Jointly-owned Plant Schedule of Jointly-owned plant (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share of Boardman	80.00%	65.00%
Jointly Owned Plant, Boardman Plant In Service Year	1980
Jointly Owned Utility Plant, Gross Ownership Amount of Boardman Plant In Service	$ 506
Jointly Owned Utility Plant, Ownership Amount of Boardman Plant Accumulated Depreciation	326
Jointly Owned Utility Plant Ownership Amount of Construction Work In Progress Boardman	1
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	515
Jointly Owned Utility Plant, Ownership Amount of Colstrip Plant Accumulated Depreciation	332
Jointly Owned Utility Plant Ownership Amount Of Construction Work In Progress Colstrip	3
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	222
Jointly Owned Utility Plant, Ownership Amount of Pelton/Round Butte Plant Accumulated Depreciation	52
Jointly Owned Utility Plant Ownership Amount Of Construction Work In Progress Pelton/Round Butte	15
Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service	1,243
Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation	710
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	$ 19