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Financial Data of Business Segments (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 Segment | Dec. 31, 2013 | Dec. 31, 2012
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Provision for Income Taxes, Equity Method Investment |  |  |  |  |  |  |  |  | $ 18.3 gmt_ProvisionForIncomeTaxesEquityMethodInvestment | $ 16.5 gmt_ProvisionForIncomeTaxesEquityMethodInvestment | $ 2.0 gmt_ProvisionForIncomeTaxesEquityMethodInvestment
Number of business segments |  |  |  |  |  |  |  |  | 4 us-gaap_NumberOfOperatingSegments |  |
Profitability |  |  |  |  |  |  |  |  |  |  |
Lease revenue |  |  |  |  |  |  |  |  | 1,086.6 gmt_LeaseIncome | 975.2 gmt_LeaseIncome | 917.0 gmt_LeaseIncome
Marine operating revenue |  |  |  |  |  |  |  |  | 286.3 us-gaap_CargoAndFreightRevenue | 275.1 us-gaap_CargoAndFreightRevenue | 265.5 us-gaap_CargoAndFreightRevenue
Other revenue |  |  |  |  |  |  |  |  | 78.1 us-gaap_OtherNonoperatingIncome | 70.7 us-gaap_OtherNonoperatingIncome | 60.7 us-gaap_OtherNonoperatingIncome
Total Revenues | 401.4 us-gaap_Revenues | 397.2 us-gaap_Revenues | 365.8 us-gaap_Revenues | 286.6 us-gaap_Revenues | 356.6 us-gaap_Revenues | 353.2 us-gaap_Revenues | 338.9 us-gaap_Revenues | 272.3 us-gaap_Revenues | 1,451.0 us-gaap_Revenues | 1,321.0 us-gaap_Revenues | 1,243.2 us-gaap_Revenues
Maintenance expense |  |  |  |  |  |  |  |  | 337.0 us-gaap_CostOfPropertyRepairsAndMaintenance | 294.0 us-gaap_CostOfPropertyRepairsAndMaintenance | 269.7 us-gaap_CostOfPropertyRepairsAndMaintenance
Marine operating expense |  |  |  |  |  |  |  |  | 197.8 gmt_MarineOperatingExpense | 189.8 gmt_MarineOperatingExpense | 182.4 gmt_MarineOperatingExpense
Depreciation |  |  |  |  |  |  |  |  | 273.5 us-gaap_DepreciationNonproduction | 255.0 us-gaap_DepreciationNonproduction | 237.4 us-gaap_DepreciationNonproduction
Operating lease expense |  |  |  |  |  |  |  |  | 108.7 us-gaap_OperatingLeasesRentExpenseNet | 129.4 us-gaap_OperatingLeasesRentExpenseNet | 130.2 us-gaap_OperatingLeasesRentExpenseNet
Other operating expense |  |  |  |  |  |  |  |  | 28.9 us-gaap_OtherCostAndExpenseOperating | 26.1 us-gaap_OtherCostAndExpenseOperating | 24.2 us-gaap_OtherCostAndExpenseOperating
Total Expenses |  |  |  |  |  |  |  |  | 945.9 us-gaap_OperatingCostsAndExpenses | 894.3 us-gaap_OperatingCostsAndExpenses | 843.9 us-gaap_OperatingCostsAndExpenses
Net gain on asset dispositions |  |  |  |  |  |  |  |  | 87.2 gmt_GainLossOnDispositionOfAssetsNet | 85.6 gmt_GainLossOnDispositionOfAssetsNet | 79.5 gmt_GainLossOnDispositionOfAssetsNet
Interest expense, net |  |  |  |  |  |  |  |  | (158.4)us-gaap_InterestExpense | (166.6)us-gaap_InterestExpense | (166.6)us-gaap_InterestExpense
Other (expense) income |  |  |  |  |  |  |  |  | (13.5)us-gaap_OtherNonoperatingIncomeExpense | (8.4)us-gaap_OtherNonoperatingIncomeExpense | (8.2)us-gaap_OtherNonoperatingIncomeExpense
Share of affiliates' earnings (pre-tax) |  |  |  |  |  |  |  |  | 67.8 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax | 92.3 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax | 21.6 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
Segment profit (loss) |  |  |  |  |  |  |  |  | 488.2 gmt_SegmentProfitLoss | 429.6 gmt_SegmentProfitLoss | 325.6 gmt_SegmentProfitLoss
SG&A |  |  |  |  |  |  |  |  | 189.2 us-gaap_SellingGeneralAndAdministrativeExpense | 178.3 us-gaap_SellingGeneralAndAdministrativeExpense | 160.2 us-gaap_SellingGeneralAndAdministrativeExpense
Income tax benefit |  |  |  |  |  |  |  |  | 75.7 us-gaap_IncomeTaxExpenseBenefit | 65.5 us-gaap_IncomeTaxExpenseBenefit | 26.1 us-gaap_IncomeTaxExpenseBenefit
Net Income | 58.5 us-gaap_NetIncomeLoss | 51.3 us-gaap_NetIncomeLoss | 53.1 us-gaap_NetIncomeLoss | 42.1 us-gaap_NetIncomeLoss | 53.3 us-gaap_NetIncomeLoss | 53.8 us-gaap_NetIncomeLoss | 35.1 us-gaap_NetIncomeLoss | 27.1 us-gaap_NetIncomeLoss | 205.0 us-gaap_NetIncomeLoss | 169.3 us-gaap_NetIncomeLoss | 137.3 us-gaap_NetIncomeLoss
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |
Investments in affiliated companies | 357.7 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures |  |  |  | 354.3 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures |  |  |  | 357.7 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures | 354.3 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures | 502.0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
Identifiable assets | 6,937.5 us-gaap_Assets |  |  |  | 6,549.6 us-gaap_Assets |  |  |  | 6,937.5 us-gaap_Assets | 6,549.6 us-gaap_Assets | 6,055.4 us-gaap_Assets
Capital Expenditures |  |  |  |  |  |  |  |  |  |  |
Portfolio investments and capital additions |  |  |  |  |  |  |  |  | (1,030.5)gmt_PortfolioInvestmentsandCapitalAdditions | (859.6)gmt_PortfolioInvestmentsandCapitalAdditions | (770.0)gmt_PortfolioInvestmentsandCapitalAdditions
Rail North America [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Leverage level expressed as a ratio of recourse debt to equity |  |  |  |  |  |  |  |  | 5 gmt_LeverageLevelExpressedAsRatioOfRecourseDebtToEquity / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember |  |
Profitability |  |  |  |  |  |  |  |  |  |  |
