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

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- Definition Equity Method Investment, Summarized Financial Information, Gain on Sale of Assets + References No definition available. + Details
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- Definition The amount of net income (loss) reported by an equity method investment of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(g)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33912-111571 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph bb -Article 1 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph g -Subparagraph 1, 2 -Article 4 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph w -Article 1 + Details
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- Definition The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity. + References No definition available. + Details
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- Definition The difference between the carrying value and the sale price of an investment. A gain would be recognized when the sale price of the investment is greater than the carrying value of the investment. This element refers to the Gain included in earnings and not to the cash proceeds of the sale. + References No definition available. + Details
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