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Income Taxes (Details Textual) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Operating Loss Carryforwards [Line Items] |  |  |
Equity Method Investment, Summarized Financial Information, Income Tax |  | $ 15.2 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax | $ 9.2 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax
Effective Income Tax Rate Reconciliation, Percent | 32.70%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 41.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations | 18.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Cumulative amount of undistributed earning of foreign subsidiaries | 727.7 us-gaap_DeferredTaxLiabilitiesUndistributedForeignEarnings |  |
Alternative minimum tax credit | 9.2 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax | 8.4 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax |
Foreign tax credit | 3.6 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign | 3.6 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign |
Foreign tax credits valuation allowance | 3.6 us-gaap_TaxCreditCarryforwardValuationAllowance | 3.6 us-gaap_TaxCreditCarryforwardValuationAllowance |
State net operating loss | 28.7 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal | 27.1 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal |
State net operating loss carryforwards valuation allowance | 11.3 gmt_DeferredTaxAssetsValuationOnStateNetOperatingLoss | 11.2 gmt_DeferredTaxAssetsValuationOnStateNetOperatingLoss |
Foreign net operating loss | 5.8 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign | 8.5 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign |
Unrecognized Tax Benefits | 5.6 us-gaap_UnrecognizedTaxBenefits | 5.7 us-gaap_UnrecognizedTaxBenefits | 4.7 us-gaap_UnrecognizedTaxBenefits
Decrease in income tax expenses unrecognized tax benefits if recognized | 3.6 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | 3.9 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities | 0.3 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities | 1.0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities |
Reductions due to expiration of the applicable statute of limitations | 0.4 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations | 0 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations |
Effective Income Tax Rate Reconciliation, Tax Contingency, Domestic, Amount | (0.2)us-gaap_IncomeTaxReconciliationTaxContingenciesDomestic | 0.6 us-gaap_IncomeTaxReconciliationTaxContingenciesDomestic | (15.5)us-gaap_IncomeTaxReconciliationTaxContingenciesDomestic
Tax On Gain Of Affiliate Sale | 0 gmt_TaxOnGainOfAffiliateSale | (23.2)gmt_TaxOnGainOfAffiliateSale | 0 gmt_TaxOnGainOfAffiliateSale
Domestic [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Amount of U.S. federal tax net operating loss | 119.3 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |  |
Foreign [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Equity Method Investment, Summarized Financial Information, Income Tax |  | 7.6 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember | 4.6 gmt_EquityMethodInvestmentSummarizedFinancialInformationIncomeTax / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember
Amount of U.S. federal tax net operating loss | $ 5.6 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_ForeignCountryMember |  |

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- Definition Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiaries and other recognized entities not within the country of domicile of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 + Details
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