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, 2022	Dec. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ (63.3)	$ 119.1	$ 13.4
Less: net gains (losses) recognized on equity securities sold during the period	(42.7)	2.2	(19.8)
Net unrealized gains (losses) recognized during the period on equity securities still held	$ (20.6)	$ 116.9	$ 33.2