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Loss and Loss Adjustment Expense Reserves Activity in Loss and Loss Adjustment Expense Reserves (Detail) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]        
Balance at January 1 $ 30,359.3 $ 26,164.1    
Less reinsurance recoverables on unpaid losses 4,941.2 6,034.0 $ 5,559.2 $ 4,733.6
Net balance at January 1 24,800.1 21,430.5    
Incurred related to:        
Current year 33,035.0 28,245.4    
Prior years 1,147.0 52.8    
Total incurred 34,182.0 28,298.2    
Paid related to:        
Current year 18,581.3 15,712.4    
Prior years 11,764.7 9,418.5    
Total paid 30,346.0 25,130.9    
Net balance at September 30 28,636.1 24,597.8    
Plus reinsurance recoverables on unpaid losses 4,941.2 6,034.0    
Balance at September 30 $ 33,577.3 $ 30,631.8