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Stock Purchase Plan Stock Purchase Plan (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2013 | Jun. 30, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
stock offering |  |  | 11,100,000 por_Stockoffering |  |  |
Proceeds from Issuance of Common Stock |  | $ 20,000,000 us-gaap_ProceedsFromIssuanceOfCommonStock | $ 47,000,000 us-gaap_ProceedsFromIssuanceOfCommonStock | $ 0 us-gaap_ProceedsFromIssuanceOfCommonStock | $ 67,000,000 us-gaap_ProceedsFromIssuanceOfCommonStock | $ 0 us-gaap_ProceedsFromIssuanceOfCommonStock
Share Price |  | $ 29.50 us-gaap_SharePrice |  |  |  |
Common Stock, Discount on Shares |  | 0.96 us-gaap_CommonStockDiscountOnShares |  |  |  |
Equity Forward Sale Agreement, Settlement Threshold | 10,400,000 por_EquityForwardSaleAgreementSettlementThreshold |  |  |  |  |
equity forward sale agreement future settlement amount | 275,000,000 por_Equityforwardsaleagreementfuturesettlementamount |  |  |  |  |
Option Indexed to Issuer's Equity, Settlement Alternatives, Cash, at Fair Value | 119,000,000 us-gaap_OptionIndexedToIssuersEquitySettlementAlternativesCashAtFairValue |  |  | 119,000,000 us-gaap_OptionIndexedToIssuersEquitySettlementAlternativesCashAtFairValue |  |
Net share settlement | 3,135,000 por_Netsharesettlement |  |  | 3,135,000 por_Netsharesettlement |  |
employee stock purchase plan shares authorized | 625,000 por_EmployeeStockPurchasePlanSharesAuthorized |  |  | 625,000 por_EmployeeStockPurchasePlanSharesAuthorized |  |
Employee Stock Purchase Plan, Base Pay Threshold |  |  |  | 10.00%por_EmployeeStockPurchasePlanBasePayThreshold |  |
Employee Stock Purchase Plan, Purchased Stock Value Limitation |  |  |  | $ 25,000 por_EmployeeStockPurchasePlanPurchasedStockValueLimitation |  |
Employee Stock Purchase Plan, Share Purchase Limitation |  |  |  | 1,500 por_EmployeeStockPurchasePlanSharePurchaseLimitation |  |
Employee Stock Purchase Plan, Purchase Price |  |  |  | 95.00%por_EmployeeStockPurchasePlanPurchasePrice |  |
Employee Stock Purchase Plan, Number of Available Shares | 427,021 por_EmployeeStockPurchasePlanNumberOfAvailableShares |  |  | 427,021 por_EmployeeStockPurchasePlanNumberOfAvailableShares |  |
Dividend Reinvestment and Direct Stock Purchase Plan Shares |  |  |  | 2,500,000 por_DividendReinvestmentAndDirectStockPurchasePlanShares |  |
Dividend Reinvestment and Direct Stock Purchase Plan, Number of Available Shares | 2,481,110 por_DividendReinvestmentAndDirectStockPurchasePlanNumberOfAvailableShares |  |  | 2,481,110 por_DividendReinvestmentAndDirectStockPurchasePlanNumberOfAvailableShares |  |
Common Stock [Member] |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  | 700,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember | 1,665,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |  | 2,365,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember |

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- Definition Dividend Reinvestment and Direct Stock Purchase Plan, Number of Available Shares + References No definition available. + Details
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- Definition Represents common shares set aside, issued and available for issue related to the Dividend Reinvestment and Direct Stock Purchase Plan + References No definition available. + Details
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- Definition Employee Stock Purchase Plan, Purchase Price reflected in percentage of FMV at Purchase date. + References No definition available. + Details
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- Definition Funds that would be received undter the equity forward sale agreement in the future. + References No definition available. + Details
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- Definition Discount on common shares, or any unamortized balance thereof, shown separately as a deduction from the applicable account(s) as circumstances require. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187103-122770 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 07 -Article 4 + Details
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- Definition Fair value of each cash settlement alternative for each freestanding option contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 40 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6445032&loc=d3e90205-114008 + Details
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