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Reserve for Losses and Loss Adjustment Expenses - Prior year development (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) $ 124 $ 136
Operating segments | Insurance    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) $ 10 $ 12
Percentage of prior year development 0.70% 0.90%
Operating segments | Insurance | Medium tailed lines    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) $ (20) $ (13)
Operating segments | Insurance | Medium tailed lines | Professional liability    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable)   (18)
Operating segments | Insurance | Medium tailed lines | Marine    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) (17) 7
Operating segments | Insurance | Long tailed lines | Executive assurance    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) 7 1
Operating segments | Insurance | Long tailed lines | Alternative markets    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) 3  
Operating segments | Insurance | Long tailed lines | Construction and national accounts    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) (7)  
Operating segments | Insurance | Short tailed lines | Property excluding marine    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) 18 9
Operating segments | Insurance | Short tailed lines | Warranty and lenders solutions    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable)   7
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 (unfavorable) 10 8
Operating segments | Insurance | Short tailed and long tailed lines    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) 30 25
Operating segments | Reinsurance    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) $ 40 $ 53
Percentage of prior year development 2.40% 4.00%
Operating segments | Reinsurance | Medium tailed lines | Marine and aviation    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) $ (2) $ 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 (unfavorable) (2) 3
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 (unfavorable) 44 49
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 (unfavorable) (8) 8
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 (unfavorable) 21 23
Operating segments | Reinsurance | Short tailed lines | Other specialty lines and other lines    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable)   18
Operating segments | Reinsurance | Short tailed lines | Other specialty lines    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) 33  
Operating segments | Reinsurance | Medium tailed and long tailed lines    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) (4) 3
Operating segments | Mortgage    
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Prior year development favorable (unfavorable) $ 74 $ 71
Percentage of prior year development 24.40% 23.90%