Fair Value Disclosure (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 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	$ (0.5)	[1]	$ 1.2	[1]	$ 1.6	[1]	$ 2.3	[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	1.2		1.3		2.4		2.5
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)	1.4		(0.6)		3.2		(0.9)
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		0.3		2.7		0.7
Other Expense [Member] | Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense	$ 0.9		$ (1.5)		$ 1.0		$ (2.0)

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