Earnings Per Share (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
Numerator:
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
Numerator for basic earnings per share — income available to common shareholders																	137.3		110.8		80.8
Effect of dilutive securities:
After-tax interest expense on convertible securities																	0		0		0.2
Numerator for diluted earnings per share — income available to common shareholders																	$ 137.3		$ 110.8		$ 81.0
Denominator:
Denominator for basic earnings per share — weighted average shares																	46.8		46.4		46.1
Effect of dilutive securities:
Equity compensation plans																	0.7		0.7		0.5
Convertible preferred stock																	0.1		0.1		0.1
Convertible securities																	0		0		0.3
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion																	47.6		47.2		47.0
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

[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] _