Leases - Components of Lease Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Operating lease revenue:
Variable lease revenue	$ 281.7	$ 260.9	$ 558.5	$ 521.2
Operating Lease, Variable Lease Income	23.9	21.7	46.4	42.5
Operating Lease, Lease Income	305.6	282.6	604.9	563.7
Finance lease revenue	3.0	2.3	5.7	4.5
Total lease revenue	308.6	284.9	610.6	568.2
Total operating lease revenue	21.8	18.3	45.7	40.9
gatxOperatingLeaseOtherIncome	$ 21.8	$ 18.3	$ 45.7	$ 40.9