v3.26.1
Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)
 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, current accident year $ 29,517,000,000 $ 28,607,000,000
Losses and loss adjustment expenses incurred, prior accident years (1,467,000,000) (240,000,000)
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | 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
Property and Casualty Reinsurance, Excluding Retroactive Reinsurance [Member] | Property and Casualty Reinsurance [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, prior accident years (869,000,000) (506,000,000)
Property and Casualty Reinsurance, Excluding Retroactive Reinsurance [Member] | Primary Insurance Businesses [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Losses and loss adjustment expenses incurred, prior accident years $ (598,000,000) $ 266,000,000