Investment Securities (Net Investment Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Investment income
Total investment income	$ 30.6	$ 32.5	$ 93.6	$ 98.2
Less third-party investment expenses	(5.9)	(5.2)	(15.5)	(13.7)
Net investment income, pre-tax	24.7	27.3	78.1	84.5
Fixed maturity investments.
Investment income
Total investment income	24.4	25.1	72.4	76.3
Short-term investments
Investment income
Total investment income	0.8	1.1	2.0	2.9
Common Stock
Investment income
Total investment income	4.3	4.4	15.2	14.0
Convertible fixed maturity investments
Investment income
Total investment income	1.1	0.7	2.3	2.1
Other long-term investments
Investment income
Total investment income	0.1	1.2	1.8	2.7
Funds Held under Reinsurance Agreements [Member]
Investment income
Total investment income	$ (0.1)	$ 0	$ (0.1)	$ 0.2