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Concentrations (Details) | 12 Months Ended
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Dec. 31, 2014
Concentration [Abstract] |
Concentration Risk, Labor Subject to Collective Bargaining Arrangements | .40
Multiemployer Plans, Collective-Bargaining Arrangement, Description | three
Canadian |
Concentration [Abstract] |
Number of service centers | 3 gmt_NumberOfServiceCenters / us-gaap_StatementGeographicalAxis = country_CA
Petroleum Industry [Member] |
Concentration [Abstract] |
Approximate percentage of revenue | 24.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = gmt_PetroleumIndustryMember
Chemical Industry [Member] |
Concentration [Abstract] |
Approximate percentage of revenue | 21.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = gmt_ChemicalIndustryMember
Transportation Industry [Member] |
Concentration [Abstract] |
Approximate percentage of revenue | 20.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = gmt_TransportationIndustryMember
Steel Industry [Member] |
Concentration [Abstract] |
Approximate percentage of revenue | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = gmt_SteelIndustryMember
Food/Agriculture [Member] |
Concentration [Abstract] |
Approximate percentage of revenue | 7.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = gmt_FoodAndAgricultureMember
Customer Concentration Risk [Member] |
Concentration [Abstract] |
Approximate percentage of revenue | 10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year [Member] |
Concentration [Abstract] |
Concentration Risk, Labor Subject to Collective Bargaining Arrangements | 0.03
ASC [Member] |
Concentration [Abstract] |
Concentration Risk, Labor Subject to Collective Bargaining Arrangements | twelve
GERMANY |
Concentration [Abstract] |
Concentration Risk, Labor Subject to Collective Bargaining Arrangements | one

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- Definition Number of Service Centers + References No definition available. + Details
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- Definition Description of any concentration of labor subject to collective bargaining agreements, including the general nature of the risk. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6327-108592 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 + Details
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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 + Details
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- Definition Description of the significant collective-bargaining arrangements within the range of expiration dates for collective-bargaining arrangements requiring contributions to a multiemployer plan as well as other information to help investors understand the significance of the collective-bargaining agreements and when they expire. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
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