Income Taxes (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Income Taxes (Textual) [Abstract]
GATX's effective tax rate	30.00%	23.00%
Effective tax rate excluding the effect of interest rate swap	29.00%	27.00%
Gross liability for unrecognized tax benefits	$ 20.8
Decrease in income tax expenses unrecognized tax benefits if recognized	20.8
Decrease in income tax expenses unrecognized tax benefits if recognized net of federal tax	18.8
Unrecognized state tax benefits that may be recognized	15.5
Unrecognized foreign tax benefits that may be recognized	$ 0.4
Estimated time period for recognition of unrecognized taxes	within the next 12 months