Regulatory Matters and Revenues Subject to Refund (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011
Regulated Operations [Abstract]
Estimated project cost of the natural gas turbine and associated facilities	$ 85.6

Estimated project cost of the natural gas turbine and associated facilities.

No definition available.

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