Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 0.0	$ 1.5	$ 48.9
Portfolio proceeds and other		230.6	272.8	269.6
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 663.3	627.0	575.1
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		10.00%
Rail North America [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges			0.0	0.0
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies				0.8
Rail North America [Member] | RailPulse
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 0.2	0.2
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		0.0	1.2	34.3
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		663.1	626.8	$ 574.3
Portfolio Management [Member] | Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 663.1	$ 626.8

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