Fair Value Disclosure - Derivative Instruments (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Interest Expense [Member] | Fair Value Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	[1]	$ 5.8	$ 2.1
Interest Expense [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Effective portion reclassified from accumulated other comprehensive loss		1.1	1.7
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other comprehensive loss (effective portion)		(11.5)	(5.1)
Other Nonoperating Income (Expense) [Member] | Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Effective portion reclassified from accumulated other comprehensive loss		8.2	4.1
Other Expense [Member] | Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		$ (0.7)	$ 4.8

[1] (1) The fair value adjustments related to the underlying debt equally offset the amounts recognized in interest expense.