Investments in Affiliated Companies (Guarantees) (Details 4) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 104.5	[1]	$ 134.0	[1]
Guarantee of Indebtedness of Others [Member]
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 42.0		$ 42.0

[1] At December 31, 2012, the carrying value of liabilities on the balance sheet for commercial commitments was $6.4 million. The expirations of these commitments range from 2013 to 2022. GATX is not aware of any event that would require it to satisfy any of these commitments.