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Risk Management Counterparties Representing 10% or More of Assets and Liabilities from price risk management activities (Details) | Sep. 30, 2018 | Dec. 31, 2017
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Assets from price risk management activities: |  |
Counterparty A | 58.00% | 39.00%
Counterparty B | 3.00% | 12.00%
Total | 61.00% | 51.00%
Liabilities from price risk management activities: |  |
Counterparty C | 67.00% | 62.00%
Total | 67.00% | 62.00%

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- Definition Assets from price risk management activities: [Abstract] + References No definition available. + Details
Name: | por_AssetsFromPriceRiskManagementActivitiesAbstract
Namespace Prefix: | por_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents aggregate of individually significant counterparties of total assets arising from derivative instruments from price risk management activities. + References No definition available. + Details
Name: | por_ConcentrationOfRiskDerivativeInstrumentsAssets
Namespace Prefix: | por_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Represents aggregate of individually significant counterparties of total liabilities arising from derivative instruments from price risk management activities. + References No definition available. + Details
Name: | por_ConcentrationOfRiskDerivativeInstrumentsLiabilities
Namespace Prefix: | por_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Individually significant counterparty of total assets arising from derivative instruments from price risk management activities. + References No definition available. + Details
Name: | por_CounterpartyA
Namespace Prefix: | por_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Individually significant counterparty of total assets arising from derivative instruments from price risk management activities. + References No definition available. + Details
Name: | por_CounterpartyB
Namespace Prefix: | por_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Counterparty D for PRM Liab + References No definition available. + Details
Name: | por_CounterpartyDforPRMLiab
Namespace Prefix: | por_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Liabilities from price risk management activities: [Abstract] + References No definition available. + Details
Name: | por_LiabilitiesFromPriceRiskManagementActivitiesAbstract
Namespace Prefix: | por_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration