Investments in Affiliated Companies (Operating Results, Affiliates) (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity Method Investments and Joint Ventures [Abstract]
Gain on Sale of Investments			$ 2.5
Operating results for all affiliated companies of GATX
Revenues	$ 339.3	$ 339.0	334.5
Gains on sales of assets	37.6	33.7	43.5
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 121.4	$ 99.6	$ 108.6