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Net realized and unrealized gains and losses on derivative transactions (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Commodity contracts: [Abstract] |  |  |
Electricity | $ 41 | $ 34 | $ 72
Natural Gas | 85 | (56) | 103
Realized And Unrealized Losses Net Commodity Contracts, Foreign Currency | $ (1) | $ 0 | $ 1

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- Definition Commodity contracts: [Abstract] + References No definition available. + Details
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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
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Namespace Prefix: | por_
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- Definition Realized And Unrealized Gains And Losses Net Commodity Contracts, Foreign Currency + References No definition available. + Details
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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
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