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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Jul. 02, 2024
Dec. 31, 2023
Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Net losses and loss adjustment expenses $ 1,413,526 $ 1,434,871 $ 4,155,146 $ 3,934,969      
Favorable (adverse) development on prior years' loss reserves     344,212 169,754      
Unpaid losses and loss adjustment expenses 25,877,478 22,144,366 25,877,478 22,144,366   $ 23,483,321 $ 20,947,898
Reinsurance recoverable on unpaid losses 10,293,842 8,119,634 10,293,842 8,119,634   $ 8,820,567 $ 7,994,884
Reinsurance recoverable for retroactive reinsurance transaction     0 (125,067)      
Hurricane Helene (2024)              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Net losses and loss adjustment expenses     62,000        
2023 Catastrophes              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Net losses and loss adjustment expenses       46,200      
Insurance [Member]              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Net losses and loss adjustment expenses 1,215,469 1,258,864 3,608,979 3,415,860      
Insurance [Member] | International professional liability product lines, general liability, property and credit and surety product lines [Member]              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Favorable (adverse) development on prior years' loss reserves     322,100        
Insurance [Member] | General liability product lines              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Favorable (adverse) development on prior years' loss reserves       (70,500)      
Reinsurance [Member]              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Net losses and loss adjustment expenses $ 198,057 $ 176,007 $ 546,167 519,109      
Reinsurance recoverable for retroactive reinsurance transaction       125,100      
Reinsurance [Member] | Public entity and general liability product lines              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Favorable (adverse) development on prior years' loss reserves       (60,900)      
Program services and other fronting [Member] | Casualty policy portfolio part of retroactive reinsurance contract [Member]              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Unpaid losses and loss adjustment expenses         $ 302,600    
Reinsurance recoverable on unpaid losses         $ 302,600    
Insurance and Reinsurance | International professional liability product lines and property, marine and energy, workers' compensation and personal lines product lines [Member]              
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]              
Favorable (adverse) development on prior years' loss reserves       $ 276,600