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Commitments and Contingencies (Tables) | 9 Months Ended
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Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract] |
Ownership and Operating Ownership Interest Percentage | Colstrip Units 3 and 4 are owned by the Company, PacifiCorp, Portland General Electric (PGE), and Puget Sound Energy (PSE) (collectively, the "Western Co-Owners"), as well as NorthWestern and Talen Montana, LLC (Talen), as tenants in common under an Ownership and Operating Agreement, dated May 6, 1981, as amended (O&O Agreement), in the percentages set forth below:
Co-Owner |  | Unit 3 |  |  | Unit 4 |
Avista |  |  | 15 | % |  |  | 15 | %
PacifiCorp |  |  | 10 | % |  |  | 10 | %
PGE |  |  | 20 | % |  |  | 20 | %
PSE |  |  | 25 | % |  |  | 25 | %
NorthWestern |  |  | — |  |  |  | 30 | %
Talen |  |  | 30 | % |  |  | — |

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