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Debt - Long-Term Debt (Details)
$ in Thousands
 12 Months Ended
Nov. 01, 2024
USD ($)
Nov. 15, 2023
USD ($)
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Jul. 11, 2024
USD ($)
Jun. 20, 2024
USD ($)
Jun. 19, 2024
USD ($)
Apr. 01, 2024
USD ($)
Oct. 18, 2023
USD ($)
May 31, 2023
USD ($)
Long-term debt outstanding [Line Items]                      
Long-term debt     $ 2,292,610 $ 2,166,223              
Repayments of long-term debt     $ 182,135 568,883 $ 38,764            
Montana-Dakota Utilities Co.                      
Long-term debt outstanding [Line Items]                      
Ratio of total debt to total capitalization     0.48                
Revolving Credit Facility | Montana-Dakota Utilities Co.                      
Long-term debt outstanding [Line Items]                      
Line of credit facility, maximum borrowing capacity                   $ 200,000  
Ratio of funded debt to total capitalization as specified in debt covenants     0.65                
Revolving Credit Facility | Cascade Natural Gas                      
Long-term debt outstanding [Line Items]                      
Ratio of total debt to total capitalization     0.65                
Revolving Credit Facility | Intermountain Gas Company                      
Long-term debt outstanding [Line Items]                      
Ratio of total debt to total capitalization     0.65                
Revolving Credit Facility | MDU Resources Group, Inc.                      
Long-term debt outstanding [Line Items]                      
Long-term debt                     $ 200,000
Ratio of total debt to total capitalization     0.65                
Senior Notes                      
Long-term debt outstanding [Line Items]                      
Long-term debt, gross     $ 1,947,000 $ 1,882,000              
Senior Notes | Montana-Dakota Utilities Co.                      
Long-term debt outstanding [Line Items]                      
Ratio of funded debt to total capitalization as specified in debt covenants           0.65          
Long-term debt           $ 125,000          
Interest rate, stated percentage           5.96%          
Senior Notes | Cascade Natural Gas                      
Long-term debt outstanding [Line Items]                      
Total debt to total capitalization     0.50                
Senior Notes | Intermountain Gas Company                      
Long-term debt outstanding [Line Items]                      
Total debt to total capitalization     0.60                
Senior Notes | MDU Resources Group, Inc.                      
Long-term debt outstanding [Line Items]                      
Total debt to total capitalization     0.46                
Revolving Credit Agreement | Cascade Natural Gas                      
Long-term debt outstanding [Line Items]                      
Line of credit facility, maximum borrowing capacity             $ 175,000 $ 100,000      
Revolving Credit Agreement | Intermountain Gas Company                      
Long-term debt outstanding [Line Items]                      
Line of credit facility, maximum borrowing capacity             $ 175,000 $ 100,000      
Term Loan Agreement | MDU Resources Group, Inc.                      
Long-term debt outstanding [Line Items]                      
Long-term debt                     $ 375,000
Repayments of long-term debt $ 190,000 $ 185,000                  
Term Loan Agreement | WBI Energy Transmission                      
Long-term debt outstanding [Line Items]                      
Long-term debt                 $ 60,000    
Interest rate, stated percentage                 4.52%    
Ratio of total debt to total capitalization                 0.65    
Debt instrument, annual principal payment                 $ 4,000    
Uncommitted Note Purchase And Private Shelf Agreement | WBI Energy Transmission                      
Long-term debt outstanding [Line Items]                      
Line of credit facility, maximum borrowing capacity     $ 350,000                
Ratio of total debt to total capitalization     0.55                
Total debt to total capitalization     0.40                
Long-term debt, gross     $ 235,000                
Line of credit facility, remaining borrowing capacity     $ 115,000