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Employee benefit plans Multiemployer Plan (Details 2) - Multiemployer plans, pension [Member] - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Jun. 30, 2015
Multiemployer Plans [Line Items] |  |
Additional Multiemployer Plan Withdrawal Obligation | $ 2.4 |
Additional Estimated Multiemployer Plan Withdrawal Obligation After Tax | $ 1.5 |
Multiemployer plans, withdrawal obligation |  | $ 16.4
Estimated multiemployer plan, withdrawal obligation, after tax |  | $ 9.8

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- Definition Estimated additional multiemployer plan, withdrawal obligation, after tax. + References No definition available. + Details
Name: | mdu_AdditionalEstimatedMultiemployerPlanWithdrawalObligationAfterTax
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of the additional obligation recognized by withdrawing from a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References No definition available. + Details
Name: | mdu_AdditionalMultiemployerPlanWithdrawalObligation
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Estimated multiemployer plan, withdrawal obligation, after tax. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of the obligation recognized by withdrawing from a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References No definition available. + Details
Name: | us-gaap_MultiemployerPlansWithdrawalObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |