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Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Assets	$ 67,391,562	$ 61,897,982
Corporate and eliminations
Segment Reporting Information [Line Items]
Assets	4,290,693	4,815,408
Segment assets [Member]
Segment Reporting Information [Line Items]
Assets	63,100,869	57,082,574
Segment assets [Member] | Markel Insurance
Segment Reporting Information [Line Items]
Assets	44,398,876	40,603,824
Segment assets [Member] | Industrial
Segment Reporting Information [Line Items]
Assets	3,931,643	3,878,918
Segment assets [Member] | Financial
Segment Reporting Information [Line Items]
Assets	12,899,959	11,059,318
Segment assets [Member] | Consumer and Other
Segment Reporting Information [Line Items]
Assets	$ 1,870,391	$ 1,540,514

X

- Definition

Amount of asset recognized for present right to economic benefit.

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