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, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ 13.4	$ 106.5	$ (43.4)
Less: net (losses) gains recognized on equity securities sold during the period	(19.8)	2.8	(3.9)
Net unrealized gains (losses) recognized during the period on equity securities still held	$ 33.2	$ 103.7	$ (39.5)