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Goodwill and other intangible assets (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period | $ 636,039 us-gaap_Goodwill | [1] | $ 636,039 us-gaap_Goodwill | [1]
Goodwill acquired during the year/other | (835)us-gaap_GoodwillOtherChanges |  |  |
Goodwill acquired during the year |  |  | 0 us-gaap_GoodwillAcquiredDuringPeriod |
Balance as of end of period | 635,204 us-gaap_Goodwill | [1] | 636,039 us-gaap_Goodwill | [1]
Natural gas distribution: |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period | 345,736 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  | 345,736 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |
Goodwill acquired during the year/other | 0 us-gaap_GoodwillOtherChanges / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  |  |
Goodwill acquired during the year |  |  | 0 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |
Balance as of end of period | 345,736 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |  | 345,736 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_NaturalGasDistributionMember |
Pipeline and energy services: |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period | 9,737 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember | [1] | 9,737 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember | [1]
Goodwill acquired during the year/other | 0 us-gaap_GoodwillOtherChanges / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  |  |
Goodwill acquired during the year |  |  | 0 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |
Balance as of end of period | 9,737 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember | [1] | 9,737 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember | [1]
Accumulated impairment which occurred in prior periods | 12,300 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |  | 12,300 us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = mdu_PipelineAndEnergyServicesMember |
Construction materials and contracting: |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period | 176,290 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  | 176,290 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |
Goodwill acquired during the year/other | 0 us-gaap_GoodwillOtherChanges / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  |  |
Goodwill acquired during the year |  |  | 0 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |
Balance as of end of period | 176,290 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |  | 176,290 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionMaterialsAndContractingMember |
Construction services: |  |  |  |
Goodwill [Roll Forward] |  |  |  |
Balance as of beginning of period | 104,276 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  | 104,276 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |
Goodwill acquired during the year/other | (835)us-gaap_GoodwillOtherChanges / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  |  |
Goodwill acquired during the year |  |  | 0 us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |
Balance as of end of period | $ 103,441 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |  | $ 104,276 us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = mdu_ConstructionServicesMember |
[1] | Balance is presented net of accumulated impairment of $12.3 million at the pipeline and energy services segment, which occurred in prior periods.

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 + Details
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- Definition Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Definition Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a,h) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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- Definition Amount of other (increase) decrease of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 + Details
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