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Commitments and Contingencies - Additional Information (Details) |  | 1 Months Ended | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 01, 2023 Lawsuit | Apr. 30, 2022 USD ($) a | Jul. 31, 2021 USD ($) | Aug. 31, 2019 USD ($) | Sep. 30, 2023 USD ($) Lawsuit Plaintiff | Aug. 29, 2023 a Building | Aug. 18, 2023 a Building | Sep. 30, 2022 | May 31, 2021 a Building
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 |  |  |  |  |
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 lawsuit filed by an individual | Lawsuit | 2 |  |  |  |  |  |  |  |
Number of lawsuit filed by a business entity | Lawsuit | 1 |  |  |  |  |  |  |  |
Number of subrogation actions filed | Lawsuit |  |  |  |  | 6 |  |  |  |
Number of actions on behalf of individual plaintiffs | Plaintiff |  |  |  |  | 4 |  |  |  |
Number of class action lawsuit | Lawsuit |  |  |  |  | 1 |  |  |  |
August Fire Activity [Member] | Damage from Fire [Member] |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Number of residential, commercial and other structures impacted | Building |  |  |  |  |  | 6 | 240 |  |
Road fire covered area | a |  |  |  |  |  | 53 |  |  |
August Fire Activity [Member] | Damage from Fire [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Road fire covered area | a |  |  |  |  |  |  | 10,000 |  |
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 discussed below, the Colstrip owners agreed to stay both the litigation and the arbitration through January 12, 2024, at which time the proceedings would resume absent further agreement between the owners. |  |  |  |
Colstrip [Member] | PSE [Member] | Unit 4 [Member] |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Owners percentage interest |  |  |  |  | 25.00% |  |  | 25.00% |

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