Income Taxes (Details 3) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Continuing Operations, Excluding Tax Cuts and Jobs Act		33.70%
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate		$ 75.0	$ 106.9	$ 94.6
Adjust for effect of:
Foreign tax credits	$ (7.1)	0.0	(7.8)	0.0
Foreign earnings taxed at lower rates		(5.5)	(9.7)	(6.2)
Corporate owned life insurance		(0.9)	(1.7)	(0.9)
State income taxes		(0.5)	6.8	7.6
State deferred tax rate change impact		5.0	0.0	14.1
Other		(0.9)	1.2	1.7
Revaluation of deferred tax liabilities	315.9	(371.4)	0.0	0.0
Transition tax on non-U.S. earnings and profits		57.2	0.0	0.0
Other		(1.7)	0.0	0.0
Total Tax Act Impact		315.9	0.0	0.0
Decrease in Income Tax Expenses Unrecognized Tax Benefits if Recognized Net of Federal Tax	$ (315.9)	(315.9)
Income taxes		$ (243.7)	$ 95.7	$ 110.9
Effective income tax rate		(113.70%)	31.30%	41.00%