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Jointly-owned Plant Schedule of Jointly-owned plant (Details)
$ in Millions
Dec. 31, 2021
USD ($)
Jointly Owned Utility Plant Interests [Line Items]  
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service $ 850
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation 486
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress 17
Accrued Capping, Closure, Post-closure and Environmental Costs, Noncurrent 23
Colstrip [Member]  
Jointly Owned Utility Plant Interests [Line Items]  
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service 576
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation 399
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress 7
Pelton/Round Butte Member  
Jointly Owned Utility Plant Interests [Line Items]  
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service 274
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation 87
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress $ 10