Derivative instruments AP (Details 9)	12 Months Ended
Dec. 31, 2014
Accounting Policies [Abstract]
Maximum length of time allowed to hedge forecasted sales	42 months
Maximum length of time allowed to hedge interest rate	24 months
Maximum length of time allowed to hedge forecasted purchases	three years
Maximum period for settlement of all interest rate derivative transactions	90 days

Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted purchase transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments.

No definition available.

Name:	mdu_MaximumLengthOfTimeAllowedToHedgeInCashFlowHedgePurchase
Namespace Prefix:	mdu_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration