Investment Income and Gains and Losses (Schedule of Pre-tax Net Realized and Unrealized Gains (Losses) on Equity Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ (5.6)	$ (63.3)	$ 119.1
Less: net gains (losses) recognized on equity securities sold during the period	(11.6)	(42.7)	2.2
Net unrealized gains (losses) recognized during the period on equity securities still held	$ 6.0	$ (20.6)	$ 116.9