Shareholders' Equity (Details) In Millions, unless otherwise specified	Dec. 31, 2013
Common stock reserved for conversion and incentive plans
Common stock reserved for conversion and incentive plans	6.3
Share-based compensation award plans [Member]
Common stock reserved for conversion and incentive plans
Common stock reserved for conversion and incentive plans	2.7
GATX Corporation 2004 Equity Incentive Compensation Plan [Member]
Common stock reserved for conversion and incentive plans
Common stock reserved for conversion and incentive plans	3.5
GATX Corp Service Awards [Member]
Common stock reserved for conversion and incentive plans
Common stock reserved for conversion and incentive plans	0.1