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Foreign Operations (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Foreign operations data |  |  |  |  |  |  |  |  |  |  |
Revenues | $ 401.4 us-gaap_Revenues | $ 397.2 us-gaap_Revenues | $ 365.8 us-gaap_Revenues | $ 286.6 us-gaap_Revenues | $ 356.6 us-gaap_Revenues | $ 353.2 us-gaap_Revenues | $ 338.9 us-gaap_Revenues | $ 272.3 us-gaap_Revenues | $ 1,451.0 us-gaap_Revenues | $ 1,321.0 us-gaap_Revenues | $ 1,243.2 us-gaap_Revenues
Identifiable assets | 6,937.5 us-gaap_Assets |  |  |  | 6,549.6 us-gaap_Assets |  |  |  | 6,937.5 us-gaap_Assets | 6,549.6 us-gaap_Assets | 6,055.4 us-gaap_Assets
Foreign Operations (Textual) [Abstract] |  |  |  |  |  |  |  |  |  |  |
Maximum percentage of consolidated revenues derive from any individual foreign country |  |  |  |  |  |  |  |  | 10.00%gmt_MaximumPercentageOfConsolidatedRevenuesDeriveFromAnyIndividualForeignCountry |  |
Percentage of company's identifiable assets | 10.00%gmt_PercentageOfCompanysIdentifiableAssets |  |  |  |  |  |  |  | 10.00%gmt_PercentageOfCompanysIdentifiableAssets |  |
Foreign [Member] |  |  |  |  |  |  |  |  |  |  |
Foreign operations data |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 346.2 us-gaap_Revenues / us-gaap_StatementGeographicalAxis = gmt_ForeignMember | 332.1 us-gaap_Revenues / us-gaap_StatementGeographicalAxis = gmt_ForeignMember | 279.8 us-gaap_Revenues / us-gaap_StatementGeographicalAxis = gmt_ForeignMember
Identifiable assets | 2,133.9 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_ForeignMember |  |  |  | 2,200.1 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_ForeignMember |  |  |  | 2,133.9 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_ForeignMember | 2,200.1 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_ForeignMember | 1,897.9 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_ForeignMember
United States [Member] |  |  |  |  |  |  |  |  |  |  |
Foreign operations data |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 1,104.8 us-gaap_Revenues / us-gaap_StatementGeographicalAxis = gmt_DomesticMember | 988.9 us-gaap_Revenues / us-gaap_StatementGeographicalAxis = gmt_DomesticMember | 963.4 us-gaap_Revenues / us-gaap_StatementGeographicalAxis = gmt_DomesticMember
Identifiable assets | $ 4,803.6 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_DomesticMember |  |  |  | $ 4,349.5 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_DomesticMember |  |  |  | $ 4,803.6 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_DomesticMember | $ 4,349.5 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_DomesticMember | $ 4,157.5 us-gaap_Assets / us-gaap_StatementGeographicalAxis = gmt_DomesticMember

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- Definition Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.18) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 18 -Article 5 + Details
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