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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Jul. 02, 2024
Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Net losses and loss adjustment expenses $ 5,508,059 $ 5,360,559 $ 4,613,035    
Unpaid losses and loss adjustment expenses 26,633,094 23,483,321 20,947,898   $ 18,178,894
Reinsurance recoverables on unpaid losses, end of year 11,120,367 8,820,567 7,994,884   $ 6,876,317
Reinsurance Recoverable, Retroactive Reinsurance Transaction $ 0 $ (125,067) 0    
IBNR reserves as a percentage of total unpaid losses and loss adjustment expenses 71.00% 72.00%      
Prior Year Claims and Claims Adjustment Expense $ 455,300 $ 38,600 167,400    
Insurance [Member]          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Unpaid losses and loss adjustment expenses 16,073,355 14,221,258      
Prior Year Claims and Claims Adjustment Expense 451,000 104,700 142,900    
Insurance [Member] | Accident Years 2014 and Prior [Member]          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Prior Year Claims and Claims Adjustment Expense 50,100        
Reinsurance [Member]          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Unpaid losses and loss adjustment expenses 3,870,059 3,726,472      
Prior Year Claims and Claims Adjustment Expense 12,300 (57,100) 26,100    
Reinsurance [Member] | 2023 U.K. Motor Retroactive Transaction          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Reinsurance Recoverable, Retroactive Reinsurance Transaction   (125,100)      
Fronting [Member]          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Reinsurance recoverables 3,500,000 2,500,000 3,000,000    
Unpaid losses and loss adjustment expenses 6,515,153 5,185,553      
Asbestos [Member]          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Asbestos-related reserves, gross loss incurred 122,100 132,500      
Asbestos-related reserves, net loss incurred 32,500 39,600      
Casualty policy portfolio part of retroactive reinsurance contract | Fronting [Member]          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Unpaid losses and loss adjustment expenses       $ 302,600  
Reinsurance recoverables on unpaid losses, end of year       $ 302,600  
2024 Catastrophes          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Net losses and loss adjustment expenses 70,600        
Reinsurance recoverables $ 60,400        
2023 Catastrophes | Insurance [Member]          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Net losses and loss adjustment expenses   40,100      
Reinsurance recoverables   $ 9,300      
Hurricane Ian          
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]          
Net losses and loss adjustment expenses     46,200    
Reinsurance recoverables     115,300    
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