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Jointly-owned Plant Schedule of Jointly-owned plant (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Sep. 30, 2013
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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 |

X
- Definition Plant in service year for Boardman, a jointly owned plant by the reporting entity. + References No definition available. + Details
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Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | instant

X
- Definition The plant in service year of Colstrip, a jointly owned plant by the reporting entity. + References No definition available. + Details
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Namespace Prefix: | por_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | instant

X
- Definition The plant in service year for Pelton, a jointly owned plant by the reporting entity. + References No definition available. + Details
Name: | por_JointlyOwnedPlantPeltonInServiceYear
Namespace Prefix: | por_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | instant

X
- Definition The plant in service year for Round Butte, a jointly owned plant by the reporting entity. + References No definition available. + Details
Name: | por_JointlyOwnedPlantRoundButteInServiceYear
Namespace Prefix: | por_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | instant

X
- Definition The period end amount owned by the reporting entity of a jointly owned utility (Boardman) plant in service. + References No definition available. + Details
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X
- Definition The period end amount owned by the reporting entity of a jointly owned utility (Colstrip) plant in service. + References No definition available. + Details
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X
- Definition The period end amount owned by the reporting entity of utility plant (Pelton/Round Butte) plant in service. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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X
- Definition The period end amount owned by the reporting entity of a jointly owned electricity generating facility (Boardman) plant accumulated depreciation. + References No definition available. + Details
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Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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X
- Definition The period end amount owned by the reporting entity of a jointly owned electricity generating facility (Colstrip) plant accumulated depreciation. + References No definition available. + Details
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X
- Definition The reporting entity's period end amount of construction work in progress at Boardman, a jointly owned electricity generation plant. + References No definition available. + Details
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- Definition The reporting entity's period end amount of construction work in progress at Colstrip, a jointly owned electricity generation plant. + References No definition available. + Details
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X
- Definition The reporting entity's period end amount of construction work in progress at Pelton/Round Butte, a jointly owned electricity generation plant. + References No definition available. + Details
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- Definition The period end amount owned by the reporting entity of a jointly owned electricity generating facility (Pelton/Round Butte) plant accumulated depreciation. + References No definition available. + Details
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- Definition The proportionate share of the reporting entity's interest (percent) in Boardman, a jointly owned utility plant. + References No definition available. + Details
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- Definition The proportionate share of the reporting entity's interest (percent) in Colstrip, a jointly owned utility plant. + References No definition available. + Details
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X
- Definition The reporting entity's interest (percent) in Pelton/Round/Butte, a jointly owned utility plant. + References No definition available. + Details
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- Definition The period end amount owned by the reporting utility of a jointly owned utility plant in service. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Definition The period end amount of construction work in progress at a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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- Definition The period end amount of accumulated depreciation in a jointly owned electricity generation plant. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section C + Details
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