Variable Interest Entities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Other revenues	$ 2,132,606	$ 1,413,275	$ 1,307,779
Equity securities	5,720,945	0
Fair value of the Company's investment in CROF	0	5,967,847
CATCo Reinsurance Opportunities Fund Ltd. (CROF) [Member]
Variable Interest Entity [Line Items]
Equity securities	$ 4,500
Fair value of the Company's investment in CROF		$ 20,500
Unconsolidated Markel CATCo Fund 1 [Member]
Variable Interest Entity [Line Items]
Percentage of total assets of the Markel Diversified Fund invested in one of the unconsolidated Funds	2.00%	7.00%
Unconsolidated Markel CATCo Fund 2 [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Investment in one of the unconsolidated funds	$ 26,200
Other [Member]
Variable Interest Entity [Line Items]
Other revenues	220,541	$ 79,995	93,330
Revenue from Contract with Customer [Member]
Variable Interest Entity [Line Items]
Other revenues	1,944,667	1,305,655	1,223,305
Revenue from Contract with Customer [Member] | Investment Management [Member]
Variable Interest Entity [Line Items]
Other revenues	91,527	28,740	56,455
Revenue from Contract with Customer [Member] | Other [Member]
Variable Interest Entity [Line Items]
Other revenues	124,763	63,486	91,439
Revenue from Contract with Customer [Member] | Other [Member] | Investment Management [Member]
Variable Interest Entity [Line Items]
Other revenues	91,527
Unconsolidated Markel CATCo Funds [Member]
Variable Interest Entity [Line Items]
Other revenues	66,200
Net investment and insurance assets under management of MCIM for unconsolidated VIEs	$ 3,400,000
Unconsolidated Markel CATCo Funds [Member] | Revenue from Contract with Customer [Member] | Other [Member] | Investment Management [Member]
Variable Interest Entity [Line Items]
Other revenues		$ 28,740	$ 56,455