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Subsequent events (Details) - Subsequent event [Member] - USD ($) $ in Millions | Dec. 10, 2015 | Oct. 29, 2015 | Oct. 01, 2015
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MDU Energy Capital, LLC [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Senior notes issued |  |  | $ 50.0
Senior notes weighted average interest rate |  |  | 4.31%
Parent Company [Member] |  |  |
Subsequent Event [Line Items] |  |  |
Senior notes issued | $ 52.0 | $ 98.0 |
Senior notes weighted average interest rate | 4.03% | 3.90% |
Note purchase agreement |  | $ 150.0 |

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

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- Definition Senior notes issued. + References No definition available. + Details
Name: | mdu_SeniorNotesIssued
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Weighted average interest rate of senior notes. + References No definition available. + Details
Name: | mdu_SeniorNotesWeightedAverageInterestRate
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
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=mdu_MduEnergyCapitalLlcMember
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: |

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