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Price Risk Management Net realized and unrealized gains and losses on derivative transactions (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Commodity contracts: |  |
Electricity | $ 1 | $ 33
Natural Gas | $ 14 | $ 34

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- Definition Commodity contracts: [Abstract] + References No definition available. + Details
Name: | por_CurrentAssetsCommodityContractsAbstract
Namespace Prefix: | por_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Realized and unrealized gains and losses, net on commodity contracts related to electricity not designated as hedging instruments included in purchased power and fuel expense in the statement of income. + References No definition available. + Details
Name: | por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsElectricity
Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Realized and unrealized gains and losses, net on commodity contracts related to natural gas not designated as hedging instruments included in purchased power and fuel expense in the statement of income. + References No definition available. + Details
Name: | por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsNaturalGas
Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration