Investment Securities (Net Investment Income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Investment income:
Total investment income	$ 167.0	$ 195.4	$ 217.8
Third-party investment expenses	(13.4)	(10.9)	(8.9)
Net investment income, pre-tax	153.6	184.5	208.9
Fixed maturity investments
Investment income:
Total investment income	132.0	166.2	194.7
Short-term investments
Investment income:
Total investment income	3.1	4.5	3.0
Common shareholders’ equity
Investment income:
Total investment income	22.2	15.4	10.2
Convertible fixed maturity investments
Investment income:
Total investment income	6.0	5.5	5.6
Other long-term investments
Investment income:
Total investment income	3.1	4.2	6.0
Interest on funds held under reinsurance treaties
Investment income:
Total investment income	$ 0.6	$ (0.4)	$ (1.7)