Leases (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating Leased Assets [Line Items]
Capital Leases, Net Investment in Direct Financing Leases, Minimum Payments to be Received	$ 1.5	$ 33.3
2022	1,004.9
Operating Leases
Operating Lease, Liability	286.2	348.6
Lease, Liability	287.7	381.9
gatxOperatingLeaseOtherIncome	74.8	$ 88.9	$ 88.2
Recourse Operating Leases [Member]
Operating Leases
2016, Operating Lease	42.4
2017, Operating Lease	40.1
2018, Operating Lease	38.0
2019, Operating Lease	35.2
2020, Operating Lease	43.9
Years thereafter, Operating Leases	135.0
Total, Operating Leases	334.6
Lessee Operating Leases Future MinimumPayments Interest Included In Payments	(48.4)
Operating Lease, Liability	286.2
Finance Leases
Operating Leased Assets [Line Items]
Capital Leases, Net Investment in Direct Financing Leases, Minimum Payments to be Received	1.5
Total Leases
Operating Leased Assets [Line Items]
Operating and Finance Leases, Future Minimum Payments Due, Next Twelve Months	43.9
Operating Leases
2017, Operating Lease	40.1
2018, Operating Lease	38.0
2019, Operating Lease	35.2
2020, Operating Lease	43.9
Years thereafter, Operating Leases	135.0
Total, Operating Leases	336.1
Lessee Operating Leases Future MinimumPayments Interest Included In Payments	(48.4)
Lease, Liability	$ 287.7