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Regulatory Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Regulatory Assets and Liabilities Disclosure [Abstract]    
Regulatory Assets Earning a Return, Other Category $ 33por_RegulatoryAssetsEarningReturnOtherCategory $ 16por_RegulatoryAssetsEarningReturnOtherCategory
Regulatory Assets Earning a Return 63por_RegulatoryAssetsEarningReturn  
Regulatory Assets Earning a Rate of Return at the Cost of Debt 19por_RegulatoryAssetsEarningaRateofReturnattheCostofDebt  
Authorized Cost of Debt 5.54%por_AuthorizedCostofDebt  
Regulatory Assets Earning a Rate of Return at the Cost of Capital 2por_RegulatoryAssetsEarningRateOfReturnAtCostOfCapital  
Authorized Cost of Capital 7.65%por_AuthorizedCostOfCapital  
Regulatory Assets Earning a Rate of Return by Inclusion in Rate Base 33por_RegulatoryAssetsEarningRateOfReturnByInclusionInRateBase  
Regulatory Assets Earning a Rate of Return at the Approved Rate 9por_RegulatoryAssetsEarningRateOfReturnAtApprovedRate  
Approved Rate for Deferred Account Under Amortization, Low End of Range 1.47%por_ApprovedRateForDeferredAccountUnderAmortizationLowEndOfRange  
Approved Rate for Deferred Accounts under Amortization, High End of Range 1.77%por_ApprovedRateForDeferredAccountsUnderAmortizationHighEndOfRange  
Trojan ISFSI pollution control tax credits   $ 8por_TrojanIsfsiPollutionControlTaxCredits