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Investments in Affiliated Companies (Share of Affiliate Earnings) (Details 2) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Pre tax share of affiliates earnings by segment      
Share of affiliates' earnings (pre-tax) $ 67.8gmt_IncomeLossFromEquityMethodInvestmentBeforeTax $ 92.3gmt_IncomeLossFromEquityMethodInvestmentBeforeTax $ 21.6gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
Provision for Income Taxes, Equity Method Investment (18.3)gmt_ProvisionForIncomeTaxesEquityMethodInvestment (16.5)gmt_ProvisionForIncomeTaxesEquityMethodInvestment (2.0)gmt_ProvisionForIncomeTaxesEquityMethodInvestment
Share of Affiliates' Earnings 49.5us-gaap_IncomeLossFromEquityMethodInvestments 75.8us-gaap_IncomeLossFromEquityMethodInvestments 19.6us-gaap_IncomeLossFromEquityMethodInvestments
Rail North America [Member]      
Pre tax share of affiliates earnings by segment      
Share of affiliates' earnings (pre-tax) 7.9gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_RailNorthAmericaMember		 10.3gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_RailNorthAmericaMember		 6.5gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_RailNorthAmericaMember
Rail International [Member]      
Pre tax share of affiliates earnings by segment      
Share of affiliates' earnings (pre-tax) (0.3)gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_RailInternationalMember		 21.1gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_RailInternationalMember		 (18.3)gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_RailInternationalMember
Portfolio Management [Member]      
Pre tax share of affiliates earnings by segment      
Share of affiliates' earnings (pre-tax) $ 60.2gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_PortfolioManagementMember		 $ 60.9gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_PortfolioManagementMember		 $ 33.4gmt_IncomeLossFromEquityMethodInvestmentBeforeTax
/ us-gaap_StatementBusinessSegmentsAxis
= gmt_PortfolioManagementMember