Employee Benefit Plans Investment Strategy (Details)	Dec. 31, 2024	Dec. 31, 2023
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	5.00%
Pension Plan [Member] | NorthWestern Energy Pension Plan (MT) [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
Pension Plan [Member] | NorthWestern Corporation Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
Pension Plan [Member] | Northwestern Energy Health and Welfare
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
Cash and cash equivalents | Pension Plan [Member] | NorthWestern Energy Pension Plan (MT) [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Cash and cash equivalents | Pension Plan [Member] | NorthWestern Corporation Pension Plan [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.80%	1.50%
Cash and cash equivalents | Pension Plan [Member] | Northwestern Energy Health and Welfare
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.30%	0.20%
Fixed income securities(1) | Pension Plan [Member] | NorthWestern Energy Pension Plan (MT) [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	45.00%	45.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	43.70%	45.30%
Fixed income securities(1) | Pension Plan [Member] | NorthWestern Corporation Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	90.00%	90.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	89.40%	88.70%
Fixed income securities(1) | Pension Plan [Member] | Northwestern Energy Health and Welfare
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	40.00%	40.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	32.20%	35.10%
Non-U.S. fixed income securities | Pension Plan [Member] | NorthWestern Energy Pension Plan (MT) [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	0.00%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Non-U.S. fixed income securities | Pension Plan [Member] | NorthWestern Corporation Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	0.00%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Non-U.S. fixed income securities | Pension Plan [Member] | Northwestern Energy Health and Welfare
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	0.00%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Global equities(1) | Pension Plan [Member] | NorthWestern Energy Pension Plan (MT) [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	38.50%	38.50%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	39.00%	37.60%
Global equities(1) | Pension Plan [Member] | NorthWestern Corporation Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	7.00%	7.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	6.90%	6.90%
Global equities(1) | Pension Plan [Member] | Northwestern Energy Health and Welfare
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	60.00%	60.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	67.50%	64.70%
Fixed Income Investments | Pension Plan [Member] | NorthWestern Energy Pension Plan (MT) [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	11.00%	11.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	11.10%	10.60%
Fixed Income Investments | Pension Plan [Member] | NorthWestern Corporation Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	3.00%	3.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	2.90%	2.90%
Fixed Income Investments | Pension Plan [Member] | Northwestern Energy Health and Welfare
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	0.00%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Private Real Estate | Pension Plan [Member] | NorthWestern Energy Pension Plan (MT) [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	5.50%	5.50%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	6.20%	6.50%
Private Real Estate | Pension Plan [Member] | NorthWestern Corporation Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	0.00%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Private Real Estate | Pension Plan [Member] | Northwestern Energy Health and Welfare
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	0.00%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%