v3.26.1
Reserve for Losses and Loss Adjustment Expenses - Prior year development (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse $ (169) $ (153) $ (389) $ (350)
Operating segments        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (169) (153) (389) (350)
Operating segments | Short tailed lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (201) (152) (437) (355)
Operating segments | Long tailed lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse 32 (1) 48 5
Operating segments | Insurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (27) (8) (41) (25)
Operating segments | Insurance | Short tailed lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (33) (13) (43) (28)
Operating segments | Insurance | Short tailed lines | Travel and accident        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse   (7)   (15)
Operating segments | Insurance | Short tailed lines | Property, energy, marine and aviation        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (22)   (29) (11)
Operating segments | Insurance | Long tailed lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse 6 5 2 3
Operating segments | Insurance | Long tailed lines | Programs        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse   14    
Operating segments | Reinsurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (97) (81) (249) (200)
Operating segments | Reinsurance | Short tailed lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (123) (75) (295) (202)
Operating segments | Reinsurance | Short tailed lines | Property other than property catastrophe        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (61) (60) (133) (89)
Operating segments | Reinsurance | Short tailed lines | Property catastrophe        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (44)   (79) (86)
Operating segments | Reinsurance | Long tailed lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse 26 (6) 46 2
Operating segments | Reinsurance | Long tailed lines | Casualty        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse 26   46  
Operating segments | Mortgage        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (45) (64) (99) (125)
Operating segments | Mortgage | Short tailed lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse (45) (64) (99) (125)
Operating segments | Mortgage | Long tailed lines        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior year development (favorable) adverse $ 0 $ 0 $ 0 $ 0