Investments (Unrealized Gains and Losses on Available-For-Sale and Other Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Benefit (provision) for deferred income taxes	$ (49.0)	$ 217.9	$ 58.9
Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year	(641.4)	184.9	428.1
Net appreciation (depreciation) on available-for-sale fixed maturities	227.6	(1,044.2)	(302.1)
Benefit (provision) for deferred income taxes	(49.0)	217.9	58.9
Total adjustment	178.6	(826.3)	(243.2)
Net appreciation (depreciation), end of year	$ (462.8)	$ (641.4)	$ 184.9