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Goodwill (Details 6)	3 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Fair value inputs discount rate, range bottom	5.00%
Fair value inputs, discount rate, range top	9.00%
Long-term growth rate projections	3.00%

X

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value, range top.

+ References

No definition available.

+ Details

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X

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value, bottom.

+ References

No definition available.

+ Details

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Namespace Prefix:	mdu_
Data Type:	num:percentItemType
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X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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X

- Definition

Percentage of assumed long-term growth in revenues, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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