Goodwill (Details 5)	3 Months Ended
Dec. 31, 2014
Accounting Policies [Abstract]
Fair value inputs discount rate, range bottom	5.00%
Fair value inputs, discount rate, range top	9.00%mdu_FairValueDiscountRateRangeTop
Long-term growth rate projections	3.00%

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

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

Name:	us-gaap_FairValueInputsLongTermRevenueGrowthRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration