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Defined contribution plans (Details 12) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Nonqualified Defined Contribution Plan [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | $ 104,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_NonqualifiedDefinedContributionPlanMember | $ 25,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_NonqualifiedDefinedContributionPlanMember |
Defined Contribution Plan [Member] |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, cost recognized | $ 34,400,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_DefinedContributionPlanMember | $ 33,200,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_DefinedContributionPlanMember | $ 29,300,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_DefinedContributionPlanNameAxis = mdu_DefinedContributionPlanMember

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_DefinedContributionPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DefinedContributionPlanNameAxis=mdu_DefinedContributionPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DefinedContributionPlanNameAxis=mdu_NonqualifiedDefinedContributionPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |