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Employee Benefit Plans Actuarial Assumptions (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Increase in projected benefit obligation due to change in discount rate   $ (29.6)    
Pension Plan [Member] | Nonunion [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Long-term rate of increase in compensation levels   4.00% 4.00% 4.00%
Pension Plan [Member] | Union [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Long-term rate of increase in compensation levels   4.00% 4.00% 4.00%
Pension Plan [Member] | Minimum [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Discount rate   5.50% 4.95% 5.20%
Expected rate of return on assets   5.15% 4.83% 2.66%
Interest credit rating   3.30% 3.30% 3.30%
Pension Plan [Member] | Maximum [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Discount rate   5.60% 5.00% 5.20%
Expected rate of return on assets   6.65% 6.44% 4.26%
Interest credit rating   6.00% 6.00% 6.00%
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Expected rate of return on assets   5.84% 5.62% 4.23%
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]        
Health care cost trend rate assumed for next year   5.00%    
Other Postretirement Benefits Plan [Member] | Nonunion [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Long-term rate of increase in compensation levels   4.00% 4.00% 4.00%
Other Postretirement Benefits Plan [Member] | Union [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Long-term rate of increase in compensation levels   4.00% 4.00% 4.00%
Other Postretirement Benefits Plan [Member] | Minimum [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Discount rate   5.30% 4.85% 5.15%
Other Postretirement Benefits Plan [Member] | Maximum [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Discount rate   5.45% 4.90% 5.20%
Forecast [Member] | NorthWestern Corporation Pension Plan [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Expected rate of return on assets 4.58%      
Forecast [Member] | NorthWestern Energy Pension Plan [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Expected rate of return on assets 6.17%