Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 3.6	$ 0.0	$ 1.5
Portfolio proceeds and other		275.0	230.6	272.8
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 732.3	663.3	627.0
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		$ 3.6	0.0	0.0
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies				0.2
Rail North America [Member] | RailPulse
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		0.0	0.2
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		0.0	0.0	1.2
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 732.3	663.1	$ 626.8
Portfolio Management [Member] | Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			$ 663.1

[1] (1) Combined investment balances of a group of 50.0% owned domestic and foreign joint ventures with Rolls-Royce