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Agreement voting requirement description.

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Litigation settlement, agreed to settle all claims and allegations.

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Number of actions on behalf of individual plaintiffs.

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Number of class action lawsuit.

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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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Commitments and Contingencies - Additional Information (Details)	1 Months Ended				3 Months Ended
	Apr. 30, 2024 USD ($)	Apr. 30, 2022 USD ($) a	Jul. 31, 2021 USD ($)	Aug. 31, 2019 USD ($)	Mar. 31, 2024 USD ($) Plaintiff Lawsuit	Aug. 31, 2023 a	Jan. 16, 2023	May 31, 2021 a Building
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			$ 2,000,000
Natural and Cultural Damage Claim [Member] \| Subsequent Event [Member]
Loss Contingencies [Line Items]
Amout of settle all claims and allegations	$ 900,000
Boyds Fire [Member] \| Damage from Fire [Member] \| Maximum [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value				$ 4,400,000
Road 11 Fire [Member] \| Damage from Fire [Member] \| Avista Corp [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value		$ 5,000,000
Road fire covered area \| a		10,000
Babb Road Fire [Member] \| Damage from Fire [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value					$ 23,000,000
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
Number of class action lawsuit \| Lawsuit					1
Orofino Fire [Member] \| Damage from Fire [Member]
Loss Contingencies [Line Items]
Road fire covered area \| a						53
System Unit Resource Protection Act [Member] \| Natural and Cultural Damage Claim [Member]
Loss Contingencies [Line Items]
Increased potential claim			$ 6,000,000
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, the Colstrip owners agreed to stay both the litigation and the arbitration through March 2024. On April 1, 2024, the agreement to stay lapsed and at least one owner, Puget Sound Energy, has indicated that they wish to resume the arbitration proceeding.
Colstrip [Member] \| PSE [Member] \| Unit 4 [Member]
Loss Contingencies [Line Items]
Owners percentage interest					25.00%
Colstrip [Member] \| NorthWestern [Member] \| Unit 4 [Member]
Loss Contingencies [Line Items]
Owners percentage interest					30.00%		15.00%