Leases (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) railcar	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Receivable, Rolling Year Three	$ 21,900,000
Repayments of Long-term Capital Lease Obligations	$ (11,300,000)	$ (1,200,000)	$ (2,400,000)
Number of railcars received	157		1,224
Lessee, Lease, Number of Railcars Leased | railcar	8,200
Direct Financing Lease, Lease Income	$ 9,800,000
Components of GATX's finance leases
Operating Lease, Lease Income, Lease Payments	1,017,700,000
Operating Lease, Variable Lease Income	65,200,000
Operating Lease, Lease Income	1,082,900,000
Lease Income	$ 1,092,700,000	$ 1,087,800,000	$ 1,098,100,000
Capital Lease Obligations [Member]
Operating Leased Assets [Line Items]
Number of railcars received	157
Assets Held under Capital Leases [Member]
Operating Leased Assets [Line Items]
Repayments of Long-term Capital Lease Obligations	$ (10,500,000)