v3.26.1
Reserve for Unpaid Losses and Loss Adjustment Expenses - P&C Prior Accident Years Reserve Development (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 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]        
Increase (Decrease) in Deferred Revenue $ 36      
Amortization of Deferred Gain on Retroactive Reinsurance     $ 36  
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   $ (24)   $ (56)
Amortization of Deferred Gain on Retroactive Reinsurance       (56)
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]     (152) (309)
Prior accident year development before change in deferred gain     (116) (253)
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]        
Increase (Decrease) in Deferred Revenue     (36)  
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       (56)
Segment Reconciling Items        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Increase (Decrease) in Deferred Revenue     (36) (56)
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]     (110) (126)
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]     23 23
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]     4 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]     (15) (23)
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]     (29) (31)
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]     (4) (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]     (39) (22)
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]     $ (11) $ (2)