Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 48.9	$ 2.4	$ 0.3
Portfolio proceeds and other		269.6	187.1	131.1
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 575.1	588.4	584.7
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		13.80%
Rail North America [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 0.0	2.4	0.3
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies				0.0
Rail North America [Member] | RailPulse
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		0.8	0.3
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		34.3	0.0	0.0
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			588.1	$ 584.7
Portfolio Management [Member] | Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 574.3	$ 588.1

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