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Segment Reporting Disclosures (Narrative) (Details) - segment	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Number of Reportable Segments	4
Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration]	Chief Executive Officer [Member]
Segment Reporting, CODM, Profit (Loss) Measure, How Used, Description	The chief operating decision maker uses segment operating income to assess the performance of each segment and determine how to allocate capital. The chief operating decision maker allocates capital where he believes it will earn an appropriate return.
Underwriting Operations [Member] | United States [Member]
Segment Reporting Information [Line Items]
Percentage of gross premiums written	75.00%	75.00%	75.00%
Markel Ventures [Member] | United States [Member]
Segment Reporting Information [Line Items]
Percentage of revenue attributable to U.S.	94.00%	95.00%	96.00%

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- Definition

Percentage of gross premiums written

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Indicates title and position of individual or name of group identified as chief operating decision maker (CODM) for segment reporting.

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Description of how chief operating decision maker (CODM) uses reported segment profit (loss) measure to assess performance and allocate resource.

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