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Balance Sheet Components (Details) (USD $)
 3 Months Ended 0 Months Ended
Mar. 31, 2015
Mar. 31, 2014
May 22, 2015
Jan. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
May 21, 2015
May 19, 2015
Valuation and Qualifying Accounts Disclosure [Line Items]                
Allowance for Doubtful Accounts Receivable, Current $ 7,000,000us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent $ 7,000,000us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent     $ 6,000,000us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent $ 6,000,000us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent    
Finite-Lived Intangible Assets, Accumulated Amortization 200,000,000us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization       191,000,000us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization      
Amortization of Intangible Assets 9,000,000us-gaap_AmortizationOfIntangibleAssets 6,000,000us-gaap_AmortizationOfIntangibleAssets            
Syndicated credit facility scheduled to expire in 2018 500,000,000por_CreditFacilityExpiringIn2017              
Line of Credit Facility, Current Borrowing Capacity 500,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity       700,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity      
Debt Instrument, Covenant Description .65              
Ratio of Indebtedness to Net Capital 0.559us-gaap_RatioOfIndebtednessToNetCapital1              
Short-term Debt 0us-gaap_ShortTermBorrowings              
Borrowings 46,000,000us-gaap_LineOfCredit              
Line of Credit Facility, Remaining Borrowing Capacity 454,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity              
Letters of Credit Outstanding, Amount 30,000,000por_Letterofcreditfacility              
Letters of credit issued 58,000,000us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForLetterOfCreditFacilities              
Authorized Short-Term Debt 900,000,000por_AuthorizedShortTermDebt              
Debt Instrument, Interest Rate, Stated Percentage 3.46%us-gaap_DebtInstrumentInterestRateStatedPercentage     3.55%us-gaap_DebtInstrumentInterestRateStatedPercentage        
Extinguishment of Debt, Amount 120,000,000us-gaap_RepaymentsOfLongTermDebt 0us-gaap_RepaymentsOfLongTermDebt            
[Member]                
Valuation and Qualifying Accounts Disclosure [Line Items]                
Proceeds from Issuance of Long-term Debt 75,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= por_A4.39August152014Member		              
Long-term Debt [Member]                
Valuation and Qualifying Accounts Disclosure [Line Items]                
Extinguishment of Debt, Amount 70,000,000us-gaap_RepaymentsOfLongTermDebt
/ us-gaap_ExtinguishmentOfDebtAxis
= us-gaap_LongTermDebtMember		              
Notes Payable to Banks [Member]                
Valuation and Qualifying Accounts Disclosure [Line Items]                
Extinguishment of Debt, Amount 50,000,000us-gaap_RepaymentsOfLongTermDebt
/ us-gaap_ExtinguishmentOfDebtAxis
= us-gaap_NotesPayableToBanksMember		              
Subsequent Event [Member]                
Valuation and Qualifying Accounts Disclosure [Line Items]                
Debt Instrument, Interest Rate, Stated Percentage             6.80%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		 3.50%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Extinguishment of Debt, Amount     67,000,000us-gaap_RepaymentsOfLongTermDebt
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		          
Subsequent Event [Member] | [Member]                
Valuation and Qualifying Accounts Disclosure [Line Items]                
Proceeds from Issuance of Long-term Debt               70,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= por_A4.39August152014Member
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember