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Significant Accounting Policies (Details Textual) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 Entity | Dec. 31, 2013 | Dec. 31, 2012
Accounting Policies [Abstract] |  |  |
Wholly-owned bankruptcy remote special-purpose corporations | 2 gmt_WhollyOwnedBankruptcyRemoteSpecialPurposeCorporations |  |
Interest expense net of interest income on bank deposits | $ 0.9 us-gaap_InterestIncomeExpenseNet | $ 1.2 us-gaap_InterestIncomeExpenseNet | $ 1.9 us-gaap_InterestIncomeExpenseNet

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- Definition Wholly-owned bankruptcy remote special-purpose corporations. + References No definition available. + Details
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Namespace Prefix: | gmt_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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

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- Definition The net amount of operating interest income (expense). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.10) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 10 -Article 9 + Details
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Period Type: | duration