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Jointly-owned Plant Schedule of Jointly-owned plant (Details) $ in Millions	Dec. 31, 2024 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	$ 797
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	565
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	25
Colstrip [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	507
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	449
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	5
Pelton/Round Butte Member
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	221
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	76
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	20
Colstrip - Transmission
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	69
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	40
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 0

X

- Definition

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 360
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479070/980-360-S99-1

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X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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X

- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 360
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479070/980-360-S99-1

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X

- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 360
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479070/980-360-S99-1

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