Investments - Schedule of Other Than Temporary Impairment (Details) - Fixed Maturities [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Cumulative Balance of Pre-tax Credit Losses Recognized in Retained Earnings at Beginning of Period	$ 1.6	$ 1.6	$ 1.6	$ 1.4
Pre-tax Credit Losses on Fixed Maturities without Pre-tax Credit Losses Included in Cumulative Balance at Beginning of Period	0.0	0.8	0.0	1.2
From Status of Credit Loss to Status of Intent-to-sell or Required-to-sell	0.0	0.0	0.0	0.0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold	0.0	0.0	0.0	(0.2)
Cumulative Balance of Pre-tax Credit Losses Recognized in Retained Earnings at End of Period	$ 1.6	$ 2.4	$ 1.6	$ 2.4