Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Assets	$ 64,599,055	$ 61,897,982
Other Insurance Operations [Member]
Segment Reporting Information [Line Items]
Assets	12,645,919	11,017,176
Segment assets [Member]
Segment Reporting Information [Line Items]
Assets	51,953,136	50,880,806
Segment assets [Member] \| Investing [Member]
Segment Reporting Information [Line Items]
Assets	34,663,214	34,272,049
Segment assets [Member] \| Underwriting Segments [Member]
Segment Reporting Information [Line Items]
Assets	11,260,596	10,784,528
Segment assets [Member] \| Markel Ventures [Member]
Segment Reporting Information [Line Items]
Assets	$ 6,029,326	$ 5,824,229

Amount of asset recognized for present right to economic benefit.

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+ Details