v3.26.3
Unpaid losses and loss expenses Unpaid losses and loss expenses (PPD table) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development $ (511) $ (361) $ (1,080) $ (865)
North America Commercial P&C Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (14) (26) (214) (246)
North America Personal P&C Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (379) (282) (553) (403)
North American Agriculture Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (2) (30) (82) (63)
Overseas General Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (162) (84) (439) (282)
Global Reinsurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (5) 0 (16) (15)
Corporate Segment [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development 51 61 224 144
Short Tail [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (575) (460) (1,290) (1,051)
Short Tail [Member] | North America Commercial P&C Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (206) (130) (358) (323)
Short Tail [Member] | North America Personal P&C Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (379) (282) (553) (403)
Short Tail [Member] | North American Agriculture Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (2) (30) (82) (63)
Short Tail [Member] | Overseas General Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development 12 (18) (276) (252)
Short Tail [Member] | Global Reinsurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development 0 0 (21) (10)
Short Tail [Member] | Corporate Segment [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development 0 0 0 0
Long Tail [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development 64 99 210 186
Long Tail [Member] | North America Commercial P&C Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development 192 104 144 77
Long Tail [Member] | North America Personal P&C Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development 0 0 0 0
Long Tail [Member] | North American Agriculture Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development 0 0 0 0
Long Tail [Member] | Overseas General Insurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (174) (66) (163) (30)
Long Tail [Member] | Global Reinsurance [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development (5) 0 5 (5)
Long Tail [Member] | Corporate Segment [Member]        
Liability for Claims and Claims Adjustment Expense [Line Items]        
Net Prior Period Development $ 51 $ 61 $ 224 $ 144