Investment Securities (Net Investment Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Investment income
Total investment income	$ 32.7	$ 43.5	$ 65.7	$ 88.3
Third-party investment expenses	(4.0)	(3.1)	(8.5)	(6.1)
Net investment income	28.7	40.4	57.2	82.2
Fixed maturity investments.
Investment income
Total investment income	25.2	35.4	51.2	72.1
Short-term investments
Investment income
Total investment income	1.1	0.8	1.8	1.8
Common Stock
Investment income
Total investment income	4.9	4.6	9.6	9.0
Convertible fixed maturity investments
Investment income
Total investment income	0.6	2.0	1.4	3.9
Other long-term investments
Investment income
Total investment income	0.8	0.7	1.5	1.5
Interest on funds held under reinsurance treaties
Investment income
Total investment income	$ 0.1	$ 0	$ 0.2	$ 0