Reserve for Known and Incurred but Not Reported Claims (Activity in Reserve for Known and Incurred but Not Reported Claims) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance at beginning of year	$ 1,282.4	$ 1,325.3	$ 1,283.8
Provision related to current year	357.3	354.6	468.3
Provision related to prior years	(37.3)	(18.3)	18.0
Total Provision	320.0	336.3	486.3
Payments, net of recoveries, related to: Current year	204.6	199.6	225.8
Payments, net of recoveries, related to: Prior years	193.2	182.2	208.5
Total Payments, net of recoveries	397.8	381.8	434.3
Other	(11.2)	2.6	(10.5)
Balance at end of year	$ 1,193.4	$ 1,282.4	$ 1,325.3