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Pension and Other Post-Retirement Benefits (Details 1) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets [Abstract]    
Projected benefit obligations $ 502.7us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateProjectedBenefitObligation $ 67.8us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateProjectedBenefitObligation
Fair value of plan assets 456.9gmt_DefinedBenefitPlanPensionPlanswithAccumulatedBenefitObligationsinExcessofPlanAssetsAggregateProjectedFairValueofPlanAssets 40.0gmt_DefinedBenefitPlanPensionPlanswithAccumulatedBenefitObligationsinExcessofPlanAssetsAggregateProjectedFairValueofPlanAssets
Pension plans with an accumulated benefit obligation in excess of plan assets    
Accumulated benefit obligations 170.6us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation 66.4us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation
Fair value of plan assets $ 140.5us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateFairValueOfPlanAssets $ 40.0us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateFairValueOfPlanAssets