Leases Lease cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Leases [Abstract]
Finance Lease, Right-of-Use Asset, Amortization	$ 0.0	$ 0.2	$ 0.6
Finance Lease, Interest Expense	0.0	0.0	0.2
Lease, Cost, Excluding Sublease Income	41.4	40.2	54.1
gatxOperatingLeaseOtherIncome	81.5	74.8	88.9
Fixed lease cost - operating leases	$ 41.4	$ 40.0	$ 53.3