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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, 2023
Mar. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]    
Balance at beginning of period $ 1,325.3 $ 1,283.8
Provision related to current year 83.1 115.2
Provision related to prior years (0.8) 7.2
Total Provision 82.3 122.4
Payments, net of recoveries, related to: Current year 31.1 32.5
Payments, net of recoveries, related to: Prior years 64.1 68.3
Total Payments, net of recoveries 95.2 100.8
Other 2.1 (4.7)
Balance at end of period $ 1,314.5 $ 1,300.7