Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 21, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	001-15811
Entity Registrant Name	MARKEL CORPORATION
Entity Incorporation, State or Country Code	VA
Entity Tax Identification Number	54-1959284
Entity Address, Address Line One	4521 Highwoods Parkway
Entity Address, City or Town	Glen Allen
Entity Address, State or Province	VA
Entity Address, Postal Zip Code	23060-6148
City Area Code	804
Local Phone Number	747-0136
Title of 12(b) Security	Common Stock, no par value
Trading Symbol	MKL
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		13,758,155
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001096343
Current Fiscal Year End Date	--12-31