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Hedging Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 financial_institution | Mar. 31, 2014 | Dec. 31, 2014 financial_institution
General Discussion of Derivative Instruments and Hedging Activities [Abstract] |  |  |
Number of financial institutions | 4 gpor_NumberOfFinancialInstitutions |  | 4 gpor_NumberOfFinancialInstitutions
Gain on fair value hedge ineffectiveness, net | $ 31.3 us-gaap_GainLossOnFairValueHedgeIneffectivenessNet | $ (8.7)us-gaap_GainLossOnFairValueHedgeIneffectivenessNet |

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- Definition Amount of gain (loss)recognized in the income statement from the hedging ineffectiveness of a fair value hedge. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986 + Details
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