Goodwill And Intangible Assets (Components Of Goodwill) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Goodwill [Line Items]
Goodwill, beginning balance		$ 1,777,464		$ 1,142,248
Acquisitions (see note 2)		548,075		626,735
Goodwill, Impairment Loss		91,910		0		$ (18,700)
Foreign currency movements and other adjustments		4,346		8,481
Goodwill, ending balance		2,237,975		1,777,464		1,142,248
Accumulated impairment losses	[1]	139,200		47,300
Insurance [Member]
Goodwill [Line Items]
Goodwill, beginning balance		771,821		672,763
Acquisitions (see note 2)		0		93,123
Goodwill, Impairment Loss		0
Foreign currency movements and other adjustments		(1,637)		5,935
Goodwill, ending balance		770,184		771,821		672,763
Reinsurance [Member]
Goodwill [Line Items]
Goodwill, beginning balance		122,745		122,745
Acquisitions (see note 2)		0		0
Goodwill, Impairment Loss		0
Foreign currency movements and other adjustments		0		0
Goodwill, ending balance		122,745		122,745		122,745
Markel Ventures [Member]
Goodwill [Line Items]
Goodwill, beginning balance		424,981		237,767
Acquisitions (see note 2)		73,174		186,194
Goodwill, Impairment Loss		0
Foreign currency movements and other adjustments		(817)		1,020
Goodwill, ending balance		497,338		424,981		237,767
Accumulated impairment losses	[1]	47,300		47,340
Other [Member]
Goodwill [Line Items]
Goodwill, beginning balance	[2]	457,917	[1]	108,973
Acquisitions (see note 2)	[2]	474,901		347,418
Goodwill, Impairment Loss	[2]	91,910
Foreign currency movements and other adjustments	[2]	6,800		1,526
Goodwill, ending balance	[2]	847,708	[1]	$ 457,917	[1]	$ 108,973
Accumulated impairment losses	[1]	$ 91,900

[1] As of December 31, 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. As of December 31, 2017, goodwill was net of accumulated impairment losses of $47.3 million, all of which was included in Markel Ventures.

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