Condensed Financial Information of Registrant - Statements of Comprehensive Income (Details) (USD $) In Millions, 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
Condensed Financial Statements, Captions [Line Items]
Lease revenue													$ 917.0	$ 900.1	$ 860.4
Other revenue													60.7	61.3	55.2
Total 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
Maintenance expense													269.7	277.6	268.2
Depreciation													237.4	226.5	217.0
Operating lease expense													130.2	132.0	140.2
Other operating expense													24.2	27.8	30.9
Selling, general and administrative													160.2	155.3	134.8
Total Expenses													1,004.1	984.8	929.1
Net gain on asset dispositions													79.5	65.8	41.1
Interest expense, net													(166.6)	(168.9)	(167.1)
Other (expense) income													(8.2)	4.1	0.4
Income before Income Taxes and Share of Affiliates’ Earnings													143.8	107.6	59.3
Income tax benefit													(26.1)	(29.2)	(7.0)
Share of affiliates' earnings (net of tax)													19.6	32.4	28.5
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
Foreign currency translation adjustments													25.0	(39.6)	(28.4)
Unrealized gain (loss) on securities													0.2	(0.2)	1.3
Unrealized gain (loss) on derivative instruments													11.7	(0.5)	(3.5)
Post-retirement benefit plans													(12.4)	(18.8)	5.1
Other comprehensive income (loss)													24.5	(59.1)	(25.5)
Comprehensive Income													161.8	51.7	55.3
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Lease revenue													504.5	477.7	458.9
Other revenue													43.5	44.4	41.0
Total Revenues													548.0	522.1	499.9
Maintenance expense													154.8	163.9	168.7
Depreciation													122.2	117.0	110.2
Operating lease expense													91.6	95.2	101.3
Other operating expense													12.0	11.5	20.2
Selling, general and administrative													118.4	113.3	97.2
Total Expenses													499.0	500.9	497.6
Net gain on asset dispositions													59.4	44.0	27.0
Interest expense, net													(65.4)	(72.8)	(74.0)
Other (expense) income													(4.4)	(2.4)	3.7
Income before Income Taxes and Share of Affiliates’ Earnings													38.6	(10.0)	(41.0)
Income tax benefit													1.0	4.8	20.8
Share of affiliates' earnings (net of tax)													97.7	116.0	101.0
Net Income													137.3	110.8	80.8
Foreign currency translation adjustments													25.0	(39.6)	(28.4)
Unrealized gain (loss) on securities													0.2	(0.2)	1.3
Unrealized gain (loss) on derivative instruments													11.7	(0.5)	(3.5)
Post-retirement benefit plans													(12.4)	(18.8)	5.1
Other comprehensive income (loss)													24.5	(59.1)	(25.5)
Comprehensive Income													$ 161.8	$ 51.7	$ 55.3

[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.