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Jointly-owned Plant Schedule of Jointly-owned plant (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2018
USD ($)
Jointly Owned Utility Plant Interests [Line Items]  
Jointly Owned Utility Plant, Proportionate Ownership Share of Boardman 90.00%
Jointly Owned Plant, Boardman Plant In Service Year 1980
Jointly Owned Utility Plant, Gross Ownership Amount of Boardman Plant In Service $ 516
Jointly Owned Utility Plant, Ownership Amount of Boardman Plant Accumulated Depreciation 451
Jointly Owned Utility Plant Ownership Amount of Construction Work In Progress Boardman $ 0
Jointly Owned Utility Plant, Proportionate Ownership Share of Colstrip 20.00%
Jointly Owned Plant, Colstrip Plant In Service Year 1986
Jointly Owned Utility Plant, Gross Ownership Amount of Colstrip Plant In Service $ 549
Jointly Owned Utility Plant, Ownership Amount of Colstrip Plant Accumulated Depreciation 363
Jointly Owned Utility Plant Ownership Amount Of Construction Work In Progress Colstrip $ 10
Jointly Owned Utility Plant, Proportionate Ownership Share of Pelton/Round Butte 66.67%
Jointly Owned Plant, Pelton Plant in Service Year 1958
Jointly Owned Plant, Round Butte in Service Year 1964
Jointly Owned Utility Plant, Gross Ownership Amount of Pelton/Round Butte Plant In Service $ 270
Jointly Owned Utility Plant, Ownership Amount of Pelton/Round Butte Plant Accumulated Depreciation 73
Jointly Owned Utility Plant Ownership Amount Of Construction Work In Progress Pelton/Round Butte 2
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service 1,335
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation 887
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress $ 12