Goodwill And Intangible Assets Goodwill And Intangible Assets (Components of Net Intangible Assets) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Intangible [Line Items]
Net intangible assets, beginning balance		$ 1,726,196		$ 1,355,681
Acquisitions (see note 2)		113,135		608,345
Amortization of intangible assets		(148,638)	[1]	(115,930)	[2]	$ (80,758)	[3]
Impairment of intangible assets				(107,288)
Foreign currency movements and other adjustments	[4]	47,781		(14,612)
Net intangible assets, ending balance		1,738,474		1,726,196		1,355,681
Underwriting [Member]
Intangible [Line Items]
Net intangible assets, beginning balance	[5]	489,088		537,957
Acquisitions (see note 2)	[5]	41,506		1,538
Amortization of intangible assets	[5]	(39,667)		(44,464)
Impairment of intangible assets	[5]			(4,431)
Foreign currency movements and other adjustments	[4],[5]	(6,767)		(1,512)
Net intangible assets, ending balance	[5]	484,160		489,088		537,957
Markel Ventures [Member]
Intangible [Line Items]
Net intangible assets, beginning balance		431,457		400,589
Acquisitions (see note 2)		71,629		85,736
Amortization of intangible assets		(41,973)	[1],[6]	(40,208)	[2],[7]	(31,429)	[3],[8]
Impairment of intangible assets				(14,904)
Foreign currency movements and other adjustments	[4]	12,009		244
Net intangible assets, ending balance		473,122		431,457		400,589
Other Operations [Member]
Intangible [Line Items]
Net intangible assets, beginning balance	[9]	805,651		417,135
Acquisitions (see note 2)	[9]	0		521,071
Amortization of intangible assets	[9]	(66,998)		(31,258)
Impairment of intangible assets	[9]			(87,953)
Foreign currency movements and other adjustments	[4],[9]	42,539		(13,344)
Net intangible assets, ending balance	[9]	$ 781,192		$ 805,651		$ 417,135

[1] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to the Company's underwriting segments.

[2] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to the Company's underwriting segments.

[3] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to the Company's underwriting segments.

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

[5] Amounts included in Underwriting reflect the intangible assets associated with the Company's underwriting segments, which are not allocated between the Insurance and Reinsurance segments.

[6] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $53.6 million for the year ended December 31, 2019.

[7] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $52.2 million for the year ended December 31, 2018

[8] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $41.7 million for the year ended December 31, 2017.

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