Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) € in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2015 EUR (€)	Dec. 31, 2015 USD ($) Joint_Venture	Dec. 31, 2014 USD ($)	Dec. 31, 2013 EUR (€) Vessel	Dec. 31, 2013 USD ($) Vessel
Investments in and Advances to Affiliates [Line Items]
Advances to Affiliate			$ 11.2	$ 16.3
Asset Impairment Charges			33.9	1.3		$ 5.9
Equity Method Investment, Net Sales Proceeds			24.7	19.4		55.6
Portfolio proceeds and other			482.2	264.0		$ 385.3
Number of vessels received | Vessel					5	5
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			348.5	357.7		$ 354.3
Fair Value of Assets Acquired				1.1		22.9
Cardinal Marine [Domain]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges			$ (19.0)
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership			100.00%
Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	[1]		$ 335.1
GATX's Percentage Ownership	[1]		50.00%
Adler Funding Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			$ 11.7
GATX's Percentage Ownership			12.50%
Other Affiliates [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			$ 1.7
RRPF Joint Ventures [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Number of joint venture investments | Joint_Venture			15
Singco/Somagas [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Fair Value of Assets Acquired						151.8
Intermodal Investment Fund V and VII [Member]
Investments in and Advances to Affiliates [Line Items]
Equity Method Investment, Net Sales Proceeds				18.3
Aae Cargo Ag [Member]
Investments in and Advances to Affiliates [Line Items]
Portfolio proceeds and other		€ 68.2	$ 77.2		€ 67.5	$ 91.1
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership						37.50%
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges			31.0	(0.1)		$ 0.5
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			335.1	$ 338.7		$ 320.9
Portfolio Management [Member] | Cardinal Marine [Domain]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges			(19.0)
Portfolio Management [Member] | Cardinal Marine [Domain]
Investments in and Advances to Affiliates [Line Items]
Portfolio proceeds and other			$ 24.7

[1] (1) Combined investment balances of fifteen separate joint ventures (collectively, the "RRPF affiliates")