Income Taxes (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate	$ 80.9	$ 55.6	$ 50.4
Adjust for effect of:
GATX income taxes on sale of AAE	0	23.2	0
Foreign tax credits	0	(3.9)	(13.7)
Foreign earnings taxed at lower rates	(8.5)	(10.3)	(4.1)
Tax effect of foreign dividends	0	0	6.3
Expiration of the applicable statute of limitations	0	0	(15.5)
Corporate owned life insurance	(0.6)	(0.5)	(0.3)
State income taxes	4.1	1.5	1.9
Other	(0.2)	(0.1)	1.1
Income taxes	$ 75.7	$ 65.5	$ 26.1
Effective income tax rate	32.70%	41.20%	18.20%