v3.25.2
Unpaid losses and loss adjustment expenses - Narrative (Detail) - Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] - USD ($)
 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, current accident year $ 28,607,000,000 $ 28,359,000,000
Losses and loss adjustment expenses incurred, prior accident years $ (240,000,000) $ (1,167,000,000)
Prior years' losses incurred as a percentage of net liabilities 0.20% 1.10%
Primary Insurance Businesses [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, prior accident years $ 266,000,000 $ (433,000,000)
Property and Casualty Reinsurance [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, prior accident years (506,000,000) (734,000,000)
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 $ 0