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Leases (Details 2) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Assets that are financed with capital lease obligations |  |
Less: allowance for depreciation | $ (75.3)us-gaap_CapitalLeasesLesseeBalanceSheetAssetsByMajorClassAccumulatedDeprecation | $ (71.4)us-gaap_CapitalLeasesLesseeBalanceSheetAssetsByMajorClassAccumulatedDeprecation
Capital leased assets, net | 7.8 us-gaap_CapitalLeasesBalanceSheetAssetsByMajorClassNet | 10.0 us-gaap_CapitalLeasesBalanceSheetAssetsByMajorClassNet
Marine vessels [Member] |  |
Assets that are financed with capital lease obligations |  |
Capital leased assets, gross | $ 83.1 us-gaap_CapitalLeasedAssetsGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = gmt_MarineVesselsMember | $ 81.4 us-gaap_CapitalLeasedAssetsGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = gmt_MarineVesselsMember

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- Definition Amount before accumulated depreciation of leased physical assets used in the normal conduct of business to produce goods and services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45014-112735 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The total gross amount less the charge for the use of the long-lived depreciable assets subject to a lease meeting the criteria for capitalization. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 + Details
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Balance Type: | debit
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- Details
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- Definition The total charge for the use of long-lived depreciable assets subject to a lease meeting the criteria for capitalization. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45014-112735 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 2) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 + Details
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- Details
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Balance Type: |
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