Investment Income and Gains and Losses (Schedule of Net Realized and Unrealized Gains (Losses) on Investments Including Impairments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ 5.0	$ 109.4	$ (50.7)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	13.4	106.5	(43.4)
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(4.2)	3.1	(5.8)
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	2.5		(1.3)
Mortgage Loans [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ (6.7)	$ (0.2)	$ (0.2)