Fair Value Disclosure (Details 2) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	$ 1.4	$ 5.2	$ (0.7)	$ 7.7
Amount recognized in other comprehensive (loss) income (effective portion)	(6.9)	(4.8)	(5.9)	(8.2)
Amount recognized in other expense (ineffective portion)	0	(0.1)	0	(0.1)
Interest Expense [Member] | Cash Flow Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Amount reclassified from accumulated other comprehensive loss	(2.0)	(1.9)	(3.9)	(3.7)
Operating Lease Expense [Member] | Cash Flow Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Amount reclassified from accumulated other comprehensive loss	$ (0.4)	$ (0.4)	$ (0.8)	$ (0.8)