Related Party Transactions Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Services and other revenues	$ 813,202	$ 635,083	$ 462,263
Reinsurance recoverables	5,432,712	5,221,947
Nephila Holdings Limited [Member]
Related Party Transaction [Line Items]
Gross written premiums on Markel's program with Nephila	425,000	322,100
Syndicate 2357 and certain other Nephila Reinsurers [Member]
Related Party Transaction [Line Items]
Reinsurance recoverables	238,800	179,800
Markel CATCo Re. Related Party Transaction 2 [Member]
Related Party Transaction [Line Items]
Ceded losses and loss adjustment expenses	4,900
Ceding fees	8,800
Reinsurance recoverables	4,900
Markel CATCo Re. Related Party Transaction 1 [Member]
Related Party Transaction [Line Items]
Gross written premiums on behalf of Markel CATCo Re.	5,500	10,900	9,700
Insurance-linked Securities Operations [Member]
Related Party Transaction [Line Items]
Services and other revenues	225,600	91,500	28,700
Insurance-linked Securities Operations [Member] | Unconsolidated entities managed by Nephila and MCIM [Member]
Related Party Transaction [Line Items]
Services and other revenues	200,800	90,600	28,700
Underwriting Operations [Member]
Related Party Transaction [Line Items]
Ceded losses and loss adjustment expenses	(556,618)	(710,568)	(856,843)
Reinsurance recoverables	$ 3,447,186	$ 3,530,790	$ 3,722,604