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Deferred tax reconciliation (Details 4) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Income Tax Disclosure [Abstract] |  |  |
Change in net deferred income tax liability from the preceding table | $ 94,349 mdu_IncomeTaxReconciliationChangeInNetDeferredIncomeTaxLiability |  |
Deferred taxes associated with other comprehensive loss | 2,360 mdu_DeferredTaxReconciliationOtherComprehensiveIncome |  |
Other | (9,694)mdu_DeferredTaxReconciliationOther |  |
Deferred income taxes | $ 87,015 us-gaap_DeferredIncomeTaxExpenseBenefit | $ 86,778 us-gaap_DeferredIncomeTaxExpenseBenefit | $ (7,503)us-gaap_DeferredIncomeTaxExpenseBenefit

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- Definition Taxes associated with other comprehensive income to reconcile net deferred income tax liability to deferred income tax expense for the period. + References No definition available. + Details
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