Income Taxes (Details 3) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate			$ 106.9	$ 94.6	$ 80.9
Adjust for effect of:
Foreign tax credits	$ (7.1)		(7.8)	0.0	0.0
Foreign earnings taxed at lower rates			(9.7)	(6.2)	(8.5)
Corporate owned life insurance			(1.7)	(0.9)	(0.6)
State income taxes			6.8	7.6	4.1
State deferred tax rate change impact		$ 14.1	0.0	14.1	0.0
Other			1.2	1.7	(0.2)
Income taxes			$ 95.7	$ 110.9	$ 75.7
Effective income tax rate			31.30%	41.00%	32.70%