Income Taxes (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent		30.10%	28.40%
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate	$ 26.3	$ 30.5	$ 35.8
Adjust for effect of:
Foreign tax credits	0.0	0.0	(1.4)
Foreign earnings taxed at lower rates	9.8	9.8	7.8
Corporate owned life insurance	(0.6)	(0.8)	(1.0)
State income taxes	1.2	3.8	4.7
Other	1.3	0.4	1.1
Revaluation of deferred tax liabilities	0.0	0.0	(9.4)
Transition tax on foreign earnings and profits	0.0	0.0	(23.1)
Other	0.0	0.0	(2.8)
Total Tax Act impact	0.0	0.0	(16.5)
Income taxes	$ 37.3	$ 40.9	$ 30.5
Effective income tax rate	29.80%	28.20%	17.90%
Canada Revenue Agency [Member]
Adjust for effect of:
Revaluation of deferred tax liabilities	$ 0.7	$ 2.8	$ 0.0