Investment Securities (Net Investment Income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Investment income:
Total investment income	$ 130.6	$ 167.0	$ 195.4
Third-party investment expenses	(19.7)	(13.4)	(10.9)
Net investment income, pre-tax	110.9	153.6	184.5
Fixed maturity investments
Investment income:
Total investment income	100.5	132.0	166.2
Short-term investments
Investment income:
Total investment income	3.4	3.1	4.5
Common Stock
Investment income:
Total investment income	20.5	22.2	15.4
Convertible fixed maturity investments
Investment income:
Total investment income	2.9	6.0	5.5
Other long-term investments
Investment income:
Total investment income	3.0	3.1	4.2
Interest on funds held under reinsurance treaties
Investment income:
Total investment income	$ 0.3	$ 0.6	$ (0.4)