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, 2023	Dec. 31, 2022	Dec. 31, 2021
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ (32.5)	$ (106.5)	$ 123.0
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(20.2)	(45.2)	1.7
Mortgage Loans on Real Estate [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(6.9)	2.3	0.7
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(5.6)	(63.3)	119.1
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ 0.2	$ (0.3)	$ 1.5