Pension and Other Post-Retirement Benefits (Details 10) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Multiemployer Plans
Contributions	$ 8.1		$ 8.5		$ 8.4
American Maritime Officers Pension Plan [Member]
Multiemployer Plans
Zone Status	Yellow	[1]
Contributions	1.5	[2]	2.1	[2]	2.2	[2]
Collective Bargaining	Jan. 15, 2017	[1]
Other Multiemployer Post Retirement Plans [Member]
Multiemployer Plans
Contributions	$ 6.6		$ 6.4		$ 6.2

[1] The American Maritime Officers Pension Plan was determined to be in endangered status (i.e. “yellow zone” as defined by the Pension Protection Act of 2006) for the plan year beginning October 1, 2012, because it has funding or liquidity problems, or both. A rehabilitation plan, as defined by the Employee Retirement Security Act of 1974, was instituted under which certain adjustable benefits were reduced or eliminated and GATX is required to contribute at a negotiated rate per day worked by each employee.

[2] GATX's contributions represented more than 5% of the total contributions to the plan during each year and no surcharge was imposed for any year.