Investments in Affiliated Companies (Operating Results, Affiliates) (Details 5) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating results for all affiliated companies of GATX
Revenues	$ 651.8	$ 645.5	$ 636.2
Gains on sales of assets	57.4	21.1	10.9
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 75.3	$ 88.5	$ 55.8