v3.26.3
Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Reinsurance Contract [Axis]: Adverse Development Cover A&E        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Change in deferred gain on retroactive reinsurance     $ (533)  
Increase (Decrease) in Deferred Revenue     36  
Gain (Loss) on Contract Termination $ 497   497  
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Increase (Decrease) in Deferred Revenue   $ (8)   $ (64)
Property, Liability and Casualty Insurance Product Line        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     (746) (412)
Prior accident year development before change in deferred gain     (213) (348)
Property, Liability and Casualty Insurance Product Line | Reinsurance Contract [Axis]: Adverse Development Cover A&E        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Change in deferred gain on retroactive reinsurance     (533)  
Property, Liability and Casualty Insurance Product Line | Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Increase (Decrease) in Deferred Revenue       (64)
Segment Reconciling Items | Catastrophe        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     (50) (39)
Segment Reconciling Items | Workers' Compensation        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     (167) (188)
Segment Reconciling Items | Change in Workers Compensation Discount Including Accretion [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     34 34
Segment Reconciling Items | General Liability [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     116 0
Segment Reconciling Items | Marine        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     1 0
Segment Reconciling Items | Property Insurance [Member] | P&C Business Insurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     (25) (28)
Segment Reconciling Items | Property Insurance [Member] | P&C Personal Insurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     (36) (36)
Segment Reconciling Items | Professional Liability Insurance [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     (9) (11)
Segment Reconciling Items | Surety Product Line [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     (32) (22)
Segment Reconciling Items | Assumed reinsurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     5 0
Segment Reconciling Items | Automobiles [Member] | P&C Business Insurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     26 0
Segment Reconciling Items | Automobiles [Member] | P&C Personal Insurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     (59) (55)
Segment Reconciling Items | Uncollectible reinsurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     0 6
Segment Reconciling Items | Insurance, Other [Member]        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Prior accident year development [1]     $ (17) $ (9)