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Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Balance as of the beginning of the period | $ 129 | $ 153 | $ 139 | $ 119
Net realized and unrealized losses (gains)/losses | (2) | (1) | (10) | 34
Transfers out of Level 3 to Level 2 | (2) | 1 | (4) | 0
Balance as of the end of the period | $ 125 | $ 153 | $ 125 | $ 153

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- Definition Net Liabilities From Price Risk Management Activities Classified as Level 3 + References No definition available. + Details
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Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Realized and Unrealized Gains and Losses, Net, Commodity Contracts + References No definition available. + Details
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Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | duration