Regulatory matters and revenues subject to refund (Details) (USD $)	Jun. 30, 2012
Regulatory matters and revenues subject to refund [Abstract]
Estimated project cost of the natural gas turbine and associated facilities	$ 85,600,000
Annual reduction in rates	$ 250,000

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:	instant