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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Reinsurance Recoverable, Retroactive Reinsurance Transaction $ (125,067) $ 0 $ 0
Net losses and loss adjustment expenses $ 5,360,559 $ 4,613,035 4,061,000
IBNR reserves as a percentage of total unpaid losses and loss adjustment expenses 72.00% 70.00%  
Prior Year Claims and Claims Adjustment Expense $ 38,600 $ 167,400 479,800
Ceded Premiums Written (5,604,662) (4,998,292) (4,318,951)
Vesttoo LOCs      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease) 65,000    
Insurance [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Net losses and loss adjustment expenses 4,690,745 3,936,425 3,311,185
Prior Year Claims and Claims Adjustment Expense 104,700 142,900 506,300
Insurance [Member] | 2013 and Prior Accident Years [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Prior Year Claims and Claims Adjustment Expense 25,900    
Reinsurance [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Reinsurance Recoverable, Retroactive Reinsurance Transaction (125,100)    
Net losses and loss adjustment expenses 669,814 676,610 749,815
Prior Year Claims and Claims Adjustment Expense (57,100) 26,100 (19,900)
Reinsurance [Member] | 2013 and Prior Accident Years [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Prior Year Claims and Claims Adjustment Expense 69,400    
Reinsurance [Member] | 2013 and Prior Accident Years [Member] | 2023 U.K. Motor Retroactive Transaction      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Prior Year Claims and Claims Adjustment Expense 28,100    
Program services and other fronting [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Reinsurance recoverables 2,500,000 3,000,000  
Ceded Premiums Written (3,725,818) (3,358,127) (2,958,188)
Asbestos [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Asbestos-related reserves, gross loss incurred 132,500 153,200  
Asbestos-related reserves, net loss incurred 39,600 54,500  
2023 Catastrophes      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Net losses and loss adjustment expenses 40,100    
Reinsurance recoverables $ 9,300    
Hurricane Ian | Insurance [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Net losses and loss adjustment expenses   46,200  
Reinsurance recoverables   115,300  
Hurricane Ian | Program services and other fronting [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Net losses and loss adjustment expenses   850,000  
Ceded Premiums Written   850,000  
Russia-Ukraine conflict      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Net losses and loss adjustment expenses   35,700  
Reinsurance recoverables   $ 44,300  
2021 Catastrophes [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Net losses and loss adjustment expenses     195,000
Reinsurance recoverables     $ 221,700