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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense $ (4,315) $ 62,000 $ 56,562 $ 62,000
Favorable (adverse) development on prior years' loss reserves     360,473 344,212
Commutation of ceded reinsurance contracts     312,244 0
Commutation of ceded reinsurance contracts, cash payments     292,600  
California Wildfires (2025)        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense     (56,600)  
Hurricane Helene (2024)        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense       (62,000)
Markel Insurance        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense $ (4,315) $ 62,000 56,562 62,000
Markel Insurance | Run-off risk-managed directors and officers product lines        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Favorable (adverse) development on prior years' loss reserves     142,300  
Markel Insurance | Marine & Energy, Property, Programs, General Liability, Worker's Compensation product lines        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Favorable (adverse) development on prior years' loss reserves     $ 314,800  
Markel Insurance | International Professional Liability, General Liability, Property, and Credit & Surety product lines        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Favorable (adverse) development on prior years' loss reserves       $ 289,900