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Subsequent event (Details) (Withdrawal from multiemployer defined benefit plan [Member], Subsequent event [Member], USD $) In Millions, unless otherwise specified	Oct. 26, 2014
Withdrawal from multiemployer defined benefit plan [Member] | Subsequent event [Member]
Subsequent event [Line Items]
Estimated multiemployer plan, withdrawal obligation	$ 14.0
Estimated multiemployer plan, withdrawal obligation, after tax	$ 8.4

X

- Definition

Estimated multiemployer plan, withdrawal obligation, after tax.

+ References

No definition available.

+ Details

Name:	mdu_EstimatedMultiemployerPlanWithdrawalObligationAfterTax
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 9

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

+ Details

Name:	us-gaap_LossContingencyEstimateOfPossibleLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

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