Goodwill And Intangible Assets (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Goodwill And Intangible Assets [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)	$ 107,300
Goodwill, Impairment Loss	91,910		$ 0		$ (18,700)
Carrying value following impairment charge	2,237,975		1,777,464		1,142,248
Amortization of intangible assets	115,930	[1]	80,758	[2]	68,533	[3]
Estimated amortization of intangible assets for 2019	140,300
Estimated amortization of intangible assets for 2020	137,200
Estimated amortization of intangible assets for 2021	133,500
Estimated amortization of intangible assets for 2022	130,100
Estimated amortization of intangible assets for 2023	128,400
Indefinite-lived intangible assets	92,400		88,200
Intangible assets acquired during period	608,300
Amortizable intangible assets acquired	$ 604,100
Definite-lived intangible assets, weighted-average amortization period	15 years
Impairment of Goodwill and Intangible Assets	$ 199,198		$ 0		18,723
Markel Ventures Operations [Member]
Goodwill And Intangible Assets [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)	14,900
Goodwill, Impairment Loss					18,700
Markel Ventures Operations [Member] | Industrial Manufacturing [Member]
Goodwill And Intangible Assets [Line Items]
Carrying value following impairment charge					$ 0
Markel CATCo IM [Member]
Goodwill And Intangible Assets [Line Items]
Impairment of Intangible Assets (Excluding Goodwill)	87,100
Goodwill, Impairment Loss	91,900
Impairment of Goodwill and Intangible Assets	179,000
Intangible Assets, Net (Including Goodwill)	$ 0
Investment Management Agreements [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	16 years
Customer Relationships [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	16 years
Trade Names [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	15 years
Agent Relationships [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	12 years
Technology [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	6 years
Other Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Definite-lived intangible assets, weighted-average amortization period	5 years

[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 any other reportable 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 any other reportable 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 any other reportable segments.