Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment income:
Total investment income	$ 66.2	$ 72.4	$ 72.2
Third-party investment expenses	(5.4)	(12.9)	(12.4)
Net investment income, pre-tax	60.8	59.5	59.8
Fixed maturity investments
Investment income:
Total investment income	52.6	51.3	48.1
Short-term investments
Investment income:
Total investment income	0.2	0.1	0.8
Common Stock
Investment income:
Total investment income	10.1	16.6	16.5
Other long-term investments
Investment income:
Total investment income	$ 3.3	$ 4.4	$ 6.8