Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Numerator:
Net income	$ 58.2		$ 39.5		$ 45.4		$ 62.2		$ 58.5		$ 51.3		$ 53.1		$ 42.1		$ 205.3	$ 205.0		$ 169.3
Denominator:
Denominator for basic earnings per share — weighted average shares																	43.1	45.0		46.4
Effect of dilutive securities:
Equity compensation plans																	0.7	0.8		0.7
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion																	43.8	45.8		47.1
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

[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.