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, 2022	Dec. 31, 2021	Dec. 31, 2020
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ (106.5)	$ 123.0	$ 5.0
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(63.3)	119.1	13.4
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(45.2)	1.7	(4.2)
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(0.3)	1.5	2.5
Mortgage Loans on Real Estate [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ 2.3	$ 0.7	$ (6.7)