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Concentrations (Details)
12 Months Ended
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 3gmt_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