Fair Value Disclosure (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Interest Expense [Member] | Fair Value Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	$ 3.2	[1]	$ 0.8	[1]	$ 4.8	[1]	$ 3.1	[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	1.3		1.3		3.7		3.8
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)	0.5		0.1		3.7		(0.8)
Operating 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	0.3		0.4		3.0		1.1
Other Expense [Member] | Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense	$ (7.0)		$ 1.4		$ (6.0)		$ (0.6)

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