Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment income:
Total investment income	$ 89.9	$ 66.2	$ 72.4
Third-party investment expenses	(3.1)	(5.4)	(12.9)
Net investment income, pre-tax	86.8	60.8	59.5
Fixed maturity investments
Investment income:
Total investment income	77.3	52.6	51.3
Short-term investments
Investment income:
Total investment income	1.1	0.2	0.1
Common Stock
Investment income:
Total investment income	7.5	10.1	16.6
Other long-term investments
Investment income:
Total investment income	$ 4.0	$ 3.3	$ 4.4