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Senior Long-Term Debt And Other Debt (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Debt Instrument [Line Items]      
Senior long-term debt and other debt $ 3,779,796 $ 4,103,629  
Senior long-term debt and other debt, estimated fair value 3,400,000 3,500,000  
Interest on senior long-term debt and other debt 187,800 197,300 $ 178,600
Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Line of credit facility, capacity available 300,000    
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 680,000    
Borrowings outstanding under the facility 169,500 238,100  
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 3,439,199    
Other Debt [Member]      
Debt Instrument [Line Items]      
Senior long-term debt and other debt 340,597 416,738  
Other Debt [Member] | Markel Ventures [Member]      
Debt Instrument [Line Items]      
Senior long-term debt and other debt $ 339,000 414,100  
3.45% Unsecured Senior Notes due May 7 2052 | Unsecured Senior Notes [Member]      
Debt Instrument [Line Items]      
Debt instrument, interest rate 3.45%    
Unsecured senior notes $ 591,000 590,689  
7.35% Unsecured Senior Notes Due August 15, 2034 [Member] | Unsecured Senior Notes [Member]      
Debt Instrument [Line Items]      
Debt instrument, interest rate 7.35%    
Unsecured senior notes $ 129,076 129,004  
3.625% Unsecured Senior Notes Due March 30, 2023 [Member] | Unsecured Senior Notes [Member]      
Debt Instrument [Line Items]      
Debt instrument, interest rate 3.625%    
Unsecured senior notes $ 0 $ 249,940