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Variable Interest Entities (Narrative) (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
Jan. 01, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019
Variable Interest Entity [Line Items] |  |  |  |  |  |  |
Investment in one of the unconsolidated funds, variable interest entity | $ 90,000 | $ 0 | $ 90,000 | $ 500 | $ 90,000 | $ 500 |
Assets |  | 39,067,626 |  |  | 39,067,626 |  | $ 37,473,815
Markel Diversified Fund [Member] | Variable Interest Entity, Primary Beneficiary [Member] |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |
Assets |  | 13,500 |  |  | 13,500 |  | 19,600
Unconsolidated Markel CATCo Fund 2 [Member] |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |
Variable interest entity, consolidated, carrying amount, investment in one of the unconsolidated funds |  | 102,500 |  |  | 102,500 |  | 26,800
Unconsolidated Markel CATCo Funds [Member] |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |
Net assets under management of MCIM for unconsolidated VIEs |  | $ 2,000,000 |  |  | $ 2,000,000 |  | $ 2,700,000

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