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Equity method investments (Details)	Aug. 31, 2013	Aug. 31, 2012	Nov. 30, 2011	Nov. 30, 2010
ECTE [Member]
Schedule of Equity Method Investments [Line Items]
Percentage ownership interest in equity method investment sold				59.96%mdu_PercentageOwnershipInterestInEquityMethodInvestmentSold / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = mdu_EcteMember
Portion of remaining interest sold	25.00%mdu_RemainingInterest / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = mdu_EcteMember	25.00%mdu_RemainingInterest / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = mdu_EcteMember	25.00%mdu_RemainingInterest / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = mdu_EcteMember
ENTE [Member]
Schedule of Equity Method Investments [Line Items]
Percentage ownership interest in equity method investment sold				100.00%mdu_PercentageOwnershipInterestInEquityMethodInvestmentSold / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = mdu_EnteMember
ERTE [Member]
Schedule of Equity Method Investments [Line Items]
Percentage ownership interest in equity method investment sold				100.00%mdu_PercentageOwnershipInterestInEquityMethodInvestmentSold / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = mdu_ErteMember

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