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Discontinued operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2011
Discontinued Operations and Disposal Groups [Abstract]
Arbitration charge related to a guarantee	$ 21.0mdu_ArbitrationChargeRelatedToGuarantee
Arbitration charge after tax	$ 13.0mdu_ArbitrationChargeAfterTax

X

- Definition

Arbitration charge after tax

+ References

No definition available.

+ Details

Name:	mdu_ArbitrationChargeAfterTax
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Arbitration charge related to a guarantee

+ References

No definition available.

+ Details

Name:	mdu_ArbitrationChargeRelatedToGuarantee
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_DiscontinuedOperationsAndDisposalGroupsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration