v3.25.4
Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
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 $ 58,207,000,000 $ 57,563,000,000 $ 59,244,000,000
Losses and loss adjustment expenses incurred, prior accident years $ (1,854,000,000) $ (2,322,000,000) $ (3,541,000,000)
Prior years' losses incurred as a percentage of net liabilities 1.70% 2.20% 3.50%
Liabilities for environmental, asbestos and latent injury exposures $ 1,800,000,000 $ 1,900,000,000  
Property and Casualty Insurance and 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 (767,000,000) (602,000,000) $ (2,100,000,000)
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Catastrophe [Member]      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Losses and loss adjustment expenses incurred, current accident year 1,100,000,000 1,500,000,000 925,000,000
Minimum losses per event each accident year 150,000,000    
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 $ (1,100,000,000) $ (1,700,000,000) $ (1,400,000,000)