Investments in Affiliated Companies (Operating Results, Affiliates) (Details 4) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating results for all affiliated companies of GATX
Revenues	$ 350.7	$ 333.7	$ 339.3
Gains on sales of assets	27.4	23.6	37.6
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 100.4	$ 99.3	$ 121.4