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Commitments and Contingencies - Additional Information (Details) | 1 Months Ended | 9 Months Ended |  |  |
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Apr. 30, 2022 USD ($) a | Jul. 31, 2021 USD ($) | Aug. 31, 2019 USD ($) | Sep. 30, 2022 Lawsuit Plaintiff | Sep. 12, 2022 | May 31, 2021 a Building | Sep. 02, 2020 Building
Avista Corp [Member] |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Percentage Of Employees, Collective Bargaining Agreement |  |  |  | 40.00% |  |  |
Majority Of Bargaining Unit Employees, Percentage |  |  |  | 90.00% |  |  |
Number of collective bargaining agreement term |  |  |  | 4 years |  |  |
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 2020 Windstorm [Member] | Damage from Fire [Member] |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Number of residential, commercial and other structures impacted | Building |  |  |  |  |  |  | 230
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] |  |  |  |  |  |  |
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 |  |  |  | 8 |  |  |
Number of subrogation actions filed | Lawsuit |  |  |  | 5 |  |  |
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 |  |  |  | These business disagreements, in turn, have led to disagreements as to the interpretation of the O&O Agreement, including, but not limited to, whether a 55 percent vote of the Owner’s Committee is sufficient or whether unanimous consent of the owners is required to either remove a Colstrip unit from service or make a determination that the project can no longer be operated consistent with prudent utility practice or the requirements of governmental agencies having jurisdiction. 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. Both the arbitration and these legal proceedings remain pending. |  |  |
Colstrip [Member] | Avista Corp [Member] |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Owners percentage interest |  |  |  | 55.00% |  |  |
Colstrip [Member] | PSE [Member] | Unit 3 [Member] |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Owners percentage interest |  |  |  | 25.00% |  |  |
Colstrip [Member] | PSE [Member] | Unit 4 [Member] |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |
Owners percentage interest |  |  |  | 25.00% | 25.00% |  |

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