Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014		Dec. 31, 2013
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property																	$ 99.7	$ 79.3		$ 80.7
State deferred tax rate change impact																	14.1	0.0		0.0
Equity Method Investment, Summarized Financial Information, Income Tax																	15.4	15.2
Special termination benefits, net of tax	$ 5.6
Selected Quarterly Financial Data (unaudited)
Revenues	378.7		$ 386.2		$ 365.3		$ 319.7		$ 401.4		$ 397.2		$ 365.8		$ 286.6		1,449.9	1,451.0		1,321.0
Net income	$ 58.2		$ 39.5		$ 45.4		$ 62.2		$ 58.5		$ 51.3		$ 53.1		$ 42.1		$ 205.3	$ 205.0		$ 169.3
Share Data
Basic earnings per share (in dollars per share)	$ 1.38	[1]	$ 0.92	[1]	$ 1.04	[1]	$ 1.41	[1]	$ 1.32	[1]	$ 1.16	[1]	$ 1.17	[1]	$ 0.92	[1]	$ 4.76	$ 4.55	[1]	$ 3.64
Diluted earnings per share (in dollars per share)	$ 1.37	[1]	$ 0.91	[1]	$ 1.03	[1]	$ 1.39	[1]	$ 1.30	[1]	$ 1.14	[1]	$ 1.15	[1]	$ 0.90	[1]	$ 4.69	$ 4.48	[1]	$ 3.59
Portfolio Management [Member]
Asset Impairment Charges			$ 31.1
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property	$ (9.0)		$ (4.5)
Selected Quarterly Financial Data (unaudited)
Revenues																	$ 92.5	$ 97.4		$ 87.2
Nordic Vessels [Member] | Portfolio Management [Member]
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property																	(21.6)
Foreign Tax Authority [Member]
Equity Method Investment, Summarized Financial Information, Income Tax																	$ 7.7			$ 7.6

[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 2015, net income included $31.1 million of net impairment losses and $4.5 million of net disposition gains, both related to the decision to exit the majority of the marine investments within the Portfolio Management segment.(3)In the fourth quarter of 2015, net income included $9.0 million of net disposition gains related to the decision to exit the majority of the marine investments within the Portfolio Management segment. In addition, we recorded $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.