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Guarantees (Details 3) (USD $) | Sep. 30, 2014
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Guarantor Obligations [Line Items] |
Amount of hedging obligations guaranteed reflected on balance sheet | $ 0
Guarantor obligations, maximum exposure, undiscounted | 114,300,000
Fixed maximum amounts guaranteed by year 2014 | 19,400,000
Fixed maximum amounts guaranteed by year 2015 | 75,100,000
Fixed maximum amounts guaranteed by year 2016 | 700,000
Fixed maximum amounts guaranteed by year 2017 | 600,000
Fixed maximum amounts guaranteed by year 2018 | 500,000
Fixed maximum amounts guaranteed by year 2019 | 500,000
Fixed maximum amounts guaranteed by year expires after certain number of days after written notice | 13,500,000
No scheduled maturity date | 4,000,000
Amount outstanding under guarantees that is reflected on balance sheet | 200,000
Letters of credit | 39,000,000
Letters of credit set to expire - 2014 | 8,200,000
Letters of credit set to expire - 2015 | 30,800,000
Outstanding letters of credit | 0
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party | 4,000,000
Amount outstanding by a related party under the guarantee | 1,000,000
Amount of surety bonds outstanding | $ 387,000,000

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

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- 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

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

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- Definition The amount outstanding under gurantees that is reflected on the balance sheet. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Natural gas transportation and storage agreement fixed maximum amount of performance guarantee. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Definition Fixed maximum amounts guaranteed by year four. + References No definition available. + Details
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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
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Fixed maximum amounts guaranteed by year one. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Fixed maximum amounts guaranteed by year three. + References No definition available. + Details
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Balance Type: | debit
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- Definition Fixed maximum amounts guaranteed by year two. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Fixed maximum amounts guaranteed, due in six years. + References No definition available. + Details
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Balance Type: | debit
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- Definition Fixed maximun amounts guaranteed by year five. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- 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

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- Definition Letters of credit set to expire current year. + References No definition available. + Details
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Balance Type: | credit
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- Definition Letters of credit set to expire in one year. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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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 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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- 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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