Income Taxes (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
Income Taxes (Textual) [Abstract]
GATX's effective tax rate	25.00%	23.00%
Reduction in unrecognized tax benefits with the close of various federal and foreign tax audits		$ 3.7
Deferred tax benefit recognized due to reduction in statutory tax rates in the United Kingdom	4.1	1.9
Effective tax rate excluding the effect of the tax benefits	29.00%	30.00%
Gross liability for unrecognized tax benefits	43.7
Decrease in income tax expenses unrecognized tax benefits if recognized	23.3
Decrease in income tax expenses unrecognized tax benefits if recognized net of federal tax	$ 21.2