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Income tax expense (benefit) (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
Federal	$ 32,726us-gaap_CurrentFederalTaxExpenseBenefit	$ 45,518us-gaap_CurrentFederalTaxExpenseBenefit	$ (26,858)us-gaap_CurrentFederalTaxExpenseBenefit
State	5,390us-gaap_CurrentStateAndLocalTaxExpenseBenefit	4,311us-gaap_CurrentStateAndLocalTaxExpenseBenefit	858us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Foreign	0us-gaap_CurrentForeignTaxExpenseBenefit	(29)us-gaap_CurrentForeignTaxExpenseBenefit	(75)us-gaap_CurrentForeignTaxExpenseBenefit
Current income taxes	38,116us-gaap_CurrentIncomeTaxExpenseBenefit	49,800us-gaap_CurrentIncomeTaxExpenseBenefit	(26,075)us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred:
Federal	81,017us-gaap_DeferredFederalIncomeTaxExpenseBenefit	78,953us-gaap_DeferredFederalIncomeTaxExpenseBenefit	(1,224)us-gaap_DeferredFederalIncomeTaxExpenseBenefit
State	4,989us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit	8,031us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit	(6,323)us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit
Investment tax credit - net	1,009us-gaap_InvestmentTaxCredit	(206)us-gaap_InvestmentTaxCredit	44us-gaap_InvestmentTaxCredit
Deferred income taxes	87,015us-gaap_DeferredIncomeTaxExpenseBenefit	86,778us-gaap_DeferredIncomeTaxExpenseBenefit	(7,503)us-gaap_DeferredIncomeTaxExpenseBenefit
Change in uncertain tax positions	(5,183)mdu_ChangeInUncertainTaxBenefits	0mdu_ChangeInUncertainTaxBenefits	1,974mdu_ChangeInUncertainTaxBenefits
Change in accrued interest	21mdu_ChangeInAccruedInterest	158mdu_ChangeInAccruedInterest	458mdu_ChangeInAccruedInterest
Total income tax expense (benefit)	$ 119,969us-gaap_IncomeTaxExpenseBenefit	$ 136,736us-gaap_IncomeTaxExpenseBenefit	$ (31,146)us-gaap_IncomeTaxExpenseBenefit

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