v3.26.1
Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
 [1]
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense $ 40,983 $ (5,187) $ 76,011 [1] $ 60,877
Retroactive reinsurance transaction with Hagerty Re, net losses and loss adjustment expenses     (62,202) 0
Favorable (adverse) development on prior years' loss reserves     270,421 228,839
Retroactive reinsurance transaction with Hagerty Re | Hagerty, Inc. [Member]        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Retroactive reinsurance transaction with Hagerty Re, net losses and loss adjustment expenses     62,200  
Retroactive reinsurance transaction with Hagerty Re, cash payments     54,100  
Middle East conflict (2026)        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense     (76,000)  
California Wildfires (2025)        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense       (60,900)
Markel Insurance excluding Global Reinsurance division | Property, marine and energy, workers' compensation, and credit and surety insurance product lines        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Favorable (adverse) development on prior years' loss reserves     $ 248,700  
Markel Insurance excluding Global Reinsurance division | Marine and energy, property, general liability, and workers' compensation insurance product lines        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Favorable (adverse) development on prior years' loss reserves       179,900
Markel Insurance excluding Global Reinsurance division | 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       $ (127,900)
[1] Catastrophe current accident year losses and loss adjustment expenses were related to the Middle East conflict.