Investments in Affiliated Companies (Textual) (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) AircraftEngines Business_Activity Joint_Venture	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Equity Method Investments [Line Items]
Loans to affiliated companies	$ 0.0	$ 6.5
Asset Impairment Charges	9.0	8.6	$ 38.5
Share of affiliates' earnings (net of tax)	50.3	43.9	47.4
Portfolio proceeds	234.4	165.6	223.7
RRPF Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
EquityMethodInvestmentSummarizedFinancialInformationAPandAccruedExpense	$ 97.9	53.9
GATX ownership percentage in joint venture	50.00%
Number of joint venture investments \| Joint_Venture	17
Number of business activities \| Business_Activity	2
Number of Aircraft Engines \| AircraftEngines	452
Number of Aircraft Engines Under Leasing Arrangement \| AircraftEngines	253
Share of affiliates' earnings (net of tax)	$ 49.8	44.8	46.6
RRPF Joint Ventures [Member] \| Minimum [Member]
Schedule of Equity Method Investments [Line Items]
Lease term	3 years
RRPF Joint Ventures [Member] \| Maximum [Member]
Schedule of Equity Method Investments [Line Items]
Lease term	12 years
RRPF Joint Ventures [Member] \| Aircraft Engines [Member]
Schedule of Equity Method Investments [Line Items]
Estimated useful lives of depreciable assets	25 years
Portfolio Management [Member]
Schedule of Equity Method Investments [Line Items]
Asset Impairment Charges	$ 4.5	3.7	6.7
Cardinal Marine [Domain] \| Portfolio Management [Member]
Schedule of Equity Method Investments [Line Items]
Portfolio proceeds		$ 1.1	$ 1.0

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