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Unrecognized tax benefits (Details 6) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |
Balance at beginning of year | $ 14,914,000 us-gaap_UnrecognizedTaxBenefits | $ 14,914,000 us-gaap_UnrecognizedTaxBenefits | $ 11,206,000 us-gaap_UnrecognizedTaxBenefits
Additions for tax positions of prior years | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 3,708,000 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Settlements | (14,777,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
Balance at end of year | 137,000 us-gaap_UnrecognizedTaxBenefits | 14,914,000 us-gaap_UnrecognizedTaxBenefits | 14,914,000 us-gaap_UnrecognizedTaxBenefits
Unrecognized tax benefits highly certain deductibility |  | 8,400,000 mdu_UnrecognizedTaxBenefitsWhereDeductibilityIsHighlyCertain |
Unrecognized tax benefits that would affect the effective tax rate | 155,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | 9,000,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |
Unrecognized tax benefits, interest and penalties | 18,000 mdu_UnrecognizedTaxBenefitsInterestAndPenalties | 2,500,000 mdu_UnrecognizedTaxBenefitsInterestAndPenalties |
Interest expense | 1,800,000 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense | 1,200,000 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense | 740,000 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense
Interest income | 540,000 mdu_InterestIncomeOnIncomeTaxes | 660,000 mdu_InterestIncomeOnIncomeTaxes | 290,000 mdu_InterestIncomeOnIncomeTaxes
Accrued liability for payment of interest | $ 1,800,000 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued | $ 2,800,000 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued |

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- Definition This element represents interest income recognized for an overpayment of income taxes computed by applying the applicable statutory rate of interest to the difference between a tax position recognized for financial reporting purposes and the amount previously taken or expected to be taken in a tax return of the entity. + References No definition available. + Details
Name: | mdu_InterestIncomeOnIncomeTaxes
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The amount of interest and penalties on unrecognized tax benefits that would affect the effective tax rate. + References No definition available. + Details
Name: | mdu_UnrecognizedTaxBenefitsInterestAndPenalties
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The gross amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns where the likelihood of deductibility is highly certain. + References No definition available. + Details
Name: | mdu_UnrecognizedTaxBenefitsWhereDeductibilityIsHighlyCertain
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of interest expense accrued for an underpayment of income taxes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount of interest expense for an underpayment of income taxes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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