Net Investment Income (Schedule of Net Investment Income) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total investment income	$ 573.0	$ 343.1	$ 217.7
Deposits and other investments	303.3	135.1	47.4
Deferred compensation plan assets	21.9	(25.3)	18.7
Equity in earnings of affiliates, net	5.4	11.0	6.9
Other	0.2	7.2	0.8
Investment expenses	(3.0)	(3.0)	(2.9)
Net investment income	570.0	340.1	214.8
Debt Securities
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total investment income	231.7	206.1	133.0
Equity Securities
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total investment income	$ 10.5	$ 9.0	$ 10.9