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Variable Interest Entities (Details 1) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Carrying amounts and maximum exposure to loss with respect to VIEs |  |
Net Carrying Amount | $ 144.3 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet | $ 126.1 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet
Maximum Exposure to Loss | 144.3 us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount | 126.1 us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
Investments in Affiliates [Member] |  |
Carrying amounts and maximum exposure to loss with respect to VIEs |  |
Net Carrying Amount | 143.9 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet / gmt_VariableInterestEntitiesByClassificationOfAssetsAxis = gmt_InvestmentsInAffiliatesMember | 125.5 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet / gmt_VariableInterestEntitiesByClassificationOfAssetsAxis = gmt_InvestmentsInAffiliatesMember
Maximum Exposure to Loss | 143.9 us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount / gmt_VariableInterestEntitiesByClassificationOfAssetsAxis = gmt_InvestmentsInAffiliatesMember | 125.5 us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount / gmt_VariableInterestEntitiesByClassificationOfAssetsAxis = gmt_InvestmentsInAffiliatesMember
Other investment [Member] |  |
Carrying amounts and maximum exposure to loss with respect to VIEs |  |
Net Carrying Amount | 0.4 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet / gmt_VariableInterestEntitiesByClassificationOfAssetsAxis = gmt_OtherInvestmentMember | 0.6 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet / gmt_VariableInterestEntitiesByClassificationOfAssetsAxis = gmt_OtherInvestmentMember
Maximum Exposure to Loss | $ 0.4 us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount / gmt_VariableInterestEntitiesByClassificationOfAssetsAxis = gmt_OtherInvestmentMember | $ 0.6 us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount / gmt_VariableInterestEntitiesByClassificationOfAssetsAxis = gmt_OtherInvestmentMember

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- Definition The net carrying amount of the consolidated Variable Interest Entity's assets and liabilities included in the reporting entity's statement of financial position. + References No definition available. + Details
Name: | us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph b -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5728-111685 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5747-111685 + Details
Name: | us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | gmt_VariableInterestEntitiesByClassificationOfAssetsAxis=gmt_OtherInvestmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |