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

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- 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
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- Definition This item represents the entity's proportionate share for the period of the net 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 9 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.12) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 7 + Details
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