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Legal Proceedings and Other Contingencies (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Oct. 02, 2013
Insurance_Provider
Dec. 14, 2012
Employee
Corporations
Jun. 29, 2009
Jan. 31, 2015
case
Loss Contingencies [Line Items]            
Accruals for losses related to probable litigation matters $ 1.7us-gaap_LitigationReserve          
Number of sites for which the Company is involved in environmental remediation 18gmt_AdministrativeAndJudicialProceedingsAtSites          
Accruals for remediation and restoration 13.8us-gaap_AccrualForEnvironmentalLossContingencies 14.7us-gaap_AccrualForEnvironmentalLossContingencies        
Viareggio [Member]            
Loss Contingencies [Line Items]            
Number of casualties         32gmt_NumberOfCasualties
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		  
Number of subsidiaries       2gmt_NumberOfSubsidiaries
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		    
Number of Italian railway companies       4gmt_NumberOfCompanies
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		    
Number of employees       18dei_EntityNumberOfEmployees
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		    
Number of insurers     3gmt_NumberOfInsuranceProviders
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		      
Asbestos - related cases pending against the company and its subsidiaries 90us-gaap_LossContingencyPendingClaimsNumber
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		          
Legal Fees $ 7.8us-gaap_LegalFees
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		          
Number of Maintenance and Supervisory employees       10gmt_NumberofMaintenanceandSupervisoryemployees
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		    
Court Proceedings 1gmt_CourtProceedings
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		          
Percentage of coverage held by Liberty Mutual Insurance Europe Limited     25.00%gmt_PercentageofcoverageheldbyLibertyMutualInsuranceEuropeLimited
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		      
Asbestos Issue [Member]            
Loss Contingencies [Line Items]            
New cases filed number 7us-gaap_LossContingencyNewClaimsFiledNumber
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_AsbestosIssueMember		          
Cases dismissed or settled 34us-gaap_LossContingencyClaimsSettledAndDismissedNumber
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_AsbestosIssueMember		          
Pending Litigation [Member] | Asbestos Cases [Member] | Subsequent Event [Member]            
Loss Contingencies [Line Items]            
Asbestos - related cases pending against the company and its subsidiaries           104us-gaap_LossContingencyPendingClaimsNumber
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_AsbestosCasesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Pending Litigation [Member] | Asbestos Cases [Member] | Subsequent Event [Member] | Jones Act claims [Member]            
Loss Contingencies [Line Items]            
Asbestos - related cases pending against the company and its subsidiaries           87us-gaap_LossContingencyPendingClaimsNumber
/ us-gaap_LitigationCaseAxis
= gmt_JonesActClaimsMember
/ us-gaap_LitigationStatusAxis
= us-gaap_PendingLitigationMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_AsbestosCasesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Punctured and Ignited [Member] | Viareggio [Member]            
Loss Contingencies [Line Items]            
number of railcars         1gmt_NumberOfRailars
/ gmt_LitigationCaseDetailsAxis
= gmt_PuncturedAndIgnitedMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		  
Overturned [Member] | Viareggio [Member]            
Loss Contingencies [Line Items]            
number of railcars         5gmt_NumberOfRailars
/ gmt_LitigationCaseDetailsAxis
= gmt_OverturnedMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember		  
Derailed [Member] | Viareggio [Member]            
Loss Contingencies [Line Items]            
number of railcars         14gmt_NumberOfRailars
/ gmt_LitigationCaseDetailsAxis
= gmt_DerailedMember
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= gmt_ViareggioMember