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Fair Value Disclosure (Details Textual) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Sep. 30, 2014
Dec. 31, 2014
Fair Value Disclosure (Textual) [Abstract]      
Derivative Asset, Notional Amount $ 600.0us-gaap_DerivativeAssetNotionalAmount    
Derivative, Notional Amount 312.6invest_DerivativeNotionalAmount   281.5invest_DerivativeNotionalAmount
Cash Flow Hedge Pre Tax Gain Loss to be Reclassified within Twelve Months (5.9)gmt_CashFlowHedgePreTaxGainLossToBeReclassifiedWithinTwelveMonths    
Expected After tax reclassification of net losses from accumulated other comprehensive income to earnings in Next Twelve Months 3.7us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths    
Aggregate fair value of all derivative instruments with Net liability position $ 0us-gaap_DerivativeNetLiabilityPositionAggregateFairValue    
Fair Value Hedging [Member]      
Fair Value Disclosure (Textual) [Abstract]      
Number of instruments, outstanding 8us-gaap_DerivativeNumberOfInstrumentsHeld
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
   
Maturity Date Range of Derivative Instruments   2015 to 2020  
Cash Flow Hedges [Member]      
Fair Value Disclosure (Textual) [Abstract]      
Number of instruments, outstanding 6us-gaap_DerivativeNumberOfInstrumentsHeld
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
  7us-gaap_DerivativeNumberOfInstrumentsHeld
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
Maturity Date Range of Derivative Instruments 2015 to 2022