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Subsequent events (Details) (USD $)	Oct. 04, 2012 Borrowing capacity amendment [Member]	Oct. 22, 2012 Refinancing of Debt [Member] MDU Energy Capital, LLC [Member]	May 15, 2013 Future issuance of debt [Member] MDU Energy Capital, LLC [Member]
Subsequent events [Line Items]
Borrowing capacity of amended revolving credit agreement	$ 125,000,000
Issuance under agreement		25,000,000
Weighted average interest rate		4.10%
Future issuance of debt			$ 25,000,000

X

- Definition

Borrowing capacity of amended revolving credit agreement.

+ References

No definition available.

+ Details

Name:	mdu_BorrowingCapacityOfAmendedRevolvingCreditAgreement
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Future issuance of long-term debt.

+ References

No definition available.

+ Details

Name:	mdu_FutureIssuanceOfDebt
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Issuance under agreement.

+ References

No definition available.

+ Details

Name:	mdu_IssuanceUnderAgreement
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Weighted average interest rate.

+ References

No definition available.

+ Details

Name:	mdu_WeightedAverageInterestRate
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_SubsequentEventLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration