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Credit facilities (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2014 | May 08, 2014 | Dec. 31, 2013 | Dec. 01, 2014
Line of credit facility and other debt [Abstract] |  |  |  |  |  |
Letters of credit at end of period | $ 36.4 mdu_LettersOfCredit |  |  |  |  |
Long-term debt [Member] | Commercial paper revolving credit agreement [Member] | MDU Resources Group, Inc [Member] |  |  |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |  |  |
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 |  |  |
Amount outstanding, end of period | 77.5 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember | [1] |  | 78.9 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember | [1] |
Expiration date | May 08, 2019 |  |  |  |  |
Long-term debt [Member] | Commercial paper revolving credit agreement [Member] | Centennial Energy Holdings, Inc [Member] |  |  |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |  |  |
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 |  |  |
Amount outstanding, end of period | 211.0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember | [1] |  | 75.0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = mdu_CommercialPaperRevolvingCreditAgreementMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember | [1] |
Expiration date | May 08, 2019 |  |  |  |  |
Long-term debt [Member] | Revolving credit facility [Member] | MDU Resources Group, Inc [Member] |  |  |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |  |  |
Letters of credit at end of period | 0 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember |  |  |  |  |
Option to increase borrowings, maximum amount | 225.0 mdu_OptionToIncreaseBorrowingsMaximumAmount / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_MduResourcesGroupIncMember |  |  |  |  |
Long-term debt [Member] | Revolving credit facility [Member] | Cascade Natural Gas Corporation [Member] |  |  |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |  |  |
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 | [2] |  |  |  |
Amount outstanding, end of period | 0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember |  |  | 11.5 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember |  |
Letters of credit at end of period | 2.2 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember | [3] |  |  |  |
Expiration date | Jul. 09, 2018 |  |  |  |  |
Option to increase borrowings, maximum amount | 75.0 mdu_OptionToIncreaseBorrowingsMaximumAmount / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CascadeNaturalGasCorporationMember |  |  |  |  |
Long-term debt [Member] | Revolving credit facility [Member] | Intermountain Gas Company [Member] |  |  |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |  |  |
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 | [4] |  |  |  |
Amount outstanding, end of period | 21.0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_IntermountainGasCompanyMember |  |  | 3.0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_IntermountainGasCompanyMember |  |
Letters of credit at end of period | 0 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_IntermountainGasCompanyMember |  |  |  |  |
Expiration date | Jul. 13, 2018 |  |  |  |  |
Option to increase borrowings, maximum amount | 90.0 mdu_OptionToIncreaseBorrowingsMaximumAmount / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_IntermountainGasCompanyMember |  |  |  |  |
Long-term debt [Member] | Revolving credit facility [Member] | Centennial Energy Holdings, Inc [Member] |  |  |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |  |  |
Letters of credit at end of period | 0 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember |  |  |  |  |
Option to increase borrowings, maximum amount | 800.0 mdu_OptionToIncreaseBorrowingsMaximumAmount / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_LongTermDebtMember / dei_LegalEntityAxis = mdu_CentennialEnergyHoldingsIncMember |  |  |  |  |
Short-term debt [Member] | Revolving credit facility [Member] | Dakota Prairie Refining, LLC [Member] |  |  |  |  |  |
Line of credit facility and other debt [Abstract] |  |  |  |  |  |
Facility limit, maximum borrowing capacity | 50.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember | [5] |  |  |  | 50.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember
Amount outstanding, end of period | 0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |  | 0 us-gaap_LineOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Letters of credit at end of period | 1.0 mdu_LettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember | [3] |  |  |  |
Expiration date | Dec. 01, 2015 |  |  |  |  |
Option to increase borrowings, maximum amount | $ 75.0 mdu_OptionToIncreaseBorrowingsMaximumAmount / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |  |  |  |
[1] | Amount outstanding under commercial paper program.
[2] | Certain provisions allow for increased borrowings, up to a maximum of $75.0 million.
[3] | An outstanding letter of credit reduces the amount available under the credit agreement.
[4] | Certain provisions allow for increased borrowings, up to a maximum of $90.0 million.
[5] | Certain provisions allow for increased borrowings up to a maximum of $75.0 million.

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- 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

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- Definition The option to increase borrowing capacity to the stated maximum amount, at the option of the company, on stated conditions. + References No definition available. + Details
Name: | mdu_OptionToIncreaseBorrowingsMaximumAmount
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_LineOfCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Definition Date the credit facility terminates, in CCYY-MM-DD format. + 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_LineOfCreditFacilityExpirationDate1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
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
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_DebtInstrumentAxis=us-gaap_LongTermDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=us-gaap_ShortTermDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |