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Segment Reporting Disclosures (Reconciliation Of Segment Assets To The Company's Consolidated Balance Sheets (Details) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Segment Reporting Information [Line Items] |  |
Assets | $ 39,067,626 | $ 37,473,815
Other [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 5,999,200 | 6,171,708
Segment assets [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 33,068,426 | 31,302,107
Segment assets [Member] | Investing [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 22,097,531 | 22,129,633
Segment assets [Member] | Underwriting [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 7,229,312 | 6,621,639
Segment assets [Member] | Markel Ventures [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | $ 3,741,583 | $ 2,550,835

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