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Schedule V - Valuation And Qualifying Accounts (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Reserve Deducted From Accounts Receivable      
Valuation And Qualifying Accounts Disclosure [Line Items]      
Balance at beginning of period $ 21.3 $ 14.0 $ 14.0
Additions Charged to costs and expenses 8.1 11.4 4.5
Additions Charged to other accounts 0.0 0.0 0.0
Deductions from reserve [1] 7.6 4.1 4.5
Balance at end of period 21.8 21.3 14.0
Reserve For Known And Incurred But Not Reported Claims      
Valuation And Qualifying Accounts Disclosure [Line Items]      
Balance at beginning of period 1,325.3 1,283.8 1,178.0
Additions Charged to costs and expenses 336.3 486.3 588.7
Additions Charged to other accounts 2.6 (10.5) (0.6)
Deductions from reserve [2] 381.8 434.3 482.3
Balance at end of period 1,282.4 1,325.3 1,283.8
Reserve deducted from notes receivable      
Valuation And Qualifying Accounts Disclosure [Line Items]      
Balance at beginning of period 6.8 0.3 0.3
Additions Charged to costs and expenses 0.0 27.7 0.0
Additions Charged to other accounts 0.0 0.0 0.0
Deductions from reserve 6.5 21.2 0.0
Balance at end of period 0.3 6.8 0.3
Reserve Deducted From Deferred Income Taxes      
Valuation And Qualifying Accounts Disclosure [Line Items]      
Balance at beginning of period 7.4 8.1 9.4
Additions Charged to costs and expenses 7.6 0.0 0.0
Additions Charged to other accounts 0.0 0.0 0.0
Deductions from reserve 1.3 0.7 1.3
Balance at end of period $ 13.7 $ 7.4 $ 8.1
[1] Amount represents accounts written off, net of recoveries.
[2] Amount represents claim payments, net of recoveries.