Goodwill And Intangible Assets (Narrative) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Goodwill And Intangible Assets [Line Items]
Impairment of goodwill		$ 0		$ 91,910		$ 0
Goodwill and intangible asset impairment		0		199,198		0
Impairment of intangible assets (excluding goodwill)				107,288
Carrying value following impairment charge		2,308,548	[1]	2,237,975	[1]	1,777,464
Amortization of intangible assets		148,638	[2]	115,930	[3]	80,758	[4]
Estimated amortization of intangible assets for 2020		150,200
Estimated amortization of intangible assets for 2021		146,300
Estimated amortization of intangible assets for 2022		142,900
Estimated amortization of intangible assets for 2023		141,000
Estimated amortization of intangible assets for 2024		139,100
Indefinite-lived intangible assets		92,400
Intangible assets acquired during period		113,135		608,345
Amortizable intangible assets acquired		$ 113,100
Definite-lived intangible assets, weighted-average amortization period		11 years
Markel CATCo IM [Member]
Goodwill And Intangible Assets [Line Items]
Impairment of goodwill				91,900
Goodwill and intangible asset impairment				179,000
Impairment of intangible assets (excluding goodwill)				87,100
Customer Relationships [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period		12 years
Trade Names [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period		12 years
Other Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period		10 years
Markel Ventures [Member]
Goodwill And Intangible Assets [Line Items]
Impairment of goodwill				0
Goodwill and intangible asset impairment	[5]			14,904
Impairment of intangible assets (excluding goodwill)				14,904
Carrying value following impairment charge		$ 606,777	[1]	497,338	[1]	424,981
Amortization of intangible assets		41,973	[2],[5]	40,208	[3],[6]	$ 31,429	[4],[7]
Intangible assets acquired during period		$ 71,629		$ 85,736

[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] 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] 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.

[5] 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.

[6] 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

[7] 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.