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Pension and Other Post-Retirement Benefits (Details 8) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Fair value of pension plan assets |  |
Assets | $ 457.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |
Fair Value, Inputs, Level 1 [Member] |  |
Fair value of pension plan assets |  |
Assets | 15.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 447.8 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fair Value, Inputs, Level 2 [Member] |  |
Fair value of pension plan assets |  |
Assets | 441.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 25.1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fair Value, Inputs, Level 3 [Member] |  |
Fair value of pension plan assets |  |
Assets |  | 422.7 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Short-term investment funds [Member] |  |
Fair value of pension plan assets |  |
Assets | 0.9 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_ShortTermInvestmentFundsMember |
Short-term investment funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair value of pension plan assets |  |
Assets | 0.9 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_ShortTermInvestmentFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 1.3 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_ShortTermInvestmentFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Short-term investment funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair value of pension plan assets |  |
Assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_ShortTermInvestmentFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 1.3 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_ShortTermInvestmentFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Short-term investment funds [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair value of pension plan assets |  |
Assets |  | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_ShortTermInvestmentFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Common stock [Member] |  |
Fair value of pension plan assets |  |
Assets |  |
Common stock [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair value of pension plan assets |  |
Assets |  |
Common stock [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair value of pension plan assets |  |
Assets |  |
Common stock [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair value of pension plan assets |  |
Assets |  |
Common stock collective funds [Member] |  |
Fair value of pension plan assets |  |
Assets | 215.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_StockCollectiveFundsMember |
Common stock collective funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair value of pension plan assets |  |
Assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_StockCollectiveFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 263.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_StockCollectiveFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Common stock collective funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair value of pension plan assets |  |
Assets | 215.2 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_StockCollectiveFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_StockCollectiveFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Common stock collective funds [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair value of pension plan assets |  |
Assets |  | 263.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_StockCollectiveFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Fixed income collective trust funds [Member] |  |
Fair value of pension plan assets |  |
Assets | 208.1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_FixedIncomeCollectiveTrustFundsMember |
Fixed income collective trust funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair value of pension plan assets |  |
Assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_FixedIncomeCollectiveTrustFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 138.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_FixedIncomeCollectiveTrustFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Fixed income collective trust funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair value of pension plan assets |  |
Assets | 208.1 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_FixedIncomeCollectiveTrustFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_FixedIncomeCollectiveTrustFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fixed income collective trust funds [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair value of pension plan assets |  |
Assets |  | 138.0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = gmt_FixedIncomeCollectiveTrustFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Real estate investment funds [Member] |  |
Fair value of pension plan assets |  |
Assets | 17.9 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember |
Real estate investment funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair value of pension plan assets |  |
Assets | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 21.7 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Real estate investment funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair value of pension plan assets |  |
Assets | 17.9 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 0 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Real estate investment funds [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair value of pension plan assets |  |
Assets |  | 21.7 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
UNITED STATES | Common stock [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair value of pension plan assets |  |
Assets | 13.3 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_StatementGeographicalAxis = country_US | 21.4 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_StatementGeographicalAxis = country_US
International [Member] | Common stock [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair value of pension plan assets |  |
Assets | $ 1.6 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_StatementGeographicalAxis = gmt_InternationalMember | $ 2.4 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_StatementGeographicalAxis = gmt_InternationalMember

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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930 + Details
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