v3.10.0.1
Document And Entity Information - USD ($)
12 Months Ended
Dec. 31, 2018
Feb. 05, 2019
Jun. 30, 2018
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Entity Registrant Name MARKEL CORP    
Entity Central Index Key 0001096343    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Common Stock, Shares Outstanding   13,874,896  
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 14,686,000,000
Entity Emerging Growth Company false    
Entity Small Business false    
Entity Shell Company false