details
Name: | us-gaap_PublicUtilitiesRequestedReturnOnEquityPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_MNPUCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_OPUCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_WYPSCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_IPUCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_MISOMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_MtpscMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_MNPUCGasCostTariffMemberMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_MTPSCPhase2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_NDPSCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PublicUtilitiesRegulatoryProceedingAxis=mdu_NDPSCRidersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_NDPSCTransmissionAdjustmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_NDPSCRenewableRiderMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=mdu_NDPSCElectricRiderMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |