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Stock-based Compensation Expense (Details) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based Compensation $ 6us-gaap_ShareBasedCompensation $ 4us-gaap_ShareBasedCompensation $ 4us-gaap_ShareBasedCompensation    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 4,687,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized        
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 3,554,884us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Share-based Liabilities Paid 1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesPaid 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesPaid 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsShareBasedLiabilitiesPaid    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Total Fair Value 3us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue    
Adjustments Related to Tax Withholding for Share-based Compensation 1us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation 2us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation 1us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation    
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 6us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized        
Stock-based Compensation, Attainment of Performance Goals That Allows Vesting 134.20%por_StockBasedCompensationAttainmentOfPerformanceGoalsThatAllowsVesting 117.50%por_StockBasedCompensationAttainmentOfPerformanceGoalsThatAllowsVesting 112.00%por_StockBasedCompensationAttainmentOfPerformanceGoalsThatAllowsVesting    
Stock-based Compensation, Forfeiture Rate 5.00%por_StockBasedCompensationForfeitureRate        
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount 0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount 0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount 0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount    
stock based compensation impact on cash flows 0por_StockBasedCompensationImpactOnCashFlows 0por_StockBasedCompensationImpactOnCashFlows 0por_StockBasedCompensationImpactOnCashFlows    
Performance based stock award percentage - minimum 0.00%por_Performancebasedstockawardpercentageminimum        
Performance based stock award percentage - maximum 150.00%por_Performancebasedstockawardpercentagemaximum        
Subsequent Event [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based Compensation       $ 2us-gaap_ShareBasedCompensation
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
$ 4us-gaap_ShareBasedCompensation
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember