Fair Value Disclosure (Details 2) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Interest Expense [Member] | Fair Value Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	[1]	$ 0.6	$ 2.6	$ 1.5	$ (3.3)
Interest Expense [Member] | Cash Flow Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Effective portion reclassified from accumulated other comprehensive loss		1.7	1.7	5.1	5.1
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other comprehensive loss (effective portion)		(11.1)	(2.8)	(34.8)	(26.8)
Operating Expense [Member] | Cash Flow Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Effective portion reclassified from accumulated other comprehensive loss		0.0	0.7	0.0	1.1
Other Nonoperating Income (Expense) [Member] | Cash Flow Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Effective portion reclassified from accumulated other comprehensive loss		10.1	3.3	33.8	13.6
Other Expense [Member] | Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		$ (2.0)	$ 2.2	$ 4.1	$ (0.1)

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