Fair Value Disclosure (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fair Value Hedging [Member]
Comprehensive income impacts of GATX's derivative instruments
Interest expense	$ (1.5)	$ 1.4	$ (2.6)	$ (0.7)
Cash Flow Hedging [Member]
Comprehensive income impacts of GATX's derivative instruments
Other comprehensive (loss) income (effective portion)	0.4	(6.9)	0.7	(5.9)
Non-designated [Member]
Comprehensive income impacts of GATX's derivative instruments
Other expense	(1.8)	0.2	(1.5)	0.3
Interest Expense [Member] | Cash Flow Hedging [Member]
Comprehensive income impacts of GATX's derivative instruments
Amount reclassified from accumulated unrealized loss on derivative instrument	(1.1)	(2.0)	(3.0)	(3.9)
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	$ (0.3)	$ (0.4)	$ (0.7)	$ (0.8)