Financial Data of Business Segments - Segment Data (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Revenues	$ 343.4	$ 337.9	$ 970.9	$ 917.5
Share of affiliates' earnings	19.1	1.8	23.3	33.9
Total gross income	362.5	339.7	994.2	951.4
Ownership costs	136.3	130.3	399.6	393.8
Other costs and expenses	136.0	127.2	345.1	340.1
Segment profit (loss)	90.2	82.2	249.5	217.5
SG&A	38.6	38.5	115.6	112.3
Income before income taxes	51.6	43.7	133.9	105.2
Portfolio investments and capital additions	131.6	200.0	523.4	460.1
Investments in Affiliated Companies	532.3		532.3		513.8
Identifiable assets	6,125.1		6,125.1		5,857.5
Rail [Member]
Segment Reporting Information [Line Items]
Revenues	245.2	242.6	736.0	717.1
Share of affiliates' earnings	4.3	(2.0)	(12.2)	13.4
Total gross income	249.5	240.6	723.8	730.5
Ownership costs	114.9	111.8	342.9	341.3
Other costs and expenses	71.1	65.8	209.2	217.9
Segment profit (loss)	63.5	63.0	171.7	171.3
Portfolio investments and capital additions	123.3	133.8	443.9	290.1
Investments in Affiliated Companies	126.2		126.2		163.0
Identifiable assets	4,553.4		4,553.4		4,448.6
ASC [Member]
Segment Reporting Information [Line Items]
Revenues	80.2	71.3	169.2	142.2
Total gross income	80.2	71.3	169.2	142.2
Ownership costs	7.1	5.6	15.9	13.5
Other costs and expenses	59.9	57.2	124.0	110.8
Segment profit (loss)	13.2	8.5	29.3	17.9
Portfolio investments and capital additions	0.3	3.3	11.7	15.9
Identifiable assets	290.6		290.6		286.3
Portfolio Management [Member]
Segment Reporting Information [Line Items]
Revenues	17.8	23.7	65.0	57.2
Share of affiliates' earnings	14.8	3.8	35.5	20.5
Total gross income	32.6	27.5	100.5	77.7
Ownership costs	12.5	12.1	37.0	36.1
Other costs and expenses	5.1	3.9	11.9	10.6
Segment profit (loss)	15.0	11.5	51.6	31.0
Portfolio investments and capital additions	6.3	62.1	62.8	151.4
Investments in Affiliated Companies	406.1		406.1		384.5
Identifiable assets	815.1		815.1		862.0
Other [Member]
Segment Reporting Information [Line Items]
Revenues	0.2	0.3	0.7	1.0
Total gross income	0.2	0.3	0.7	1.0
Ownership costs	1.8	0.8	3.8	2.9
Other costs and expenses	(0.1)	0.3		0.8
Segment profit (loss)	(1.5)	(0.8)	(3.1)	(2.7)
Portfolio investments and capital additions	1.7	0.8	5.0	2.7
Identifiable assets	$ 466.0		$ 466.0		$ 159.6