Selected Quarterly Financial Data (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Selected Quarterly Financial Data (unaudited)
Revenues	$ 331.7		$ 331.9		$ 323.1		$ 256.5		$ 330.0		$ 315.2		$ 296.8		$ 249.4		$ 1,243.2		$ 1,191.4		$ 1,114.0
Net income	29.7		53.8	[1]	23.5	[2]	30.3	[3]	31.6		32.9		26.4		19.9	[3]	137.3		110.8		80.8
Share Data
Basic earnings per share (in dollars per share)	$ 0.63	[4]	$ 1.15	[4]	$ 0.50	[4]	$ 0.65	[3],[4]	$ 0.68	[4]	$ 0.71	[4]	$ 0.57	[4]	$ 0.43	[3],[4]	$ 2.93	[4]	$ 2.39	[4]	$ 1.75
Diluted earnings per share (in dollars per share)	$ 0.62	[4]	$ 1.13	[4]	$ 0.49	[4]	$ 0.64	[3],[4]	$ 0.67	[4]	$ 0.70	[4]	$ 0.56	[4]	$ 0.42	[3],[4]	$ 2.88	[4]	$ 2.35	[4]	$ 1.72
Share of affiliates' earnings (net of tax)																	19.6		32.4		28.5
Expiration of the applicable statute of limitations																	15.5				0
Cash Flow Hedges [Member] | AAE Cargo AG [Member]
Share Data
Share of affiliates' earnings (net of tax)					$ 12.1

[1] The increase in income from the second quarter of 2012 was primarily due to the absence of the hedge loss at AAE and the recognition of $15.5 million of tax benefits.

[2] The decrease in income from the first quarter of 2012 was primarily due to a $12.1 million loss on the termination of a qualified hedge at GATX's AAE Cargo affiliate, partially offset by an increase in seasonal income at ASC.

[3] Total revenues and net income in the first quarter of each year are typically lower than the subsequent quarters due to seasonal inactivity at ASC.

[4] _