v3.26.1
Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)
 3 Months Ended
Mar. 31, 2026
Mar. 31, 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 $ 14,452,000,000 $ 14,638,000,000
Losses and loss adjustment expenses incurred, prior accident years (499,000,000) (163,000,000)
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 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 (239,000,000) 167,000,000
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 $ (260,000,000) $ (330,000,000)