Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Investment income
Total investment income	$ 28.4	$ 34.4	$ 55.5	$ 63.9
Third-party investment expenses	(4.6)	(4.8)	(8.4)	(9.6)
Net investment income, pre-tax	23.8	29.6	47.1	54.3
Fixed maturity investments.
Investment income
Total investment income	23.4	25.6	46.5	48.0
Short-term investments
Investment income
Total investment income	0.7	0.6	1.3	1.2
Common Stock
Investment income
Total investment income	3.3	5.5	6.7	10.9
Convertible fixed maturity investments
Investment income
Total investment income	0.1	0.8	0.3	1.2
Other long-term investments
Investment income
Total investment income	0.9	1.9	0.8	2.6
Funds Held under Reinsurance Agreements [Member]
Investment income
Total investment income	$ 0.0	$ 0.0	$ (0.1)	$ 0.0