Investments (Credit Loss Impairments Recognized) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance, beginning of year	$ 18.1	$ 24.9	$ 32.4
Additions for credit loss impairments recognized in the current period on securities previously impaired	0.0	0.0	0.5
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	(2.6)	(2.4)	(3.6)
Reductions for credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period	0.0	(4.4)	(4.4)
Balance, end of year	$ 15.5	$ 18.1	$ 24.9