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Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Sales | $ 0 | $ (4,027) | $ 0 | $ (52,807)
Unconsolidated Markel CATCo Fund 2 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Sales |  |  | (41,300) |
Fair value, nonrecurring [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Assets measured at fair value on a nonrecurring basis | 0 | 0 | 0 | 0
Liabilities measured at fair value on a nonrecurring basis | $ 0 | $ 0 | $ 0 | $ 0

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