Fair Value Disclosure (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Fair Value Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	$ (1.2)	[1]	$ (1.5)	[1]	$ (2.3)	[1]	$ (2.6)	[1]
Cash Flow Hedges [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Unrealized gain on derivative instruments (effective portion)	0.6		0.4		0.9		0.7
Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense	1.5		(1.8)		2.0		(1.5)
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 on derivative instruments	(1.3)		(1.1)		(2.5)		(3.0)
Operating Lease 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 on derivative instruments	$ (0.3)		$ (0.3)		$ (0.7)		$ (0.7)

[1] The fair value adjustments related to the underlying debt equally offsets the amounts recognized in interest expense