Income Taxes (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate	$ 37.2	$ 41.0	$ 26.3
Adjust for effect of:
Foreign earnings taxed at lower rates	18.0	10.2	9.8
State income taxes	2.6	1.9	0.2
State deferred tax rate change impact	(8.3)	0.0	0.0
Other	(2.0)	1.0	1.3
Effective Income Tax Rate Reconciliation, Nondeductible compensation	2.6	1.9	0.6
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Amount	(2.3)	(3.5)	(1.2)
State net operating loss valuation allowance	9.9	1.0	1.0
Income tax benefit	$ 54.8	$ 53.2	$ 37.3
Effective income tax rate	30.90%	27.30%	29.80%
Deferred Tax Liabilities, Intangible amortization	$ 1.8	$ 1.9
Canada Revenue Agency [Member]
Adjust for effect of:
Revaluation of deferred tax liabilities	$ 2.9	$ 0.3	$ 0.7