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Investments in Affiliated Companies (Textual) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) AircraftEngines Business_Activity Joint_Venture	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Loans to affiliated companies		$ 6.5	$ 9.7
Asset Impairment Charges		8.6	38.5	$ 33.9
Share of affiliates' earnings (net of tax)		43.9	47.4	45.9
Portfolio proceeds		$ 165.6	223.7	482.2
RRPF Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
GATX ownership percentage in joint venture		50.00%
Number of joint venture investments \| Joint_Venture		16
Number of business activities \| Business_Activity		2
Number of Aircraft Engines \| AircraftEngines		432
Number of Aircraft Engines Under Leasing Arrangement \| AircraftEngines		229
Share of affiliates' earnings (net of tax)	$ 58.4	$ 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		$ 3.7	6.7	31.0
Cardinal Marine [Domain] \| Portfolio Management [Member]
Schedule of Equity Method Investments [Line Items]
Portfolio proceeds		$ 1.1	$ 1.0	$ 24.7