Investment Securities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Investment income
Total investment income	$ 195.4	$ 217.8	$ 258.6
Third-party investment expenses	(10.9)	(8.9)	(8.5)
Net investment income	184.5	208.9	250.1
Fixed maturity investments.
Investment income
Total investment income	166.2	194.7	243.3
Short-term investments
Investment income
Total investment income	4.5	3.0	5.5
Common shareholders' equity.
Investment income
Total investment income	15.4	10.2	4.9
Convertible fixed maturities
Investment income
Total investment income	5.5	5.6	8.0
Other long-term investments
Investment income
Total investment income	4.2	6.0	4.2
Interest on funds held under reinsurance treaties
Investment income
Total investment income	$ (0.4)	$ (1.7)	$ (7.3)