Investment Securities (Net Investment Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Investment income
Total investment income	$ 41.0	$ 45.7	$ 129.3	$ 145.5
Third-party investment expenses	(3.4)	(2.9)	(9.5)	(7.4)
Net investment income	37.6	42.8	119.8	138.1
Fixed maturity investments.
Investment income
Total investment income	33.0	38.5	105.1	127.0
Short-term investments
Investment income
Total investment income	0.5	0.9	2.3	3.2
Common shareholders' equity.
Investment income
Total investment income	4.8	3.6	13.8	10.1
Convertible fixed maturity investments
Investment income
Total investment income	2.0	1.3	5.9	3.9
Other long-term investments
Investment income
Total investment income	0.7	1.2	2.2	1.9
Interest on funds held under reinsurance treaties
Investment income
Total investment income	$ 0	$ 0.2	$ 0	$ (0.6)