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Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022 | Mar. 31, 2022
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Unconsolidated Markel CATCo Fund 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Redemption of remaining investment, Markel CATCo Re Fund |  |  | $ 41,300
Fair Value, Nonrecurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets, Fair Value Disclosure | $ 0 | $ 0 |
Financial and Nonfinancial Liabilities, Fair Value Disclosure | $ 0 | $ 0 |

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