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Subsquent events (Details) (Subsequent event [Member], USD $) In Millions, unless otherwise specified | 0 Months Ended
--- | ---
Oct. 01, 2013 MDU Energy Capital [Member] | Oct. 30, 2013 Cascade Natural Gas Co. [Member] | Oct. 30, 2013 Intermountain Gas Company [Member] | Nov. 01, 2013 Centennial Energy Holdings, Inc [Member]
Subsequent Event [Line Items] |  |  |  |
Credit agreement | $ 30.0 |  |  |
Senior notes issued |  | $ 50.0 | $ 50.0 | $ 25.0
Senior notes weighted average interest rate |  | 4.24% | 4.21% | 4.84%

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- Definition Credit agreement. + References No definition available. + Details
Name: | mdu_CreditAgreement
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

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- Definition Describes the event or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued. + References No definition available. + Details
Name: | mdu_SeniorNotesIssued
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

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- Definition Describes the event or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued. + References No definition available. + Details
Name: | mdu_SeniorNotesWeightedAverageInterestRate
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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