Leases (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Operating Leased Assets [Line Items]
Capital Leases, Net Investment in Direct Financing Leases, Minimum Payments to be Received	$ 0.0	$ 1.5
2023	1,010.5
Operating Leases
Operating Lease, Liability	257.9	286.2
Lease, Liability	257.9	287.7
gatxOperatingLeaseOtherIncome	81.5	$ 74.8	$ 88.9
Recourse Operating Leases [Member]
Operating Leases
2016, Operating Lease	41.0
2017, Operating Lease	38.9
2018, Operating Lease	36.2
2019, Operating Lease	44.8
2020, Operating Lease	38.7
Years thereafter, Operating Leases	98.4
Total, Operating Leases	298.0
Lessee Operating Leases Future MinimumPayments Interest Included In Payments	(40.1)
Operating Lease, Liability	$ 257.9