Acquisitions (Summary Of Net Intangible Assets Acquired) (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended						13 Months Ended
	Nov. 17, 2017	Mar. 31, 2018	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	[3]	Dec. 31, 2018
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Economic useful lives of finite-lived intangible assets, years			15 years
Amortization (from the Acquisition Date through December 31, 2017)			$ 115,930	[1]	$ 80,758	[2]	$ 68,533
Net intangible assets as of December 31, 2017			$ 1,726,196		$ 1,355,681				$ 1,726,196
Customer Relationships [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Economic useful lives of finite-lived intangible assets, years			16 years
Trade Names [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Economic useful lives of finite-lived intangible assets, years			15 years
Technology [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Economic useful lives of finite-lived intangible assets, years			6 years
State National Companies, Inc. [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets		$ 338,500
Indefinite-lived intangible assets		$ 32,000
Economic useful lives of finite-lived intangible assets, years		13 years
Intangible assets, before amortization, as of the Acquisition Date	$ 370,500
Amortization (from the Acquisition Date through December 31, 2017)									32,336
Net intangible assets as of December 31, 2017			$ 338,164						$ 338,164
State National Companies, Inc. [Member] | Insurance Licenses [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	$ 32,000
Economic useful lives of indefinite-lived intangible assets, years	Indefinite
State National Companies, Inc. [Member] | Customer Relationships [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets	$ 289,000
Economic useful lives of finite-lived intangible assets, years	13 years
State National Companies, Inc. [Member] | Trade Names [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets	$ 22,500
Economic useful lives of finite-lived intangible assets, years	13 years
State National Companies, Inc. [Member] | Technology [Member]
Schedule of Indefinite-Lived And Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets	$ 27,000
Economic useful lives of finite-lived intangible assets, years	9 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.