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Senior Long-Term Debt And Other Debt (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
May 15, 2024
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Debt Instrument [Line Items]        
Senior long-term debt and other debt   $ 4,330,341 $ 3,779,796  
Senior long-term debt and other debt, estimated fair value   $ 3,800,000 3,400,000  
Line of credit facility, initiation date   Jun. 23, 2023    
Line of credit facility, expiration date   Jun. 23, 2028    
Interest on senior long-term debt and other debt   $ 197,400 187,800 $ 197,300
Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Line of credit facility, increase (decrease), net   $ 200,000    
Line of credit facility, unused capacity, commitment fee percentage   0.20%    
Borrowings outstanding under the facility   $ 0 0  
Revolving Credit Facility [Member] | Markel Ventures [Member]        
Debt Instrument [Line Items]        
Line of credit facility, capacity available   692,000    
Borrowings outstanding under the facility   150,000 169,500  
Revolving Credit Facility [Member] | Secured Letters Of Credit [Member]        
Debt Instrument [Line Items]        
Line of credit facility, capacity available   200,000    
Previous line of credit facility        
Debt Instrument [Line Items]        
Line of credit facility, capacity available   300,000    
Unsecured Senior Notes [Member]        
Debt Instrument [Line Items]        
Senior long-term debt and other debt   4,032,959    
Other Debt [Member]        
Debt Instrument [Line Items]        
Senior long-term debt and other debt   297,382 340,597  
Other Debt [Member] | Markel Ventures [Member]        
Debt Instrument [Line Items]        
Senior long-term debt and other debt   $ 297,400 339,000  
7.35% Unsecured Senior Notes Due August 15, 2034 [Member] | Unsecured Senior Notes [Member]        
Debt Instrument [Line Items]        
Debt instrument, interest rate   7.35%    
3.625% Unsecured Senior Notes Due March 30, 2023 [Member] | Unsecured Senior Notes [Member]        
Debt Instrument [Line Items]        
Debt instrument, face amount     $ 250,000  
Debt instrument, interest rate     3.625%  
6.0% Unsecured Senior Notes due May 16 2054        
Debt Instrument [Line Items]        
Net proceeds from issuance of unsecured senior notes $ 592,600      
6.0% Unsecured Senior Notes due May 16 2054 | Unsecured Senior Notes [Member]        
Debt Instrument [Line Items]        
Debt instrument, face amount $ 600,000      
Debt instrument, interest rate 6.00% 6.00%