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Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Balance as of the beginning of the period | $ 153 | $ 158 | $ 119 | $ 119
Net realized and unrealized losses (gains)/losses | (1) | 0 | 34 | 40
Transfers out of Level 3 to Level 2 | 1 | 2 | 0 | 1
Balance as of the end of the period | $ 153 | $ 160 | $ 153 | $ 160

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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Period Type: | duration