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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

Debt instrument [Line Items]

Letters of credit

$ 36.4mdu_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

175.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= mdu_CommercialPaperRevolvingCreditAgreementMember
/ us-gaap_DebtInstrumentAxis
= us-gaap_LongTermDebtMember
/ dei_LegalEntityAxis
= mdu_MduResourcesGroupIncMember

175.0us-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

0mdu_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

50.0us-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

2.2mdu_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

65.0us-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

0mdu_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

650.0us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= mdu_CommercialPaperRevolvingCreditAgreementMember
/ us-gaap_DebtInstrumentAxis
= us-gaap_LongTermDebtMember
/ dei_LegalEntityAxis
= mdu_CentennialEnergyHoldingsIncMember

650.0us-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

0mdu_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

175mdu_LongtermPrivateShelfAgreement
/ us-gaap_CreditFacilityAxis
= mdu_UncommittedLongTermPrivateShelfAgreementMember
/ us-gaap_DebtInstrumentAxis
= us-gaap_LongTermDebtMember
/ dei_LegalEntityAxis
= mdu_WbiEnergyTransmissionIncMember

Notes outstanding

100.0mdu_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

25.0mdu_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

$ 28.0mdu_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.

X

- Definition

Letters of credit.

+ References

No definition available.

+ Details

Name:	mdu_LettersOfCredit
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- 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

X

- Definition

Notes outstanding which reduce capacity under the uncommitted private shelf agreement.

+ References

No definition available.

+ Details

Name:	mdu_Notesoutstanding
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Ratio - EBITDA to interest expense.

+ References

No definition available.

+ Details

Name:	mdu_RatioEbitdaToInterestExpense
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Ratio - EBIT to interest expense.

+ References

No definition available.

+ Details

Name:	mdu_RatioEbitToInterestExpense
Namespace Prefix:	mdu_
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

Name:	mdu_RatioOfFundedDebtCapitalizationCompanyAloneAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Ratio of funded debt to total capitalization as specified in debt convenants.

+ References

No definition available.

+ Details

Name:	mdu_RatioOfFundedDebtToTotalCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants.

+ References

No definition available.

+ Details

Name:	mdu_RatioOfSubsidiaryDebtToSubsidiaryCapitalizationAsSpecifiedInDebtConvenants
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

Name:	mdu_RatioOfTotalDebtToAdjustedTotalCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Ratio of total debt to total capitalization as specified in debt convenants.

+ References

No definition available.

+ Details

Name:	mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Senior notes issued.

+ References

No definition available.

+ Details

Name:	mdu_SeniorNotesIssued
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Weighted average interest rate of senior notes issued.

+ References

No definition available.

+ Details

Name:	mdu_SeniorNotesWeightedAverageInterestRate
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	dei_LegalEntityAxis=mdu_CascadeNaturalGasCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=mdu_CentennialEnergyHoldingsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=mdu_IntermountainGasCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=mdu_MduEnergyCapitalLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=mdu_MduResourcesGroupIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=mdu_WbiEnergyTransmissionIncMember
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: