Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Investment income
Total investment income	$ 17.1	$ 17.3	$ 47.2	$ 54.2
External investment expenses	(0.3)	(3.5)	(4.6)	(9.5)
Net investment income, pre-tax	16.8	13.8	42.6	44.7
Fixed maturity investments.
Investment income
Total investment income	13.7	12.7	38.5	38.4
Short-term investments
Investment income
Total investment income	0.0	0.1	0.1	0.2
Common Stock
Investment income
Total investment income	1.4	3.3	5.7	11.9
Convertible fixed maturity investments
Investment income
Total investment income	0.1	1.0	0.4	1.5
Other long-term investments
Investment income
Total investment income	$ 1.9	$ 0.2	$ 2.5	$ 2.2