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Short-term borrowings (Details 2) (Revolving credit facility [Member], Short-term debt [Member], Dakota Prairie Refining, LLC [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 01, 2014
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Revolving credit facility [Member] | Short-term debt [Member] | Dakota Prairie Refining, LLC [Member] |  |  |
Short-term debt [Line Items] |  |  |
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 | [1] | $ 50.0 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember
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_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
Ratio of total funded debt to total capitalization as specified in debt covenants | 65.00%mdu_Ratiooftotalfundeddebttototalcapitalizationasspecifiedindebtcovenants / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_DebtInstrumentAxis = us-gaap_ShortTermDebtMember / dei_LegalEntityAxis = mdu_DakotaPrairieRefiningLlcMember |  |
[1] | Certain provisions allow for increased borrowings up to a maximum of $75.0 million.

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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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Period Type: | instant

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

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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_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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