Goodwill And Intangible Assets (Components Of Goodwill) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Goodwill [Line Items]
Goodwill, beginning balance		$ 2,237,975	[1]	$ 1,777,464
Acquisitions (see note 2)		118,878		548,075
Impairment of goodwill		0		(91,910)		$ 0
Amortizable intangible assets acquired		113,100
Foreign currency movements and other adjustments	[2]	(48,305)		4,346
Goodwill, ending balance		2,308,548	[1]	2,237,975	[1]	1,777,464
Accumulated impairment losses	[1]	139,200		139,200
Indefinite-lived intangible assets		92,400
Markel Ventures [Member]
Goodwill [Line Items]
Goodwill, beginning balance		497,338	[1]	424,981
Acquisitions (see note 2)		118,878		73,174
Impairment of goodwill				0
Foreign currency movements and other adjustments	[2]	(9,439)		(817)
Goodwill, ending balance		606,777	[1]	497,338	[1]	424,981
Accumulated impairment losses	[1]	47,300		47,300
Insurance [Member]
Goodwill [Line Items]
Goodwill, beginning balance		770,184	[1]	771,821
Acquisitions (see note 2)		0		0
Impairment of goodwill				0
Foreign currency movements and other adjustments	[2]	1,263		(1,637)
Goodwill, ending balance		771,447	[1]	770,184	[1]	771,821
Reinsurance [Member]
Goodwill [Line Items]
Goodwill, beginning balance		122,745	[1]	122,745
Acquisitions (see note 2)		0		0
Impairment of goodwill				0
Foreign currency movements and other adjustments	[2]	0		0
Goodwill, ending balance		122,745	[1]	122,745	[1]	122,745
Other Operations [Member]
Goodwill [Line Items]
Goodwill, beginning balance	[3]	847,708	[1]	457,917
Acquisitions (see note 2)	[3]	0		474,901
Impairment of goodwill	[3]			(91,910)
Foreign currency movements and other adjustments	[2],[3]	(40,129)		6,800
Goodwill, ending balance	[3]	807,579	[1]	847,708	[1]	$ 457,917
Accumulated impairment losses	[1]	$ 91,900		$ 91,900

[1] As of December 31, 2019 and 2018, goodwill was net of accumulated impairment losses of $139.2 million, of which $91.9 million was in Other and $47.3 million was in Markel Ventures.

[2] Foreign currency movements and other adjustments includes adjustments to goodwill resulting from changes to the preliminary purchase price allocation, if any, for acquisitions that occurred in the prior year.

[3] Amounts included in Other reflect the Company's operations that are not included in a reportable segment.