Income From Investments - Net Impairment Losses Recognized in Earnings (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Investment Income [Line Items]
Net Impairment Losses Recognized in Earnings	$ (2.2)	$ (1.8)	$ 0.0	$ (0.5)	$ (3.8)	$ (2.9)	$ (2.6)	$ (5.0)	$ (4.5)	$ (14.3)	$ (32.7)
Fixed Maturities [Member]
Net Investment Income [Line Items]
Net Impairment Losses Recognized in Earnings									(2.0)	(12.1)	(26.6)
Equity Securities [Member]
Net Investment Income [Line Items]
Net Impairment Losses Recognized in Earnings									(2.5)	(2.2)	(5.6)
Real Estate [Member]
Net Investment Income [Line Items]
Net Impairment Losses Recognized in Earnings									$ 0.0	$ 0.0	$ (0.5)