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Stock awards (Details 2) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Nonemployee director stock compensation plan [Member] |  |  |
Share-based compensation arrangement by share-based payment award [Line Items] |  |  |
Shares issued for stock-based compensation | 43,088 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = mdu_NonemployeeDirectorStockCompensationPlanMember | 36,713 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = mdu_NonemployeeDirectorStockCompensationPlanMember | 53,888 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = mdu_NonemployeeDirectorStockCompensationPlanMember
Fair value of vested shares | $ 1,100,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = mdu_NonemployeeDirectorStockCompensationPlanMember | $ 1,100,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = mdu_NonemployeeDirectorStockCompensationPlanMember | $ 1,100,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = mdu_NonemployeeDirectorStockCompensationPlanMember
Long-term incentive plan [Member] |  |  |
Share-based compensation arrangement by share-based payment award [Line Items] |  |  |
Shares issued for stock-based compensation |  |  | 43,103 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod / us-gaap_AwardTypeAxis = mdu_LongTermIncentivePlanMember
Fair value of vested shares |  |  | $ 930,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = mdu_LongTermIncentivePlanMember

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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_AwardTypeAxis=mdu_NonemployeeDirectorStockCompensationPlanMember
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Data Type: | na
Balance Type: |
Period Type: |