Investments in Affiliated Companies (Textual) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) AircraftEngines Business_Activity Joint_Venture	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Equity Method Investments [Line Items]
Noncash or Part Noncash Acquisition, Value of Assets Acquired	$ 0.0	$ 0.0	$ 1.1
Loans to affiliated companies	9.7	11.2
Asset Impairment Charges	38.5	33.9	1.3
Share of affiliates' earnings (net of tax)	47.4	45.9	49.5
Portfolio proceeds	$ 223.7	482.2	264.0
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	15
Number of business activities | Business_Activity	2
Number of Aircraft Engines | AircraftEngines	407
Number of Aircraft Engines Under Leasing Arrangement | AircraftEngines	211
Share of affiliates' earnings (net of tax)	$ 46.6	$ 58.4	$ 42.2
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