Investments (Schedule of Pre-tax Net Realized and Unrealized Gains (Losses) on Equity Securities) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments Debt And Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ 0.3	$ 30.3	$ 65.9	$ (44.4)
Less: net gains (losses) recognized on equity securities sold during the period	0.9	(3.5)	1.4	(19.9)
Net unrealized (losses) gains recognized during the period on equity securities still held	$ (0.6)	$ 33.8	$ 64.5	$ (24.5)