Fair Value Disclosure (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other comprehensive (loss) income (effective portion)	$ (5.9)	$ (8.1)	$ 15.8
Fair Value Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Interest expense	(2.3)	7.4	(9.3)
Cash Flow Hedging [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense	0	(0.1)	(0.4)
Non-designated [Member]
Impact of GATX's Derivative Instrument on Income Statement and Other comprehensive income (loss)
Other expense	(1.8)	(0.4)	(0.2)
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	(2.5)	(7.7)	(6.2)
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	$ (1.5)	$ (1.5)	$ (1.4)