Employee Benefit Plans Investment Strategy (Details)	Dec. 31, 2022	Dec. 31, 2021
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%	0.10%
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	1.10%	0.40%
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	0.60%	0.10%
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	45.00%	55.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	44.50%	53.80%
Fixed income securities | 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	88.60%	89.50%
Fixed income securities | 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	36.70%	33.70%
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%	4.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	3.90%
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	1.00%	1.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.90%	0.90%
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 | Pension Plan [Member] | NorthWestern Energy Pension Plan (MT) [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	44.00%	41.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	43.40%	42.20%
Global equities | Pension Plan [Member] | NorthWestern Corporation Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Allocations [Abstract]
Target allocation of investments by plan	9.00%	9.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	9.40%	9.20%
Global equities | 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	62.70%	66.20%
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	5.50%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	5.50%	0.00%
Fixed Income Investments | 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%
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%	0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	6.60%	0.00%
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%