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Employee Benefit Plans Actuarial Assumptions (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Increase in projected benefit obligation due to change in discount rate   $ 87.6    
Pension Plan [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Expected rate of return on assets       4.70%
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   2.84% 2.84% 2.89%
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   2.00% 2.03% 2.03%
Pension Plan [Member] | Minimum [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Discount rate   3.10% 4.15% 3.50%
Expected rate of return on assets   4.23% 4.47% 4.70%
Interest credit rating   3.60% 4.00% 4.00%
Pension Plan [Member] | Maximum [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Discount rate   3.20% 4.20% 3.60%
Expected rate of return on assets   5.06% 4.97% 4.70%
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   4.79% 4.82% 4.70%
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   2.84% 2.84% 2.89%
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   2.00% 2.03% 2.03%
Other Postretirement Benefits Plan [Member] | Minimum [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Discount rate   2.80% 3.90% 3.20%
Other Postretirement Benefits Plan [Member] | Maximum [Member]        
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]        
Discount rate   2.80% 3.95% 3.30%
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 3.45%      
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 4.49%