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Guarantees (Details 5) (USD $)
Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]  
Amount of hedging obligations guaranteed reflected on balance sheet $ 0mdu_AmountOfHedgingObligationsGuaranteedReflectedOnBalanceSheet
Guarantor obligations, maximum exposure, undiscounted 86,700,000us-gaap_GuaranteeObligationsMaximumExposure
Fixed maximum amounts guaranteed by year 2015 66,900,000mdu_FixedMaximumAmountsGuaranteedDueNextFiscalYear
Fixed maximum amounts guaranteed by year 2016 700,000mdu_FixedMaximumAmountsGuaranteedDueinTwoYears
Fixed maximum amounts guaranteed by year 2017 600,000mdu_FixedMaximumAmountsGuaranteedDueinThreeYears
Fixed maximum amounts guaranteed by year 2018 500,000mdu_FixedMaximumAmountsGuaranteedDueInFourYears
Fixed maximum amounts guaranteed by year 2019 500,000mdu_FixedMaximumAmountsGuaranteedDueinFiveYears
Expires on a specified number of days after the receipt of written notice 13,500,000mdu_FixedMaximumAmountsGuaranteedByYearExpiresAfterCertainNumberOfDaysAfterWrittenNotice
Fixed maximum amounts guaranteed by year no maturity date 4,000,000mdu_FixedMaximumAmountsGuaranteedByYearNoMaturityDate
Amount outstanding under guarantees that are reflected on balance sheet 200,000mdu_AmountOutstandingUnderGuaranteesThatIsReflectedOnBalanceSheet
Letters of credit 36,400,000mdu_LettersOfCredit
Outstanding letters of credit 0us-gaap_LettersOfCreditOutstandingAmount
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party 4,000,000mdu_FixedMaximumAmountOfGuarantee
The amount outstanding by a related party under the guarantee 1,100,000mdu_AmountOutstandingByRelatedPartyUnderGuarantee
Amount of surety bonds outstanding $ 423,500,000mdu_AmountOfSuretyBondsOutstanding