Agreement voting requirement description.

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Area where fire occurred and damaged several residential structures.

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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 class action lawsuit.

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Number of lawsuits filed seeking unspecified damages.

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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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Road fire covered area.

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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI https://asc.fasb.org/extlink&oid=122136288&loc=d3e659983-123033

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Amount awarded to other party in judgment or settlement of litigation.

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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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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=121557415&loc=d3e14435-108349

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-Name Accounting Standards Codification
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Commitments and Contingencies - Additional Information (Details)	1 Months Ended				12 Months Ended
	Apr. 30, 2022 USD ($) a	Jul. 31, 2021 USD ($)	Sep. 30, 2020 a	Aug. 31, 2019 USD ($)	Dec. 31, 2022 Lawsuit Plaintiff	Jan. 16, 2023	Sep. 30, 2022	May 31, 2021 a Building	Sep. 02, 2020 a Building
Loss Contingencies [Line Items]
Owners percentage interest					15.00%
Natural and Cultural Damage Claim [Member]
Loss Contingencies [Line Items]
Litigation settlement damages claim amount \| $		$ 2,000,000
Boyds Fire [Member] \| Damage from Fire [Member] \| Maximum [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value \| $				$ 4,400,000
Labor Day Windstorm [Member] \| Damage from Fire [Member]
Loss Contingencies [Line Items]
Number of residential, commercial and other structures impacted \| Building									230
Road fire covered area \| a									25,000
Babb Road Fire [Member] \| Damage from Fire [Member]
Loss Contingencies [Line Items]
Number of residential, commercial and other structures impacted \| Building								220
Road fire covered area \| a								15,000
Number of lawsuits filed seeking unspecified damages \| Lawsuit					9
Number of subrogation actions filed \| Lawsuit					6
Number of actions on behalf of individual plaintiffs \| Plaintiff					2
Number of class action lawsuit \| Lawsuit					1
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 agreements discussed below, the Colstrip owners agreed to stay both the litigation and the arbitration until March 2023, at which time the proceedings would resume absent additional agreement between the owners.
Washington [Member]
Loss Contingencies [Line Items]
Wildfires covered area which damaged several residential structures, (in acres) \| a			5
Avista Corp [Member] \| Road Eleven Fire [Member] \| Damage from Fire [Member]
Loss Contingencies [Line Items]
Loss contingency, damages sought, value \| $	$ 5,000,000
Road fire covered area \| a	10,000
PSE [Member] \| Colstrip [Member] \| Unit 3 [Member]
Loss Contingencies [Line Items]
Owners percentage interest					25.00%		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 3 [Member] \| Subsequent Event [Member]
Loss Contingencies [Line Items]
Owners percentage interest						15.00%
NorthWestern [Member] \| Colstrip [Member] \| Unit 4 [Member]
Loss Contingencies [Line Items]
Owners percentage interest					30.00%
NorthWestern [Member] \| Colstrip [Member] \| Unit 4 [Member] \| Subsequent Event [Member]
Loss Contingencies [Line Items]
Owners percentage interest						15.00%