Lease revenue |  |  |  |  |  |  |  |  | 864.1 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 758.9 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 713.9 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Marine operating revenue |  |  |  |  |  |  |  |  | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Other revenue |  |  |  |  |  |  |  |  | 63.4 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 58.2 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 51.4 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Total Revenues |  |  |  |  |  |  |  |  | 927.5 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 817.1 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 765.3 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Maintenance expense |  |  |  |  |  |  |  |  | 265.5 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 228.2 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 201.4 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Marine operating expense |  |  |  |  |  |  |  |  | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Depreciation |  |  |  |  |  |  |  |  | 190.0 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 176.7 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 167.7 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Operating lease expense |  |  |  |  |  |  |  |  | 103.7 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 124.4 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 126.5 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Other operating expense |  |  |  |  |  |  |  |  | 21.9 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 18.4 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 18.5 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Total Expenses |  |  |  |  |  |  |  |  | 581.1 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 547.7 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 514.1 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Net gain on asset dispositions |  |  |  |  |  |  |  |  | 72.3 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 67.7 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 58.6 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Interest expense, net |  |  |  |  |  |  |  |  | (98.4)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | (106.0)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | (101.9)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Other (expense) income |  |  |  |  |  |  |  |  | (7.2)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | (9.8)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | (5.1)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Share of affiliates' earnings (pre-tax) |  |  |  |  |  |  |  |  | 7.9 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 10.3 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 6.5 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Segment profit (loss) |  |  |  |  |  |  |  |  | 321.0 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 231.6 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 209.3 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |
Investments in affiliated companies | 17.2 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember |  |  |  | 31.4 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember |  |  |  | 17.2 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 31.4 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 46.9 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Identifiable assets | 4,358.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember |  |  |  | 3,710.5 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember |  |  |  | 4,358.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 3,710.5 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | 3,601.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Capital Expenditures |  |  |  |  |  |  |  |  |  |  |
Portfolio investments and capital additions |  |  |  |  |  |  |  |  | (810.6)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | (502.4)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember | (465.9)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_RailNorthAmericaMember
Rail International [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Leverage level expressed as a ratio of recourse debt to equity |  |  |  |  |  |  |  |  | 2 gmt_LeverageLevelExpressedAsRatioOfRecourseDebtToEquity / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember |  |
Profitability |  |  |  |  |  |  |  |  |  |  |
Lease revenue |  |  |  |  |  |  |  |  | 188.6 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 180.2 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 161.2 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Marine operating revenue |  |  |  |  |  |  |  |  | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Other revenue |  |  |  |  |  |  |  |  | 10.3 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 8.8 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 6.5 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Total Revenues |  |  |  |  |  |  |  |  | 198.9 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 189.0 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 167.7 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Maintenance expense |  |  |  |  |  |  |  |  | 45.9 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 42.9 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 46.6 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Marine operating expense |  |  |  |  |  |  |  |  | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Depreciation |  |  |  |  |  |  |  |  | 47.1 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 43.2 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 36.1 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Operating lease expense |  |  |  |  |  |  |  |  | 0 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 0 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 0 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Other operating expense |  |  |  |  |  |  |  |  | 5.1 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 5.3 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 5.1 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Total Expenses |  |  |  |  |  |  |  |  | 98.1 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 91.4 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 87.8 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Net gain on asset dispositions |  |  |  |  |  |  |  |  | 6.0 