Income Taxes (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate	$ 41.0	$ 26.3	$ 30.5
Adjust for effect of:
Foreign earnings taxed at lower rates	10.2	9.8	9.8
State income taxes	2.9	1.2	3.8
Other	1.0	1.3	0.8
Effective Income Tax Rate Reconciliation, Nondeductible compensation	1.9	0.6	0.2
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Amount	(3.5)	(1.2)	(1.4)
Income taxes	$ 53.2	$ 37.3	$ 40.9
Effective income tax rate	27.30%	29.80%	28.20%
Deferred Tax Liabilities, Intangible amortization	$ 1.9
Canada Revenue Agency [Member]
Adjust for effect of:
Revaluation of deferred tax liabilities	$ 0.3	$ 0.7	$ 2.8