Income Taxes (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate	$ 51.6	$ 37.2	$ 41.0
Adjust for effect of:
Foreign earnings taxed at applicable statutory rates	9.8	18.0	10.2
Nondeductible compensation	2.7	2.6	1.9
Share-based compensation	(1.7)	(2.3)	(3.5)
State income taxes	3.7	2.6	1.9
State deferred tax rate change impact	(3.0)	(8.3)	0.0
State net operating loss valuation allowance	(2.3)	9.9	1.0
Other	(2.1)	(2.0)	1.0
Income tax benefit	$ 58.7	$ 54.8	$ 53.2
Effective income tax rate	23.90%	30.90%	27.30%
Canada Revenue Agency [Member]
Adjust for effect of:
Foreign deferred tax rate change impact	$ 0.0	$ 2.9	$ 0.3