Reserve for Known and Incurred but Not Reported Claims (Activity in Reserve for Known and Incurred but Not Reported Claims) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance at beginning of period	$ 1,282.4	$ 1,325.3
Provision related to current year	77.0	83.1
Provision related to prior years	(7.5)	(0.8)
Total Provision	69.5	82.3
Payments, net of recoveries, related to: Current year	30.6	31.1
Payments, net of recoveries, related to: Prior years	61.0	64.1
Total Payments, net of recoveries	91.6	95.2
Other	1.3	2.1
Balance at end of period	$ 1,261.6	$ 1,314.5