Quarterly report pursuant to Section 13 or 15(d)

Hedging Activities (Narrative) (Details)

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Hedging Activities (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
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 4gpor_NumberOfFinancialInstitutions   4gpor_NumberOfFinancialInstitutions
Gain on fair value hedge ineffectiveness, net $ 31.3us-gaap_GainLossOnFairValueHedgeIneffectivenessNet $ (8.7)us-gaap_GainLossOnFairValueHedgeIneffectivenessNet