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

X
- Definition The amount of hedging obligations outstanding reflected on the balance sheet and guaranteed by a related party. + References No definition available. + Details
Name: | mdu_AmountOfHedgingObligationsGuaranteedReflectedOnBalanceSheet
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of surety bonds outstanding related to construction contracts and reclamation obligations. + References No definition available. + Details
Name: | mdu_AmountOfSuretyBondsOutstanding
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount outstanding related to a natural gas transportation and storage agreement. + References No definition available. + Details
Name: | mdu_AmountOutstandingByRelatedPartyUnderGuarantee
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount outstanding under gurantees that is reflected on the balance sheet. + References No definition available. + Details
Name: | mdu_AmountOutstandingUnderGuaranteesThatIsReflectedOnBalanceSheet
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Natural gas transportation and storage agreement fixed maximum amount of performance guarantee. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountOfGuarantee
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed that expires after a certain number of days written notice. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYearExpiresAfterCertainNumberOfDaysAfterWrittenNotice
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed with no maturity date. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYearNoMaturityDate
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed, due in five years. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedDueinFiveYears
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed, due in four years. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedDueInFourYears
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed, due in three years. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedDueinThreeYears
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed, due in two years. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedDueinTwoYears
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed, due next fiscal year. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedDueNextFiscalYear
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Letters of credit. + References No definition available. + Details
Name: | mdu_LettersOfCredit
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248 + Details
Name: | us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant