Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 1.5	$ 48.9	$ 2.4
Portfolio proceeds and other		272.8	269.6	187.1
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 627.0	575.1	588.4
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		12.90%
Rail North America [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 0.0	0.0	2.4
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies				0.3
Rail North America [Member] | RailPulse
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		0.2	0.8
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		1.2	34.3	0.0
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			574.3	$ 588.1
Portfolio Management [Member] | Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 626.8	$ 574.3

[1] (1) Combined investment balances of a group of 50% owned domestic and foreign joint ventures with Rolls-Royce