Investments in Affiliated Companies (Operating Results, Affiliates) (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Schedule of Equity Method Investments [Line Items]
Revenues	$ 1,740.4	$ 1,585.5	$ 1,410.9
Net income attributable to GATX	333.3	284.2	259.2
Asset Impairment Charges	3.6	0.0	1.5
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Revenues	643.4	514.0	487.2
Gains on sales of assets	134.6	75.2	91.7
Net income attributable to GATX	$ 238.4	$ 164.0	$ 150.6