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Hedging Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 financial_institution | Dec. 31, 2013 financial_institution | Dec. 31, 2012
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 | $ 121.1 us-gaap_GainLossOnFairValueHedgeIneffectivenessNet | $ (18.2)us-gaap_GainLossOnFairValueHedgeIneffectivenessNet | $ (0.1)us-gaap_GainLossOnFairValueHedgeIneffectivenessNet
Gain on fair value hedge ineffectiveness |  | (9.1)us-gaap_LossOnFairValueHedgeIneffectiveness |
Loss related to amortization of OCI from hedging activity |  | $ (9.1)us-gaap_OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax |
Production delivered under fixed price swaps, percentage | 62.00%gpor_ProductionDeliveredUnderFixedPriceSwapsPercentage |  |

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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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- Definition Amount of recognized loss 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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- Definition Amount after tax and reclassification adjustments, of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=28358780&loc=SL7669646-108580 + Details
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