v3.26.3
Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2026
 [1]
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense $ 74,555 $ (4,315) $ 150,566 [1] $ 56,562
Retroactive reinsurance transaction with Hagerty Re, net losses and loss adjustment expenses     (62,202) 0
Favorable (adverse) development on prior years' loss reserves     385,941 360,473
Commutation of ceded reinsurance contracts     0 312,244
Commutation of ceded reinsurance contracts, cash payments       292,600
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     (150,600)  
Ceded losses     270,200  
California Wildfires (2025)        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Catastrophe Current Year Claims and Claims Adjustment Expense       (56,600)
Markel Insurance excluding Global Reinsurance division | Property, workers' compensation, marine and energy, professional liability, credit and surety, and general liability insurance product lines.        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Favorable (adverse) development on prior years' loss reserves     $ 408,100  
Markel Insurance excluding Global Reinsurance division | Marine and energy, general liability, property, programs, and workers' compensation insurance product lines        
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Favorable (adverse) development on prior years' loss reserves       364,500
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       $ 142,300
[1] Catastrophe current accident year losses and loss adjustment expenses were related to the Middle East conflict.