Leases Lease cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Finance Lease, Right-of-Use Asset, Amortization	$ 0.2	$ 0.6	$ 0.7
Finance Lease, Interest Expense	0.0	0.2	0.3
Operating Lease, Cost, Variable Leases	40.0	53.3	59.7
Lease, Cost, Excluding Sublease Income	40.2	54.1	60.7
gatxOperatingLeaseOtherIncome	$ 74.8	$ 88.9	$ 88.2