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Employee Benefits Schedule of Changes in Fair Value of Plan Assets (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Defined Benefit Plan, Fair Value of Plan Assets $ 591us-gaap_DefinedBenefitPlanFairValueOfPlanAssets $ 596us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Unrealized gain on assets 2us-gaap_UnrealizedGainLossOnInvestments 4us-gaap_UnrealizedGainLossOnInvestments
Available-for-sale Securities, Gross Realized Gain (Loss) 3us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet (2)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
Purchases and sales, net $ (7)por_DefinedBenefitPlanPurchasesAndSalesNet $ (3)por_DefinedBenefitPlanPurchasesAndSalesNet