Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) $ in Millions		12 Months Ended
		Dec. 31, 2018 USD ($) Joint_Venture	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2015
Investments in and Advances to Affiliates [Line Items]
Advances to Affiliate		$ 0.0	$ 6.5
Asset Impairment Charges		9.0	8.6	$ 38.5
Equity Method Investment, Net Sales Proceeds		0.0	2.3	1.0
Portfolio proceeds and other		234.4	165.6	223.7
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		464.5	441.0	387.0
Cardinal Marine [Domain]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership					50.00%
Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies	[1]	$ 464.3
GATX's Percentage Ownership	[1]	50.00%
Adler Funding Llc [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 0.0	3.0	0.0
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 0.2
GATX's Percentage Ownership		12.50%
RRPF Joint Ventures [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Number of joint venture investments | Joint_Venture		17
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 4.5	3.7	6.7
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 464.3	434.2	375.3
Portfolio Management [Member] | Cardinal Marine [Domain]
Investments in and Advances to Affiliates [Line Items]
Portfolio proceeds and other			$ 1.1	$ 1.0

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