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Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Assets	$ 62,283,238	$ 55,045,710
Other Insurance Operations [Member]
Segment Reporting Information [Line Items]
Assets	11,140,935	9,086,197
Segment assets [Member]
Segment Reporting Information [Line Items]
Assets	51,142,303	45,959,513
Segment assets [Member] | Investing [Member]
Segment Reporting Information [Line Items]
Assets	34,442,388	30,542,282
Segment assets [Member] | Underwriting Segments [Member]
Segment Reporting Information [Line Items]
Assets	10,818,395	9,897,689
Segment assets [Member] | Markel Ventures [Member]
Segment Reporting Information [Line Items]
Assets	$ 5,881,520	$ 5,519,542

X

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Amount of asset recognized for present right to economic benefit.

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