Income taxes (Details 9)	12 Months Ended
Dec. 31, 2017
Accounting Policies [Abstract]
Threshold of likelihood of tax positions being realized upon ultimate settlement with a taxing authority	50.00%

Threshold of likelihood that tax benefits will be realized upon ultimate settlement with a taxing authority.

No definition available.

Name:	mdu_ThresholdOfLikelihoodOfTaxPositionsBeingRealizedUponUltimateSettlementWithTaxingAuthority
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration