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Income taxes (Details 9) | 12 Months Ended
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Dec. 31, 2017
Accounting Policies [Abstract] |
Threshold of likelihood of tax positions being realized upon ultimate settlement with a taxing authority | 50.00%

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- Definition Threshold of likelihood that tax benefits will be realized upon ultimate settlement with a taxing authority. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration