Employee Benefit Plans Investment Strategy (Details)	Dec. 31, 2021	Dec. 31, 2020
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	0.10%	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	0.40%	0.70%
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	0.10%	1.00%
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	55.00%	55.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	53.80%	52.70%
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%	80.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	89.50%	77.30%
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	33.70%	37.90%
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	4.00%	4.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	3.90%	3.80%
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%	2.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.90%	2.60%
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	41.00%	41.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	42.20%	43.50%
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%	18.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	9.20%	19.40%
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	66.20%	61.10%