| Cover page | Aug. 04, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Aug. 4, 2021 |
| Entity Registrant Name | MDU Resources Group Inc |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 1-03480 |
| Entity Tax Identification Number | 30-1133956 |
| Entity Address, Address Line One | 1200 West Century Avenue |
| Entity Address, Address Line Two | P.O. Box 5650 |
| Entity Address, City or Town | Bismarck |
| Entity Address, State or Province | ND |
| Entity Address, Postal Zip Code | 58506-5650 |
| City Area Code | 701 |
| Local Phone Number | 530-1000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $1.00 per share |
| Trading Symbol | MDU |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000067716 |
| Amendment Flag | false |
