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Income Taxes (Details 1) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Income Tax Disclosure [Abstract] |  |  |
Provision for Income Taxes, Equity Method Investment | $ 18.3 gmt_ProvisionForIncomeTaxesEquityMethodInvestment | $ 16.5 gmt_ProvisionForIncomeTaxesEquityMethodInvestment | $ 2.0 gmt_ProvisionForIncomeTaxesEquityMethodInvestment
Equity Method Investment, Summarized Financial Information, Income Tax |  | 15.2 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax | 9.2 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax
Income before income taxes |  |  |
Domestic | 137.9 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic | 66.0 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic | 80.5 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic
Foreign | 93.3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign | 93.0 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign | 63.3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign
Income before Income Taxes and Share of Affiliates’ Earnings | $ 231.2 gmt_IncomeBeforeIncomeTaxes | $ 159.0 gmt_IncomeBeforeIncomeTaxes | $ 143.8 gmt_IncomeBeforeIncomeTaxes

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- Definition Equity Method Investment, Summarized Financial Information, Income Tax + References No definition available. + Details
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- Definition Income before income taxes. + References No definition available. + Details
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- Definition Provision for Income Taxes, Equity Method Investment + References No definition available. + Details
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- Definition The portion of earnings or loss from continuing operations before income taxes that is attributable to domestic operations. + 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.(h)(1)(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition The portion of earnings or loss from continuing operations before income taxes that is attributable to foreign operations, which is defined as Income or Loss generated from operations located outside the entity's country of domicile. + 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.(h)(1)(i)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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