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Impairment of long-lived assets (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Asset Impairment Charges [Abstract]
Asset impairment charges	$ 14.5	$ 2.7
Asset impairment charges (after tax)	$ 9.0	$ 1.7

X

- Definition

Asset impairment charges (after tax).

+ References

No definition available.

+ Details

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

X

- Definition

Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 45

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Details

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