Investments - Net Realized Gains on Sales of Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fixed Maturities [Member]
Net Investment Income [Line Items]
Realized Gains	$ 2.4	$ 2.7	$ 7.3	$ 14.1
Realized Losses	0.0	(0.8)	(0.4)	(1.1)
Equity Securities [Member]
Net Investment Income [Line Items]
Realized Gains	3.9	9.3	31.1	9.8
Realized Losses	$ 0.0	$ 0.0	$ 0.0	$ (0.1)