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Commitments and Contingencies - Ownership and Operating Interest Percentage (Details) - Colstrip [Member] | Sep. 30, 2023 | Jan. 16, 2023 | Sep. 30, 2022
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Avista [Member] | Unit 3 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 15.00% |  |
Avista [Member] | Unit 4 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 15.00% |  |
PacifiCorp [Member] | Unit 3 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 10.00% |  |
PacifiCorp [Member] | Unit 4 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 10.00% |  |
PGE [Member] | Unit 3 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 20.00% |  |
PGE [Member] | Unit 4 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 20.00% |  |
PSE [Member] | Unit 3 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 25.00% |  |
PSE [Member] | Unit 4 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 25.00% |  | 25.00%
NorthWestern [Member] | Unit 4 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 30.00% | 15.00% |
Talen [Member] | Unit 3 [Member] |  |  |
Loss Contingencies [Line Items] |  |  |
Owners percentage interest | 30.00% |  |

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