Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
GATX's effective tax rate	20.00%	25.00%
Loss related to interest rate swaps at AAE	$ 20.9	$ 20.9
Gain related to interest rate swaps at AAE	11.0	11.0
Swiss statutory tax rate	10.00%
Reduction in unrecognized tax benefits with the close of federal tax audits	15.5
Effective tax rate excluding the effect of interest rate swap	31.00%	31.00%
Gross liability for unrecognized tax benefits	4.7
Decrease in income tax expenses unrecognized tax benefits if recognized	4.7
Decrease in income tax expenses unrecognized tax benefits if recognized net of federal tax	3.2
Unrecognized foreign tax benefits that may be recognized	0.4
Estimated time period for recognition of unrecognized taxes	Within the next 12 months
United Kingdom [Member]
Deferred tax benefit recognized due to reduction in statutory tax rate	4.6	4.1
Canada [Member]
Deferred tax expense recognized due to increase in statutory tax rate	$ 0.7