Investments in Affiliated Companies (Significant Investments in Affiliates) (Details 1) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 0.3	$ 6.6	$ 9.0
Portfolio proceeds and other		131.1	250.3	234.4
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 584.7	512.6	464.5
Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership	[1]	50.00%
Adler Funding Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
GATX's Percentage Ownership		12.50%
Rail North America [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		$ 0.3	0.4	0.6
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			0.2	0.2
Rail North America [Member] | Adler Funding Llc [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		0.0	0.2
Portfolio Management [Member]
Investments in and Advances to Affiliates [Line Items]
Asset Impairment Charges		0.0	6.2	4.5
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies			512.4	$ 464.3
Portfolio Management [Member] | Rolls Royce Partners Finance [Member]
Investments in and Advances to Affiliates, Schedule of Investments [Abstract]
Investments in affiliated companies		$ 584.7	$ 512.4

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