Income From Investments - Net Impairment Losses Recognized in Earnings (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Net Impairment Losses Recognized in Earnings	$ (3.8)	$ (2.9)	$ (2.6)	$ (5.0)	$ (8.7)	$ (8.3)	$ (6.4)	$ (9.3)	$ (14.3)	$ (32.7)	$ (27.2)
Fixed Maturities [Member]
Net Investment Income [Line Items]
Net Impairment Losses Recognized in Earnings									(12.1)	(26.6)	(11.5)
Equity Securities [Member]
Net Investment Income [Line Items]
Net Impairment Losses Recognized in Earnings									(2.2)	(5.6)	(15.7)
Real Estate [Member]
Net Investment Income [Line Items]
Net Impairment Losses Recognized in Earnings									$ 0.0	$ (0.5)	$ 0.0