Fair Value Disclosure (Details 2) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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]	$ (1.2)	$ (1.1)	$ 0.9	$ (5.9)
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	3.4	3.4
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)		(18.6)	(0.8)	(23.7)	(24.0)
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	0.1	0.0	0.4
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		19.6	(6.7)	23.7	10.3
Other Expense [Member] | Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense		$ 1.3	$ (3.6)	$ 6.1	$ (2.3)

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