Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2026	Dec. 31, 2025
Segment Reporting [Line Items]
Assets	$ 72,076,856	$ 68,905,050
Corporate and eliminations
Segment Reporting [Line Items]
Assets	3,552,631	4,397,391
Segment assets [Member]
Segment Reporting [Line Items]
Assets	68,524,225	64,507,659
Segment assets [Member] \| Markel Insurance
Segment Reporting [Line Items]
Assets	45,424,421	44,314,601
Segment assets [Member] \| Industrial
Segment Reporting [Line Items]
Assets	4,191,123	3,965,449
Segment assets [Member] \| Financial
Segment Reporting [Line Items]
Assets	17,139,760	14,444,426
Segment assets [Member] \| Consumer and Other
Segment Reporting [Line Items]
Assets	$ 1,768,921	$ 1,783,183

Amount of asset recognized for present right to economic benefit.

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+ Details

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details