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Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Regulatory Assets and Liabilities Disclosure [Abstract] |  |
Regulatory Assets Earning a Return, Other Category | $ 51 | $ 44
Regulatory Assets Earning a Return | 51 |
Regulatory Assets Earning a Rate of Return at the Cost of Capital | $ 10 |
Public Utilities, Approved Return on Equity, Percentage | 7.51% |
Authorized Cost of Capital | 2.87% |
Regulatory Assets Earning a Rate of Return by Inclusion in Rate Base | $ 14 |
Regulatory Assets Earning a Rate of Return at the Approved Rate | $ 25 |
Approved Rate for Deferred Account Under Amortization, Low End of Range | 1.47% |
Approved Rate for Deferred Accounts under Amortization, High End of Range | 2.38% |
Types of Net Regulatory Assets Earning Returns | $ 2 |
Increase in net deferred tax regulatory liabilitily | $ 357 |

X
- Definition Low end of the range of approved rates of return for amortizing deferred accounts (regulatory assets). + References No definition available. + Details
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X
- Definition High end of the range of approved rates of return for amortizing deferred accounts (regulatory assets). + References No definition available. + Details
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Namespace Prefix: | por_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The Company's cost of capital rate. + References No definition available. + Details
Name: | por_Costofcapitalrate
Namespace Prefix: | por_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Increase in net deferred tax regulatory liability resulting from the Tax Cuts and Jobs Act of 2017. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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X
- Definition Regulatory assets earning a rate of return at the approved rate. + References No definition available. + Details
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Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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X
- Definition Regulatory assets earning a rate of return at the cost of capital. + References No definition available. + Details
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Namespace Prefix: | por_
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X
- Definition Regulatory assets earning a rate of return by inclusion in rate base. + References No definition available. + Details
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Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Regulatory assets earning a rate of return. + References No definition available. + Details
Name: | por_RegulatoryAssetsEarningReturn
Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Regulatory assets in 'other' category earning a rate of return. + References No definition available. + Details
Name: | por_RegulatoryAssetsEarningReturnOtherCategory
Namespace Prefix: | por_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of regulatory costs not yet approved by the regulatory authority. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Percentage of public utility's approved debt to capital, including debt and equity. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
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