Income Taxes (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Summary of reasons for difference between GATX's effective income tax rate and federal statutory income tax
Income taxes at federal statutory rate	$ 55.6	$ 50.4	$ 37.7
gmt_TaxOnGainOfAffiliateSale	23.2	0	0
Adjust for effect of:
Foreign tax credits	(3.9)	(13.7)	(0.1)
Foreign earnings taxed at lower rates	(10.3)	(4.1)	(6.7)
Tax effect of foreign dividends	0	6.3	0
Expiration of the applicable statute of limitations	0.6	(15.5)	0
Settlement of audit issues	0	0	(4.8)
Corporate owned life insurance	(0.5)	(0.3)	(0.2)
State income taxes	1.5	1.9	1.1
Other	(0.1)	1.1	2.2
Income taxes	$ 65.5	$ 26.1	$ 29.2
Effective income tax rate	41.20%	18.20%	27.10%