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Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]    
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) $ 1,120  
Net Prior Period Development (255) $ (207)
Long Tail [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Net Prior Period Development 58 104
Short Tail [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Net Prior Period Development (313) (311)
North America Commercial P&C Insurance [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Net Prior Period Development (114) (48)
North America Commercial P&C Insurance [Member] | Long Tail [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Net Prior Period Development 49 96
North America Commercial P&C Insurance [Member] | Short Tail [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Net Prior Period Development (163) (144)
Overseas General Insurance [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Net Prior Period Development (121) (89)
Overseas General Insurance [Member] | Long Tail [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Net Prior Period Development 1 (1)
Overseas General Insurance [Member] | Short Tail [Member]    
Liability for Claims and Claims Adjustment Expense [Line Items]    
Net Prior Period Development $ (122) $ (88)