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Subsequent event (Details) - Jul. 21, 2015 - Parent Company [Member] - Subsequent Event [Member] - USD ($) $ in Millions | Total
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Subsequent Event [Line Items] |
Term loan agreement | $ 75.0
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00%

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- Definition Ratio of total debt to total capitalization as specified in debt convenants. + References No definition available. + Details
Name: | mdu_RatioOfTotalDebtToTotalCapitalizationAsSpecifiedInDebtConvenants
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Term loan agreement. + References No definition available. + Details
Name: | mdu_Termloanagreement
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=us-gaap_ParentCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |