Investments (Schedule of Pre-tax Net Realized and Unrealized Gains (Losses) on Equity Securities) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Investments Debt And Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ 26.5	$ 61.5	$ 65.6	$ (74.7)
Less: net (losses) gains recognized on equity securities sold during the period	(0.1)	(15.8)	0.5	(16.4)
Net unrealized gains (losses) recognized during the period on equity securities still held	$ 26.6	$ 77.3	$ 65.1	$ (58.3)