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Commitments and Contingencies - Additional Information (Details) | 1 Months Ended | 3 Months Ended |  |  |
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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% |

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