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Fair Value Disclosure (Details 2) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Interest Expense [Member] | Fair Value Hedging [Member]    
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)    
Interest expense $ (2.3)us-gaap_ChangeInUnrealizedGainLossOnFairValueHedgingInstruments1
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember		 [1] $ 2.1us-gaap_ChangeInUnrealizedGainLossOnFairValueHedgingInstruments1
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember		 [1]
Interest Expense [Member] | Cash Flow Hedges [Member]    
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)    
Effective portion reclassified from accumulated other comprehensive loss 1.3us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember		 1.2us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_InterestExpenseMember
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member]    
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)    
Other comprehensive loss (effective portion) (8.4)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherComprehensiveIncomeMember		 1.8us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherComprehensiveIncomeMember
Operating Expense [Member] | Cash Flow Hedges [Member]    
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)    
Effective portion reclassified from accumulated other comprehensive loss 0.1us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OperatingExpenseMember		 0.2us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OperatingExpenseMember
Other Expense [Member] | Cash Flow Hedges [Member]    
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)    
Other expense (8.4)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherExpenseMember		 2.2us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherExpenseMember
Not Designated as Hedging Instrument [Member] | Other Expense [Member]    
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)    
Other expense (4.2)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherExpenseMember		 0.1us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherExpenseMember
Not Designated as Hedging Instrument [Member] | Other Expense [Member] | Aae Cargo Ag [Member]    
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)    
Other expense 5.1us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= gmt_AaeCargoAgMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherExpenseMember		  
Foreign Currency Gain (Loss) [Member] | Other Expense [Member] | Cash Flow Hedges [Member]    
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)    
Other expense $ 9.9us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignCurrencyGainLossMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherExpenseMember		  
[1] (1) The fair value adjustments related to the underlying debt equally offset the amounts recognized in interest expense.