Document And Entity Information (USD $)	12 Months Ended
	Dec. 31, 2011	Feb. 17, 2012	Jun. 30, 2011
Entity Information [Line Items]
Entity Registrant Name	MDU RESOURCES GROUP INC
Entity Central Index Key	0000067716
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 4,247,855,190
Entity Common Stock, Shares Outstanding		188,819,307
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2011