Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Nonoperating Income, Residual Sharing Income																	$ 83.6	$ 13.4		$ 9.4
Effective Income Tax Rate Reconciliation, Tax Credit, Foreign, Amount	$ 7.1																7.8	0.0		0.0
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property																	52.9	99.7		79.3
State deferred tax rate change impact									$ 14.1								0.0	14.1		0.0
Equity Method Investment, Summarized Financial Information, Income Tax																	7.8	15.4
Special termination benefits, net of tax									5.6
Selected Quarterly Financial Data (unaudited)
Revenues	362.1		$ 362.9		$ 358.9		$ 334.4		378.7		$ 386.2		$ 365.3		$ 319.7		1,418.3	1,449.9		1,451.0
Net income	$ 30.9		$ 95.7		$ 61.2		$ 69.3		$ 58.2		$ 39.5		$ 45.4		$ 62.2		$ 257.1	$ 205.3		$ 205.0
Share Data
Basic earnings per share (in dollars per share)	$ 0.78	[1]	$ 2.39	[1]	$ 1.51	[1]	$ 1.67	[1]	$ 1.38	[1]	$ 0.92	[1]	$ 1.04	[1]	$ 1.41	[1]	$ 6.35	$ 4.76	[1]	$ 4.55
Diluted earnings per share (in dollars per share)	$ 0.77	[1]	$ 2.36	[1]	$ 1.49	[1]	$ 1.66	[1]	$ 1.37	[1]	$ 0.91	[1]	$ 1.03	[1]	$ 1.39	[1]	$ 6.29	$ 4.69	[1]	$ 4.48
Portfolio Management [Member]
Nonoperating Income, Residual Sharing Income			$ 30.3														$ 82.8	$ 12.6		$ 4.2
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property											$ (26.6)
Selected Quarterly Financial Data (unaudited)
Revenues																	56.6	92.5		97.4
Rail North America [Member]
Nonoperating Income, Residual Sharing Income																	0.8	0.8		5.2
Asset Impairment Charges	$ 19.2
Selected Quarterly Financial Data (unaudited)
Revenues																	1,018.5	1,006.8		$ 927.5
Marine vessels [Member] | Portfolio Management [Member]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property									$ (9.0)
Foreign Tax Authority [Member]
Equity Method Investment, Summarized Financial Information, Income Tax			$ 3.9						$ 7.7								$ 3.9	$ 7.7

[1] (1)Quarterly earnings per share may not be additive, as per share amounts are computed independently for each quarter and the full year is based on the respective weighted average common shares and common stock equivalents outstanding.(2)In the third quarter of 2016, net income included $30.3 million of income related to the settlement of a residual sharing agreement and $3.9 million of benefit resulting from a reduction in the statutory income tax rate in the UK. In the third quarter of 2015, net income included $26.6 million of net loss related to the exit of marine investments at Portfolio Management.(3)In the fourth quarter of 2016, net income included $19.2 million of impairment losses related to certain railcars at Rail North America and $7.1 million of benefit related to the utilization of foreign tax credits. In the fourth quarter of 2015, net income included $9.0 million of net gain related to the exit of marine investments at Portfolio Management. In addition, net income included $14.1 million of expense attributable to an increase in our effective state income tax rate, $7.7 million of benefit resulting from a reduction in the statutory income tax rate in the UK, and $5.6 million of net expenses associated with an early retirement program offered to certain employees.