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 3.7 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 1.7 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Interest expense, net |  |  |  |  |  |  |  |  | (24.7)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | (23.9)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | (24.5)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Other (expense) income |  |  |  |  |  |  |  |  | (3.1)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | (1.1)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | (6.1)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Share of affiliates' earnings (pre-tax) |  |  |  |  |  |  |  |  | (0.3)gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 21.1 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | (18.3)gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Segment profit (loss) |  |  |  |  |  |  |  |  | 78.7 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 97.4 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 32.7 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |
Investments in affiliated companies | 1.8 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember |  |  |  | 2.0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember |  |  |  | 1.8 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 2.0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 77.2 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Identifiable assets | 1,229.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember |  |  |  | 1,297.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember |  |  |  | 1,229.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 1,297.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | 1,105.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
Capital Expenditures |  |  |  |  |  |  |  |  |  |  |
Portfolio investments and capital additions |  |  |  |  |  |  |  |  | (163.6)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | (168.5)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember | (200.1)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_RailInternationalMember
ASC [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Leverage level expressed as a ratio of recourse debt to equity |  |  |  |  |  |  |  |  | 1.5 gmt_LeverageLevelExpressedAsRatioOfRecourseDebtToEquity / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember |  |
Profitability |  |  |  |  |  |  |  |  |  |  |
Lease revenue |  |  |  |  |  |  |  |  | 4.2 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 4.2 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 4.3 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Marine operating revenue |  |  |  |  |  |  |  |  | 223.0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 223.5 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 239.1 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Other revenue |  |  |  |  |  |  |  |  | 0 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Total Revenues |  |  |  |  |  |  |  |  | 227.2 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 227.7 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 243.4 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Maintenance expense |  |  |  |  |  |  |  |  | 25.6 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 22.9 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 21.7 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Marine operating expense |  |  |  |  |  |  |  |  | 149.2 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 151.3 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 160.3 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Depreciation |  |  |  |  |  |  |  |  | 13.6 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 12.1 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 11.9 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Operating lease expense |  |  |  |  |  |  |  |  | 5.2 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 5.2 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 3.8 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Other operating expense |  |  |  |  |  |  |  |  | 0 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | (0.3)us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Total Expenses |  |  |  |  |  |  |  |  | 193.6 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 191.5 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 197.4 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Net gain on asset dispositions |  |  |  |  |  |  |  |  | (0.5)gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | (1.3)gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Interest expense, net |  |  |  |  |  |  |  |  | (5.6)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | (6.2)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | (7.1)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Other (expense) income |  |  |  |  |  |  |  |  | (0.2)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0.2 us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | (1.4)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Share of affiliates' earnings (pre-tax) |  |  |  |  |  |  |  |  | 0 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Segment profit (loss) |  |  |  |  |  |  |  |  | 27.3 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 28.9 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 37.5 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |
Investments in affiliated companies | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember |  |  |  | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember |  |  |  | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Identifiable assets | 286.7 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember |  |  |  | 271.0 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember |  |  |  | 286.7 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 271.0 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | 284.2 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Capital Expenditures |  |  |  |  |  |  |  |  |  |  |
