Commitments and Contingencies - Ownership and Operating Interest Percentage (Details) - Colstrip [Member]	Jun. 30, 2023	Jan. 16, 2023	Sep. 30, 2022
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%