Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2012	Feb. 15, 2013	Jun. 30, 2012
Document and entity information [Abstract]
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			$ 4,080,627,732
Entity Common Stock, Shares Outstanding		188,830,529
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2012