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Guarantees (Details 3) (USD $) | Jun. 30, 2012
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Guarantor obligations [Line Items] |
Amount of hedging obligations guaranteed reflected on balance sheet | $ 0
Guarantor Obligations, Maximum Exposure, Undiscounted | 87,300,000
Fixed maximum amounts guaranteed by year 2012 | 7,900,000
Fixed maximum amounts guaranteed by year 2013 | 62,400,000
Fixed maximum amounts guaranteed by year 2014 | 300,000
Fixed maximum amounts guaranteed by year 2015 | 100,000
Fixed maximum amounts guaranteed by year 2016 | 100,000
Fixed maximum amounts guaranteed by year 2018 | 700,000
Fixed maximum amounts guaranteed by year 2019 | 300,000
Expires on a specified number of days after the receipt of written notice | 11,500,000
No scheduled maturity date | 4,000,000
Amount outstanding under guarantees that is reflected on balance sheet | 500,000
Letters of credit | 27,500,000
Letters of credit set to expire in 2012 | 20,200,000
Letters of credit set to expire in 2013 | 7,300,000
Outstanding letters of credit | 0
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party | 5,000,000
The amount outstanding by a related party under the guarantee | 1,100,000
Amount of surety bonds outstanding | $ 604,000,000

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: | na
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 for 2012. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYear2012
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed for 2013. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYear2013
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed for 2014. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYear2014
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed for 2015. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYear2015
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed for 2016. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYear2016
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed for 2018. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYear2018
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed for 2019. + References No definition available. + Details
Name: | mdu_FixedMaximumAmountsGuaranteedByYear2019
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

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

X
- Definition Outstanding letters of credit set to expire in 2012. + References No definition available. + Details
Name: | mdu_LettersOfCreditSetToExpireIn2012
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Outstanding letters of credit set to expire in 2013. + References No definition available. + Details
Name: | mdu_LettersOfCreditSetToExpireIn2013
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- 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 FASB Interpretation (FIN) -Number 45 -Paragraph 13 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: 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
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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant