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Enviromental matters (Details 2) (USD $) | 12 Months Ended
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Dec. 31, 2014
Portland Harbor site [Member] |
Site Contingency [Line Items] |
Environmental matters investigative costs | $ 70,000,000 mdu_EnvironmentalMattersInvestigativeCosts / us-gaap_EnvironmentalRemediationSiteAxis = mdu_PortlandHarborSiteMember
Eugene, OR manufactured gas plant site [Member] |
Site Contingency [Line Items] |
Site contingency, low estimate, loss exposure | 500,000 mdu_SiteContingencyLowEstimateLossExposure / us-gaap_EnvironmentalRemediationSiteAxis = mdu_EugeneORManufacturedGasPlantSiteMember
Site contingency, high estimate, loss exposure | 11,000,000 mdu_SiteContingencyHighEstimateLossExposure / us-gaap_EnvironmentalRemediationSiteAxis = mdu_EugeneORManufacturedGasPlantSiteMember
Estimated proportional share of cleanup liability | 50.00%mdu_EstimatedProportionalShareOfCleanupLiability / us-gaap_EnvironmentalRemediationSiteAxis = mdu_EugeneORManufacturedGasPlantSiteMember
Accrual for environmental loss contingencies | 1,700,000 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = mdu_EugeneORManufacturedGasPlantSiteMember
Bremerton, WA manufactured gas plant site [Member] |
Site Contingency [Line Items] |
Site contingency, low estimate, loss exposure | 340,000 mdu_SiteContingencyLowEstimateLossExposure / us-gaap_EnvironmentalRemediationSiteAxis = mdu_BremertonWAManufacturedGasPlantSiteMember
Site contingency, high estimate, loss exposure | 6,400,000 mdu_SiteContingencyHighEstimateLossExposure / us-gaap_EnvironmentalRemediationSiteAxis = mdu_BremertonWAManufacturedGasPlantSiteMember
Accrual for environmental loss contingencies | 12,500,000 us-gaap_AccrualForEnvironmentalLossContingencies / us-gaap_EnvironmentalRemediationSiteAxis = mdu_BremertonWAManufacturedGasPlantSiteMember
Bellingham, WA manufactured gas plant site [Member] |
Site Contingency [Line Items] |
Site contingency, loss exposure not accrued, best estimate | $ 8,000,000 us-gaap_SiteContingencyLossExposureNotAccruedBestEstimate / us-gaap_EnvironmentalRemediationSiteAxis = mdu_BellinghamWAManufacturedGasPlantSiteMember

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- Definition Estimated investigative costs. + References No definition available. + Details
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Namespace Prefix: | mdu_
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Balance Type: | debit
Period Type: | instant

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- Definition The estimated proportional share of the clean-up costs. + References No definition available. + Details
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Balance Type: | na
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- Definition High-end of the range estimate of the loss exposure for reasonably possible environmental contingencies at an individual site. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Low-end of the range estimate of the loss exposure for reasonable possible environmental contingencies at an individual site. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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- Details
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- Definition Best estimate of the loss exposure for reasonably possible environmental contingencies at an individual site for which no accrual has been recorded. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (c),(d)(2) -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859 + Details
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- Details
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- Details
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Period Type: |