Income Taxes - Tax Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate	$ 58.6	$ 54.9	$ 51.6
Other	3.7	4.2	4.0
Foreign tax effects	(3.0)	9.3	9.6
Nondeductible officers compensation	4.3	2.8	2.3
Share-based compensation	(3.2)	(2.2)	(1.7)
Other	0.7	0.0	(1.7)
State and local income tax, net of federal (national) income tax effect (1)	6.0	(4.8)	(1.6)
Changes in unrecognized tax benefits	(0.3)	0.0	0.2
Income tax benefit	$ 63.1	$ 60.0	$ 58.7
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income taxes at federal statutory rate	21.00%	21.00%	21.00%
Other	1.30%	1.60%	1.60%
Foreign tax effects	(1.10%)	3.60%	3.90%
Nondeductible officers compensation	1.50%	1.10%	0.90%
Share-based compensation	(1.10%)	(0.80%)	(0.70%)
Other	0.20%	0.00%	(0.70%)
State and local income tax, net of federal (national) income tax effect	2.20%	(1.90%)	(0.60%)
Changes in unrecognized tax benefits	(0.10%)	0.00%	0.10%
Income taxes at effective income tax rate	22.60%	23.00%	23.90%
GERMANY
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Foreign tax rate change impact	$ (13.3)	$ 0.0	$ 0.0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Foreign tax rate change impact	(4.80%)	0.00%	0.00%
Other foreign jurisdictions
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Foreign tax rate change impact	$ 6.6	$ 5.1	$ 5.6
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Foreign tax rate change impact	2.40%	2.00%	2.30%