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Fair Value of FInancial Instruments Schedule of Fair Value (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds $ 65us-gaap_MoneyMarketFundsAtCarryingValue $ 59us-gaap_MoneyMarketFundsAtCarryingValue
Domestic government, Debt securities 14us-gaap_USGovernmentSecuritiesAtCarryingValue 14us-gaap_USGovernmentSecuritiesAtCarryingValue
Corporate debt securities held in decommissioning trust assets 11por_CorporateDebtSecuritiesHeldInDecommissioningTrustAssets 9por_CorporateDebtSecuritiesHeldInDecommissioningTrustAssets
Domestic Equity Securities 5por_DomesticEquitySecurities 7por_DomesticEquitySecurities
International Equity Securities 1por_InternationalEquitySecurities 1por_InternationalEquitySecurities
Debt securities - domestic government   1por_DebtSecuritiesDomesticGovernment
Electricity 5por_CommodityContractAssetElectricity 10por_CommodityContractAssetElectricity
Natural gas 2por_CommodityContractAssetNaturalGas 4por_CommodityContractAssetNaturalGas
Financial Instruments, Owned, at Fair Value 103us-gaap_FinancialInstrumentsOwnedAtFairValue 105us-gaap_FinancialInstrumentsOwnedAtFairValue
Liabilities from price risk management activities: [Abstract]    
Electricity 112por_CommodityContractLiabilitiesElectricity 127por_CommodityContractLiabilitiesElectricity
Natural gas 116por_CommodityContractLiabilitiesNaturalGas 63por_CommodityContractLiabilitiesNaturalGas
Liabilities, Fair Value Disclosure 228us-gaap_LiabilitiesFairValueDisclosure 190us-gaap_LiabilitiesFairValueDisclosure
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Domestic government, Debt securities 7us-gaap_USGovernmentSecuritiesAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
6us-gaap_USGovernmentSecuritiesAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Corporate debt securities held in decommissioning trust assets 0por_CorporateDebtSecuritiesHeldInDecommissioningTrustAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0por_CorporateDebtSecuritiesHeldInDecommissioningTrustAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Domestic Equity Securities 4por_DomesticEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
4por_DomesticEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
International Equity Securities 1por_InternationalEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
1por_InternationalEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Debt securities - domestic government   1por_DebtSecuritiesDomesticGovernment
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Electricity 0por_CommodityContractAssetElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0por_CommodityContractAssetElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Natural gas 0por_CommodityContractAssetNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0por_CommodityContractAssetNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Financial Instruments, Owned, at Fair Value 12us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
12us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Liabilities from price risk management activities: [Abstract]    
Electricity 0por_CommodityContractLiabilitiesElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0por_CommodityContractLiabilitiesElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Natural gas 0por_CommodityContractLiabilitiesNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0por_CommodityContractLiabilitiesNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Liabilities, Fair Value Disclosure 0us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
0us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 65us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
59us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Domestic government, Debt securities 7us-gaap_USGovernmentSecuritiesAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
8us-gaap_USGovernmentSecuritiesAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Corporate debt securities held in decommissioning trust assets 11por_CorporateDebtSecuritiesHeldInDecommissioningTrustAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
9por_CorporateDebtSecuritiesHeldInDecommissioningTrustAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Domestic Equity Securities 1por_DomesticEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
3por_DomesticEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
International Equity Securities 0por_InternationalEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
0por_InternationalEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Debt securities - domestic government   0por_DebtSecuritiesDomesticGovernment
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Electricity 4por_CommodityContractAssetElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
9por_CommodityContractAssetElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Natural gas 2por_CommodityContractAssetNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
4por_CommodityContractAssetNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Financial Instruments, Owned, at Fair Value 90us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
92us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Liabilities from price risk management activities: [Abstract]    
Electricity 32por_CommodityContractLiabilitiesElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
10por_CommodityContractLiabilitiesElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Natural gas 95por_CommodityContractLiabilitiesNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
40por_CommodityContractLiabilitiesNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Liabilities, Fair Value Disclosure 127us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
50us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0us-gaap_MoneyMarketFundsAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Domestic government, Debt securities 0us-gaap_USGovernmentSecuritiesAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0us-gaap_USGovernmentSecuritiesAtCarryingValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Corporate debt securities held in decommissioning trust assets 0por_CorporateDebtSecuritiesHeldInDecommissioningTrustAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0por_CorporateDebtSecuritiesHeldInDecommissioningTrustAssets
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Domestic Equity Securities 0por_DomesticEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0por_DomesticEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
International Equity Securities 0por_InternationalEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0por_InternationalEquitySecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Debt securities - domestic government   0por_DebtSecuritiesDomesticGovernment
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Electricity 1por_CommodityContractAssetElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
1por_CommodityContractAssetElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Natural gas 0por_CommodityContractAssetNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
0por_CommodityContractAssetNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Financial Instruments, Owned, at Fair Value 1us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
1us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Liabilities from price risk management activities: [Abstract]    
Electricity 80por_CommodityContractLiabilitiesElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
117por_CommodityContractLiabilitiesElectricity
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Natural gas 21por_CommodityContractLiabilitiesNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
23por_CommodityContractLiabilitiesNaturalGas
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Liabilities, Fair Value Disclosure 101us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
140us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Non Qualified Benefit Plans [Member]    
Liabilities from price risk management activities: [Abstract]    
Insurance contracts, at cash surrender value $ 26us-gaap_CashSurrenderValueFairValueDisclosure
/ por_TrustAxis
= por_NonQualifiedBenefitPlansMember
$ 26us-gaap_CashSurrenderValueFairValueDisclosure
/ por_TrustAxis
= por_NonQualifiedBenefitPlansMember