Portfolio investments and capital additions |  |  |  |  |  |  |  |  | (18.4)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | (11.2)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember | (12.6)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_AscMember
Portfolio Management [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Leverage level expressed as a ratio of recourse debt to equity |  |  |  |  |  |  |  |  | 3 gmt_LeverageLevelExpressedAsRatioOfRecourseDebtToEquity / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember |  |
Profitability |  |  |  |  |  |  |  |  |  |  |
Lease revenue |  |  |  |  |  |  |  |  | 29.7 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 31.9 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 37.6 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Marine operating revenue |  |  |  |  |  |  |  |  | 63.3 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 51.6 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 26.4 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Other revenue |  |  |  |  |  |  |  |  | 4.4 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 3.7 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 2.8 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Total Revenues |  |  |  |  |  |  |  |  | 97.4 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 87.2 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 66.8 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Maintenance expense |  |  |  |  |  |  |  |  | 0 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 0 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 0 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Marine operating expense |  |  |  |  |  |  |  |  | 48.6 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 38.5 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 22.1 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Depreciation |  |  |  |  |  |  |  |  | 22.8 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 23.0 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 21.7 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Operating lease expense |  |  |  |  |  |  |  |  | 0 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 0 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 0.2 us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Other operating expense |  |  |  |  |  |  |  |  | 1.9 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 2.4 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 0.9 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Total Expenses |  |  |  |  |  |  |  |  | 73.3 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 63.9 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 44.9 us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Net gain on asset dispositions |  |  |  |  |  |  |  |  | 9.4 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 15.5 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 19.2 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Interest expense, net |  |  |  |  |  |  |  |  | (24.3)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | (26.7)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | (27.7)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Other (expense) income |  |  |  |  |  |  |  |  | (1.2)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 1.4 us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 3.4 us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Share of affiliates' earnings (pre-tax) |  |  |  |  |  |  |  |  | 60.2 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 60.9 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 33.4 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Segment profit (loss) |  |  |  |  |  |  |  |  | 68.2 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 74.4 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 50.2 gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |
Investments in affiliated companies | 338.7 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember |  |  |  | 320.9 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember |  |  |  | 338.7 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 320.9 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 377.9 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Identifiable assets | 813.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember |  |  |  | 856.9 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember |  |  |  | 813.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 856.9 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | 797.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Capital Expenditures |  |  |  |  |  |  |  |  |  |  |
Portfolio investments and capital additions |  |  |  |  |  |  |  |  | (32.3)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | (170.5)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember | (83.5)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_PortfolioManagementMember
Other [Member] |  |  |  |  |  |  |  |  |  |  |
Profitability |  |  |  |  |  |  |  |  |  |  |
