Financial Data of Business Segments (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 Segment	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Provision for Income Taxes, Equity Method Investment													$ 2.0	$ 8.2	$ 9.6
Number of business segments													4
Profitability
Lease revenue													917.0	900.1	860.4
Marine operating revenue													265.5	230.0	198.4
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
Marine operating expense													182.4	165.6	138.0
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
Total Expenses													843.9	829.5	794.3
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
Share of affiliates' earnings (pre-tax)													21.6	40.6	38.1
Segment profit (loss)													325.6	303.5	232.2
SG&A													160.2	155.3	134.8
Income tax benefit													26.1	29.2	7.0
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
Selected Balance Sheet Data
Investments in affiliated companies	502.0							513.8					502.0	513.8	486.1
Identifiable assets	6,055.4							5,857.5					6,055.4	5,857.5	5,442.4
Capital Expenditures
Portfolio investments and capital additions													770.0	614.6	585.1
Rail North America [Member]
Segment Reporting Information [Line Items]
Leverage level expressed as a ratio of recourse debt to equity													5
Profitability
Lease revenue													713.9	690.9	671.5
Marine operating revenue													0	0	0
Other revenue													51.4	51.5	48.5
Total Revenues													765.3	742.4	720.0
Maintenance expense													201.4	206.1	205.5
Marine operating expense													0	0	0
Depreciation													167.7	162.5	158.6
Operating lease expense													126.5	130.9	139.1
Other operating expense													18.5	18.8	24.2
Total Expenses													514.1	518.3	527.4
Net gain on asset dispositions													58.6	52.1	29.6
Interest expense, net													(101.9)	(101.7)	(103.8)
Other (expense) income													(5.1)	(5.7)	(5.5)
Share of affiliates' earnings (pre-tax)													6.5	3.9	7.2
Segment profit (loss)													209.3	172.7	120.1
Selected Balance Sheet Data
Investments in affiliated companies	46.9							54.0					46.9	54.0	59.2
Identifiable assets	3,601.1							3,540.2					3,601.1	3,540.2	3,226.8
Capital Expenditures
Portfolio investments and capital additions													465.9	280.5	367.1
Rail International [Member]
Segment Reporting Information [Line Items]
Leverage level expressed as a ratio of recourse debt to equity													2
Profitability
Lease revenue													161.2	160.5	141.8
Marine operating revenue													0	0	0
Other revenue													6.5	7.8	5.7
Total Revenues													167.7	168.3	147.5
Maintenance expense													46.6	52.1	49.8
Marine operating expense													0	0	0
Depreciation													36.1	33.6	30.2
Operating lease expense													0	0	0
Other operating expense													5.1	4.2	5.1
Total Expenses													87.8	89.9	85.1
Net gain on asset dispositions													1.7	0	(0.6)
Interest expense, net													(24.5)	(25.4)	(23.3)
Other (expense) income													(6.1)	7.2	(2.0)
Share of affiliates' earnings (pre-tax)													(18.3)	0.5	(6.0)
Segment profit (loss)													32.7	60.7	30.5
Selected Balance Sheet Data
Investments in affiliated companies	77.2							88.2					77.2	88.2	81.8
Identifiable assets	1,105.8							903.2					1,105.8	903.2	1,065.6
Capital Expenditures
Portfolio investments and capital additions													200.1	140.8	107.5
ASC [Member]
Segment Reporting Information [Line Items]
Leverage level expressed as a ratio of recourse debt to equity													1.5
Profitability
Lease revenue													4.3	4.2	4.1
Marine operating revenue													239.1	212.2	185.3
Other revenue													0	0	0
Total Revenues													243.4	216.4	189.4
Maintenance expense													21.7	19.4	12.9
Marine operating expense													160.3	151.7	129.1
Depreciation													11.9	11.3	10.7
Operating lease expense													3.8	0	0
Other operating expense													(0.3)	0	0
Total Expenses													197.4	182.4	152.7
Net gain on asset dispositions													0	1.1	0
Interest expense, net													(7.1)	(7.7)	(8.3)
Other (expense) income													(1.4)	(0.1)	0.2
Share of affiliates' earnings (pre-tax)													0	0	0
Segment profit (loss)													37.5	27.3	28.6
Selected Balance Sheet Data
Investments in affiliated companies	0							0					0	0	0
Identifiable assets	284.2							276.1					284.2	276.1	271.3
Capital Expenditures
Portfolio investments and capital additions													12.6	17.4	9.0
Portfolio Management [Member]
Segment Reporting Information [Line Items]
Leverage level expressed as a ratio of recourse debt to equity													3
Profitability
Lease revenue													37.6	44.5	43.0
Marine operating revenue													26.4	17.8	13.1
Other revenue													2.8	2.0	1.0
Total Revenues													66.8	64.3	57.1
Maintenance expense													0	0	0
Marine operating expense													22.1	13.9	8.9
Depreciation													21.7	19.1	17.5
Operating lease expense													0.2	1.4	1.4
Other operating expense													0.9	4.3	1.3
Total Expenses													44.9	38.7	29.1
Net gain on asset dispositions													19.2	12.6	12.1
Interest expense, net													(27.7)	(29.6)	(28.2)
Other (expense) income													3.4	2.8	(0.1)
Share of affiliates' earnings (pre-tax)													33.4	36.2	36.9
Segment profit (loss)													50.2	47.6	48.7
Selected Balance Sheet Data
Investments in affiliated companies	377.9							371.6					377.9	371.6	345.1
Identifiable assets	797.4							844.0					797.4	844.0	741.0
Capital Expenditures
Portfolio investments and capital additions													83.5	172.0	97.4
Other [Member]
Profitability
Lease revenue													0	0	0
Marine operating revenue													0	0	0
Other revenue													0	0	0
Total Revenues													0	0	0
Maintenance expense													0	0	0
Marine operating expense													0	0	0
Depreciation													0	0	0
Operating lease expense													(0.3)	(0.3)	(0.3)
Other operating expense													0	0.5	0.3
Total Expenses													(0.3)	0.2	0
Net gain on asset dispositions													0	0	0
Interest expense, net													(5.4)	(4.5)	(3.5)
Other (expense) income													1.0	(0.1)	7.8
Share of affiliates' earnings (pre-tax)													0	0	0
Segment profit (loss)													(4.1)	(4.8)	4.3
Selected Balance Sheet Data
Investments in affiliated companies	0							0					0	0	0
Identifiable assets	266.9							294.0					266.9	294.0	137.7
Capital Expenditures
Portfolio investments and capital additions													7.9	3.9	4.1
Parent [Member]
Profitability
Income tax benefit													$ 28.1	$ 37.4	$ 16.6

[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.