Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2015
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 6.6	$ 9.0	$ 8.6
Equity Method Investment, Net Sales Proceeds				2.3
Portfolio proceeds and other		250.3	234.4	165.6
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 512.6	464.5	441.0
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]
GATX's Percentage Ownership	[1]	50.00%
Adler Funding Llc [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 0.0	0.0	3.0
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership		12.50%
Rail North America [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 0.4	0.6	4.6
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			0.2	6.8
Rail North America [Member] | Adler Funding Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		0.2
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		6.2	4.5	3.7
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			$ 464.3	434.2
Portfolio Management [Member] | Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 512.4
Portfolio Management [Member] | Cardinal Marine [Domain]
Investments in and Advances to Affiliates [Line Items]
Portfolio proceeds and other				$ 1.1

[1] (1) Combined investment balances of a group of 50% owned domestic and foreign joint ventures with Rolls-Royce plc (collectively, the "RRPF affiliates")