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Investments in Affiliated Companies (Operating Results, Affiliates) (Details 4) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Equity Method Investments and Joint Ventures [Abstract]      
Gain on Sale of Investments   $ 2.5us-gaap_GainOnSaleOfInvestments  
Operating results for all affiliated companies of GATX      
Revenues 339.0us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue 334.5us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue 651.8us-gaap_EquityMethodInvestmentSummarizedFinancialInformationRevenue
Gains on sales of assets 33.7gmt_EquityMethodInvestmentSummarizedFinancialInformationGainOnSaleOfAssets 43.5gmt_EquityMethodInvestmentSummarizedFinancialInformationGainOnSaleOfAssets 57.4gmt_EquityMethodInvestmentSummarizedFinancialInformationGainOnSaleOfAssets
Equity Method Investment, Summarized Financial Information, Net Income (Loss) $ 99.6us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss $ 108.6us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss $ 75.3us-gaap_EquityMethodInvestmentSummarizedFinancialInformationNetIncomeLoss