Investment Securities (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Investment income
Total investment income	$ 47.9	$ 55.4	$ 99.7	$ 113.1
Third-party investment expenses	(2.1)	(2.7)	(4.4)	(4.6)
Net investment income	45.8	52.7	95.3	108.5
Fixed maturity investments
Investment income
Total investment income	42.6	50.2	88.5	103.1
Short-term investments
Investment income
Total investment income	1.6	2.3	2.3	2.8
Common shareholders' equity
Investment income
Total investment income	3.0	2.2	6.4	4.3
Convertible fixed maturity investments
Investment income
Total investment income	1.1	1.9	2.6	4.0
Other long-term investments.
Investment income
Total investment income	(0.2)	(0.4)	0.7	0.5
Interest on funds held under reinsurance treaties
Investment income
Total investment income	$ (0.2)	$ (0.8)	$ (0.8)	$ (1.6)