Fair Value Disclosure (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair Value Hedging [Member]
Comprehensive income impacts of GATX's derivative instruments
Interest expense	$ (1.1)	$ (2.1)
Cash Flow Hedging [Member]
Comprehensive income impacts of GATX's derivative instruments
Other comprehensive (loss) income (effective portion)	0.3	1.0
Non-designated [Member]
Comprehensive income impacts of GATX's derivative instruments
Other expense	0.3	0.1
Interest Expense [Member] | Cash Flow Hedging [Member]
Comprehensive income impacts of GATX's derivative instruments
Amount reclassified from accumulated unrealized loss on derivative instrument	(1.9)	(1.9)
Operating Lease Expense [Member] | Cash Flow Hedging [Member]
Comprehensive income impacts of GATX's derivative instruments
Amount reclassified from accumulated unrealized loss on derivative instrument	$ (0.4)	$ (0.4)