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Price Risk Management Counterparties Representing 10% or More of Assets and Liabilities from price risk management activities (Details) | Sep. 30, 2017 | Dec. 31, 2016
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Assets from price risk management activities: |  |
Counterparty A | 53.00% | 22.00%
Counterparty B | 3.00% | 17.00%
Counterparty C | 1.00% | 12.00%
Counterparty D | 15.00% | 0.00%
Counterparty E | 10.00% | 0.00%
Total | 82.00% | 51.00%
Liabilities from price risk management activities: |  |
Counterparty F | 72.00% | 66.00%
Total | 72.00% | 66.00%

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- Definition Assets from price risk management activities: [Abstract] + References No definition available. + Details
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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
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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
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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 Individually significant counterparty of total assets arising from derivative instruments from price risk management activities. + References No definition available. + Details
Name: | por_CounterpartyC
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_CounterpartyD
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
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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_CounterpartyE
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