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Segment Reporting Disclosures (Narrative) (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2025 USD ($) segment	Dec. 31, 2024 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment	4
Markel Insurance
Segment Reporting Information [Line Items]
Goodwill, Impairment Loss	$ 0
Global Reinsurance Division [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 1,200,000

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Amount of impairment loss from asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

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