v3.26.1
Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Catastrophe Current Year Claims and Claims Adjustment Expense $ 35,028 [1] $ 66,064
Retroactive reinsurance transaction with Hagerty Re, net losses and loss adjustment expenses (62,202) 0
Favorable (adverse) development on prior years' loss reserves 104,063 150,107
Retroactive reinsurance transaction with Hagerty Re    
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Retroactive reinsurance transaction with Hagerty Re, cash payments 54,100  
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  
Middle East conflict (2026)    
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Catastrophe Current Year Claims and Claims Adjustment Expense (35,000)  
California Wildfires (2025)    
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Catastrophe Current Year Claims and Claims Adjustment Expense   (66,100)
Markel Insurance excluding Global Reinsurance division | Professional liability, credit and surety, and workers' compensation insurance product lines [Member]    
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Favorable (adverse) development on prior years' loss reserves $ 64,400  
Markel Insurance excluding Global Reinsurance division | Professional liability, general liability, marine and energy, and workers' compensation insurance product lines [Member]    
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Favorable (adverse) development on prior years' loss reserves   $ 125,200
[1] Catastrophe current accident year losses and loss adjustment expenses were attributable to the Middle East conflict.