Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 2.4	$ 0.3	$ 6.6
Portfolio proceeds and other		187.1	131.1	250.3
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 588.4	584.7	512.6
Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership	[1]	50.00%
RailPulse
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership		17.00%
Rail North America [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 2.4	0.3	0.4
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			0.0	0.2
Rail North America [Member] | Adler Funding Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		0.3
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		0.0	0.0	6.2
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			584.7	$ 512.4
Portfolio Management [Member] | Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 588.1	$ 584.7

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