Leases (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Components of GATX's finance leases
Total minimum lease payments receivable, Leveraged Leases	$ 617.0	$ 637.4
Principal and interest on third-party nonrecourse debt, Leveraged Leases	(518.6)	(535.4)
Net minimum future lease receivable, Leveraged Leases	98.4	102.0
Estimated non-guaranteed residual value of leased assets, Leveraged Leases	20.9	20.8
Unearned income, Leveraged Leases	(31.8)	(39.3)
Finance leases, Leveraged Leases	87.5	83.5
Allowance for losses, Leveraged Leases	(9.0)	(9.4)
Deferred taxes, Leveraged Leases	(62.7)	(61.8)
Net investment in finance leases, Leveraged Leases	15.8	12.3
Total minimum lease payments receivable, Direct Financing	341.4	363.7
Net minimum future lease receivable, Direct Financing	341.4	363.7
Estimated non-guaranteed residual value of leased assets, Direct Financing	93.7	93.4
unearned income, direct financing	187.7	192.9
Finance leases, Direct Financing	247.4	264.2
allowance for possible losses, direct financing	0	0
Deferred taxes, Direct Financing	0	0
Net investment in finance leases, Direct Financing	247.4	264.2
Total minimum lease payments receivable, Total Finance Leases	958.4	1,001.1
Principal and interest on third-party nonrecourse debt, Total Finance Leases	(518.6)	(535.4)
Net minimum future lease receivable, Total Finance Leases	439.8	465.7
Estimated non-guaranteed residual value of leased assets, Total Finance Leases	114.6	114.2
Unearned income, Total Finance Leases	(219.5)	(232.2)
Finance leases total financing leases	334.9	347.7
Allowance for losses, Total Finance Leases	(9.0)	(9.4)
Deferred taxes, Total Finance Leases	(62.7)	(61.8)
Net investment in finance leases, Total Finance Leases	$ 263.2	$ 276.5