Leases - Components of Lease Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Operating lease revenue:
Variable lease revenue	$ 320.8	$ 289.7	$ 937.0	$ 848.2
Operating Lease, Variable Lease Income	27.6	24.2	77.3	70.6
Operating Lease, Lease Income	348.4	313.9	1,014.3	918.8
Finance lease revenue	3.3	3.3	10.3	9.0
Total lease revenue	351.7	317.2	1,024.6	927.8
Total operating lease revenue	29.7	23.1	86.5	68.8
gatxOperatingLeaseOtherIncome	$ 29.7	$ 23.1	$ 86.5	$ 68.8