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Long-term borrowings (Details 3) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | May 08, 2014 | Sep. 12, 2013 | Jan. 15, 2015 | Jan. 09, 2015
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Debt instrument [Line Items] |  |  |  |  |  |
Letters of credit | $ 36.4 mdu_LettersOfCredit |  |  |  |  |
MDU Resources Group, Inc [Member] | Commercial paper revolving credit agreement [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Facility limit, maximum borrowing capacity | 175.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember |  | 175.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember |  |  |
Ratio of funded debt to total capitalization as specified in debt convenants | 65.00%mdu_RatioOfFundedDebtToTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember |  |  |  |  |
Ratio of funded debt to capitalization - Company alone, as specified in debt convenants | 65.00%mdu_RatioOfFundedDebtCapitalizationCompanyAloneAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember |  |  |  |  |
MDU Resources Group, Inc [Member] | Revolving credit facility [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Letters of credit | 0 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember |  |  |  |  |
MDU Energy Capital, LLC [Member] | Master shelf agreement [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Ratio of total debt to adjusted total capitalization as specified in debt convenants | 70.00%mdu_RatioOfTotalDebtToAdjustedTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = mdu_MasterShelfAgreementMemberMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduEnergyCapitalLlcMember |  |  |  |  |
Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants | 65.00%mdu_RatioOfSubsidiaryDebtToSubsidiaryCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = mdu_MasterShelfAgreementMemberMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduEnergyCapitalLlcMember |  |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00%mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = mdu_MasterShelfAgreementMemberMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduEnergyCapitalLlcMember |  |  |  |  |
Ratio - EBIT to interest expense | 150.00%mdu_RatioEbitToInterestExpense / us-gaap_CreditFacilityAxis = mdu_MasterShelfAgreementMemberMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduEnergyCapitalLlcMember |  |  |  |  |
Cascade Natural Gas Corporation [Member] | Revolving credit facility [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Facility limit, maximum borrowing capacity | 50.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember | [1] |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00%mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember |  |  |  |  |
Letters of credit | 2.2 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember | [2] |  |  |  |
Intermountain Gas Company [Member] | Revolving credit facility [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Facility limit, maximum borrowing capacity | 65.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_IntermountainGasCompanyMember | [3] |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00%mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_IntermountainGasCompanyMember |  |  |  |  |
Letters of credit | 0 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_IntermountainGasCompanyMember |  |  |  |  |
Centennial Energy Holdings, Inc [Member] | Commercial paper revolving credit agreement [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Facility limit, maximum borrowing capacity | 650.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember |  | 650.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember |  |  |
Centennial Energy Holdings, Inc [Member] | Master shelf agreement [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 60.00%mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = mdu_MasterShelfAgreementMemberMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember |  |  |  |  |
Ratio of EBITDA to interest expense | 175.00%mdu_RatioEbitdaToInterestExpense / us-gaap_CreditFacilityAxis = mdu_MasterShelfAgreementMemberMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember |  |  |  |  |
Centennial Energy Holdings, Inc [Member] | Revolving credit facility [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00%mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember |  |  |  |  |
Letters of credit | 0 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember |  |  |  |  |
WBI Energy Transmission, Inc. [Member] | Uncommitted Long Term Private Shelf Agreement [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 55.00%mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants / us-gaap_CreditFacilityAxis = mdu_UncommittedLongTermPrivateShelfAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_WbiEnergyTransmissionIncMember |  |  |  |  |
Long-term private shelf agreement |  |  |  | 175 mdu_LongtermPrivateShelfAgreement / us-gaap_CreditFacilityAxis = mdu_UncommittedLongTermPrivateShelfAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_WbiEnergyTransmissionIncMember |  |
Notes outstanding | 100.0 mdu_Notesoutstanding / us-gaap_CreditFacilityAxis = mdu_UncommittedLongTermPrivateShelfAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_WbiEnergyTransmissionIncMember |  |  |  |  |
Subsequent Event [Member] | Cascade Natural Gas Corporation [Member] | Senior notes [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Senior notes issued |  |  |  |  | 25.0 mdu_SeniorNotesIssued / us-gaap_CreditFacilityAxis = us-gaap_SeniorNotesMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Senior notes weighted average interest rate |  |  |  |  | 4.20%mdu_SeniorNotesWeightedAverageInterestRate / us-gaap_CreditFacilityAxis = us-gaap_SeniorNotesMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Subsequent Event [Member] | Centennial Energy Holdings, Inc [Member] | Letter of credit [Member] | Long-term debt [Member] |  |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |  |
Letters of credit |  |  |  |  |  | $ 28.0 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_LetterOfCreditMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
[1] | Certain provisions allow for increased borrowings, up to a maximum of $75.0 million.
[2] | An outstanding letter of credit reduces the amount available under the credit agreement.
[3] | Certain provisions allow for increased borrowings, up to a maximum of $90.0 million.

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- Definition Letters of credit. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Long-term private shelf agreement. + References No definition available. + Details
Name: | mdu_LongtermPrivateShelfAgreement
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Notes outstanding which reduce capacity under the uncommitted private shelf agreement. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Ratio - EBITDA to interest expense. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Ratio - EBIT to interest expense. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio of funded debt capitalization - Company alone, as specified in debt convenants. + References No definition available. + Details
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- Definition Ratio of funded debt to total capitalization as specified in debt convenants. + References No definition available. + Details
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Namespace Prefix: | mdu_
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- Definition Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio of total debt to adjusted total capitalization as specified in debt convenants. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Ratio of total debt to total capitalization as specified in debt convenants. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
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- Definition Senior notes issued. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Weighted average interest rate of senior notes issued. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
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Balance Type: | na
Period Type: | duration

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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Namespace Prefix: | us-gaap_
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- Details
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- Details
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- Details
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- Details
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