Agreement expiration month and year.

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Agreement voting requirement description.

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Litigation settlement increased potential claim

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Number of actions on behalf of individual plaintiffs.

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Number of collective bargaining agreement term.

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Number of residential, commercial and other structures impacted.

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Number of subrogation actions filed.

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Percentage of employees represented.

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Percentage of largest group of employees represented.

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Road fire covered area.

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The proportionate share of interest (percent) in a jointly owned utility plant.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The value (monetary amount) of the award the plaintiff seeks in the legal matter.

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Commitments and Contingencies - Additional Information (Details) $ in Millions	1 Months Ended			12 Months Ended
	Apr. 30, 2022 USD ($) a	Jul. 31, 2021 USD ($)	Aug. 31, 2019 USD ($)	Dec. 31, 2023 USD ($) Plaintiff Lawsuit	Aug. 31, 2023 a	Jan. 16, 2023	Sep. 30, 2022	May 31, 2021 a Building
Loss Contingencies [Line Items]
Owners percentage interest				15.00%
Collective Bargaining Agreements Avista Utilities Employees [Member] \| IBEW [Member]
Loss Contingencies [Line Items]
Percentage of employees represented				36.00%
Collective Bargaining Agreements Avista Utilities Bargaining Unit Employees [Member] \| IBEW [Member]
Loss Contingencies [Line Items]
Percentage of largest group of employees represented				90.00%
Collective Bargaining Agreements [Member] \| IBEW [Member]
Loss Contingencies [Line Items]
Agreement term				4 years
Agreement expiration month and year				2025-03
Natural and Cultural Damage Claim [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value		$ 2.0
Boyds Fire [Member] \| Damage from Fire [Member] \| Maximum [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value			$ 4.4
Babb Road Fire [Member] \| Damage from Fire [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value				$ 23.0
Number of residential, commercial and other structures impacted \| Building								220
Road fire covered area \| a								15,000
Number of subrogation actions filed \| Lawsuit				6
Number of actions on behalf of individual plaintiffs \| Plaintiff				4
System Unit Resource Protection Act [Member] \| Natural and Cultural Damage Claim [Member]
Loss Contingencies [Line Items]
Increased potential claim		$ 6.0
Colstrip [Member]
Loss Contingencies [Line Items]
Agreement voting requirement				NorthWestern has initiated arbitration pursuant to the O&O Agreement to resolve these business disagreements, and two actions have been initiated to compel arbitration of those disputes: one by Talen in the Montana Thirteenth Judicial District Court for Yellowstone County, and one by the Western Co-Owners, which is pending in Montana Federal District Court. In light of the ownership transfer agreements discussed below, the Colstrip owners agreed to stay both the litigation and the arbitration through March 2024
Orofino Fire [Member] \| Damage from Fire [Member]
Loss Contingencies [Line Items]
Road fire covered area \| a					3
Avista Corp [Member] \| Road 11 Fire [Member] \| Damage from Fire [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value	$ 5.0
Road fire covered area \| a	10,000
PSE [Member] \| Colstrip [Member] \| Unit 3 [Member]
Loss Contingencies [Line Items]
Owners percentage interest				25.00%
PSE [Member] \| Colstrip [Member] \| Unit 4 [Member]
Loss Contingencies [Line Items]
Owners percentage interest				25.00%			25.00%
NorthWestern [Member] \| Colstrip [Member] \| Unit 4 [Member]
Loss Contingencies [Line Items]
Owners percentage interest				30.00%		15.00%