Net Investment Income (Schedule of Net Investment Income) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total investment income	$ 564.2	$ 573.0	$ 343.1
Deposits and other investments	282.3	303.5	142.3
Deferred compensation plan assets	19.5	21.9	(25.3)
Equity in earnings of affiliates, net	7.4	5.4	11.0
Investment expenses	(3.2)	(3.0)	(3.0)
Net investment income	561.0	570.0	340.1
Debt Securities
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total investment income	244.6	231.7	206.1
Equity Securities
Schedule Of Investment Income Reported Amounts By Category [Line Items]
Total investment income	$ 10.4	$ 10.5	$ 9.0