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Unpaid losses and loss adjustment expenses - Narrative (Detail) - Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] - USD ($)
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, current accident year $ 43,124,000,000 $ 43,166,000,000
Losses and loss adjustment expenses incurred, prior accident years $ (1,182,000,000) $ (992,000,000)
Prior years' losses incurred as a percentage of net liabilities 1.10% 0.90%
Primary Insurance Businesses [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, prior accident years $ (284,000,000) $ 222,000,000
Property and Casualty Reinsurance [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, prior accident years (898,000,000) (1,200,000,000)
Catastrophe [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Minimum losses per event each accident year 150,000,000  
Losses and loss adjustment expenses incurred, current accident year $ 1,100,000,000 $ 715,000,000