Investments in Affiliated Companies (Operating Results, Affiliates) (Details 4) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating results for all affiliated companies of GATX
Revenues	$ 469.8	$ 436.9	$ 350.7
Gains on sales of assets	86.4	12.2	27.4
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 161.4	$ 120.5	$ 100.4

The amount of net income (loss) reported by an equity method investment of the entity.

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