Leases - Components of Lease Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Operating lease revenue:
Variable lease revenue	$ 289.7	$ 267.5	$ 848.2	$ 788.7
Operating Lease, Variable Lease Income	24.2	22.6	70.6	65.1
Operating Lease, Lease Income	313.9	290.1	918.8	853.8
Finance lease revenue	3.3	2.3	9.0	6.8
Total lease revenue	317.2	292.4	927.8	860.6
Total operating lease revenue	23.1	20.3	68.8	61.2
gatxOperatingLeaseOtherIncome	$ 23.1	$ 20.3	$ 68.8	$ 61.2