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Guarantees (Details 5) (USD $) | Dec. 31, 2012
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Guarantor Obligations [Line Items] |
Guarantor Obligations, Maximum Exposure, Undiscounted | $ 59,700,000
Fixed maximum amounts guaranteed, due next fiscal year | 38,700,000
Fixed maximum amounts guaranteed, due in two years | 2,200,000
Fixed maximum amounts guaranteed, due in three years | 300,000
Fixed maximum amounts guaranteed, due in four years | 100,000
Fixed maximum amounts guaranteed, due in six years | 600,000
Fixed maximum amounts guaranteed, due in seven years | 300,000
Expires on a specified number of days after the receipt of written notice | 13,500,000
Fixed maximum amounts guaranteed by year no maturity date | 4,000,000
Amount outstanding under guarantees that are reflected on balance sheet | 600,000
Letters of credit | 25,000,000
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 | 900,000
Amount of surety bonds outstanding | $ 488,000,000

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- Definition Amount of surety bonds outstanding related to construction contracts and reclamation obligations. + References No definition available. + Details
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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

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

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

X
- Definition Fixed maximum amounts guaranteed, due in six years. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Fixed maximum amounts guaranteed, due in three years. + References No definition available. + Details
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Balance Type: | debit
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- Definition Fixed maximum amounts guaranteed, due in two years. + 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, due next fiscal year. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Letters of credit. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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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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- 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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