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