Leases (Details 1) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
gatxOperatingLeaseOtherIncome	$ 88.2
Direct Financing Lease, Lease Receivable	84.4	$ 119.5
Minimum Future Receipts
2016, Finance Leases	19.7
2017, Finance Leases	13.7
2019, Finance Leases	7.2
2020, Finance Leases	7.7
Years thereafter, Finance Leases	14.2
Total, Finance Leases	84.4
2016, Operating Leases	938.4
2017, Operating Leases	726.4
2018, Operating Leases	532.9
Operating Leases, Future Minimum Payments Receivable, in Four Years	399.3
2020, Operating Leases	261.5
Years thereafter, Operating Leases	289.9
Total future receipts from leases	(3,148.4)
2016, Total	958.1
2017, Total	740.1
Leases Future Minimum Payments Receivable in Three Years	554.8
2019, Total	406.5
2020, Total	269.2
Years thereafter, Total	304.1
Total	3,232.8
Direct Financing Lease, Unguaranteed Residual Asset	40.7	57.5
Direct Financing Lease, Deferred Selling Profit	(34.8)	(50.6)
Direct Financing Lease, Net Investment in Lease	$ 90.3	$ 126.4