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Loss and Loss Adjustment Expense Reserves Activity in Loss and Loss Adjustment Expense Reserves (Detail) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Dec. 31, 2023
Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]        
Balance at January 1 $ 34,389.2 $ 30,359.3    
Less reinsurance recoverables on unpaid losses 4,515.0 5,064.4 $ 4,789.0 $ 5,559.2
Net balance at January 1 29,600.2 24,800.1    
Incurred related to:        
Current year 23,629.4 21,683.6    
Prior years (62.5) 1,110.5    
Total incurred 23,566.9 22,794.1    
Paid related to:        
Current year 11,470.3 10,789.5    
Prior years 9,606.6 9,115.8    
Total paid 21,076.9 19,905.3    
Net balance at June 30 32,090.2 27,688.9    
Plus reinsurance recoverables on unpaid losses 4,515.0 5,064.4    
Balance at June 30 $ 36,605.2 $ 32,753.3