Jointly-owned Plant Schedule of Jointly-owned plant (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share of Boardman	65.00%
Jointly Owned Plant, Boardman Plant In Service Year	1980
Jointly Owned Utility Plant, Gross Ownership Amount of Boardman Plant In Service	$ 467
Jointly Owned Utility Plant, Ownership Amount of Boardman Plant Accumulated Depreciation	292
Jointly Owned Utility Plant Ownership Amount of Construction Work In Progress Boardman	2
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	507
Jointly Owned Utility Plant, Ownership Amount of Colstrip Plant Accumulated Depreciation	326
Jointly Owned Utility Plant Ownership Amount Of Construction Work In Progress Colstrip	2
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	206
Jointly Owned Utility Plant, Ownership Amount of Pelton/Round Butte Plant Accumulated Depreciation	46
Jointly Owned Utility Plant Ownership Amount Of Construction Work In Progress Pelton/Round Butte	11
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	1,180
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	664
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 15