Lease revenue |  |  |  |  |  |  |  |  | 0 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 gmt_LeaseIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Marine operating revenue |  |  |  |  |  |  |  |  | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_CargoAndFreightRevenue / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Other revenue |  |  |  |  |  |  |  |  | 0 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_OtherNonoperatingIncome / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Total Revenues |  |  |  |  |  |  |  |  | 0 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Maintenance expense |  |  |  |  |  |  |  |  | 0 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_CostOfPropertyRepairsAndMaintenance / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Marine operating expense |  |  |  |  |  |  |  |  | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 gmt_MarineOperatingExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Depreciation |  |  |  |  |  |  |  |  | 0 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_DepreciationNonproduction / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Operating lease expense |  |  |  |  |  |  |  |  | (0.2)us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (0.2)us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (0.3)us-gaap_OperatingLeasesRentExpenseNet / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Other operating expense |  |  |  |  |  |  |  |  | 0 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_OtherCostAndExpenseOperating / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Total Expenses |  |  |  |  |  |  |  |  | (0.2)us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (0.2)us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (0.3)us-gaap_OperatingCostsAndExpenses / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Net gain on asset dispositions |  |  |  |  |  |  |  |  | 0 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 gmt_GainLossOnDispositionOfAssetsNet / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Interest expense, net |  |  |  |  |  |  |  |  | (5.4)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (3.8)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (5.4)us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Other (expense) income |  |  |  |  |  |  |  |  | (1.8)us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0.9 us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 1.0 us-gaap_OtherNonoperatingIncomeExpense / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Share of affiliates' earnings (pre-tax) |  |  |  |  |  |  |  |  | 0 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 gmt_IncomeLossFromEquityMethodInvestmentBeforeTax / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Segment profit (loss) |  |  |  |  |  |  |  |  | (7.0)gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (2.7)gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (4.1)gmt_SegmentProfitLoss / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Selected Balance Sheet Data |  |  |  |  |  |  |  |  |  |  |
Investments in affiliated companies | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember |  |  |  | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember |  |  |  | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 0 us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Identifiable assets | 249.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember |  |  |  | 414.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember |  |  |  | 249.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 414.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | 266.9 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Capital Expenditures |  |  |  |  |  |  |  |  |  |  |
Portfolio investments and capital additions |  |  |  |  |  |  |  |  | (5.6)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (7.0)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember | (7.9)gmt_PortfolioInvestmentsandCapitalAdditions / us-gaap_StatementBusinessSegmentsAxis = gmt_OtherBusinessSegmentsMember
Parent [Member] |  |  |  |  |  |  |  |  |  |  |
Profitability |  |  |  |  |  |  |  |  |  |  |
Income tax benefit |  |  |  |  |  |  |  |  | $ 94.0 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = gmt_ParentAndEquityMethodInvestmentsMember | $ 82.0 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = gmt_ParentAndEquityMethodInvestmentsMember | $ 28.1 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementEquityComponentsAxis = gmt_ParentAndEquityMethodInvestmentsMember

X
- Definition Gain (Loss) on Disposition of Assets, Net + References No definition available. + Details
Name: | gmt_GainLossOnDispositionOfAssetsNet
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents the entity's proportionate share for the period of the pre-tax income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee. Income (loss) from equity method investment, pre-tax. + References No definition available. + Details
Name: | gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Lease income. + References No definition available. + Details
Name: | gmt_LeaseIncome
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Leverage level expressed as a ratio of recourse debt to equity. + References No definition available. + Details
Name: | gmt_LeverageLevelExpressedAsRatioOfRecourseDebtToEquity
Namespace Prefix: | gmt_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition Direct cost of labor, materials and fuel used in waterborne transportation. + References No definition available. + Details
Name: | gmt_MarineOperatingExpense
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Portfolio Investments and Capital Additions + References No definition available. + Details
Name: | gmt_PortfolioInvestmentsandCapitalAdditions
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Provision for Income Taxes, Equity Method Investment + References No definition available. + Details
Name: | gmt_ProvisionForIncomeTaxesEquityMethodInvestment
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Segment profit (loss). + References No definition available. + Details
Name: | gmt_SegmentProfitLoss
Namespace Prefix: | gmt_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | gmt_SelectedBalanceSheetData
Namespace Prefix: | gmt_
Data Type: | xbrli:stringItemType
Balance Type: | na
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