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Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions | Apr. 30, 2020 | Jun. 30, 2020 | Feb. 21, 2019 | Dec. 31, 2018
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Markel Ventures [Member] | Industrial products [Member] |  |  |  |
Loss Contingencies [Line Items] |  |  |  |
Loss contingency, estimate of possible loss |  | $ 20.2 |  | $ 33.5
Anthony Belisle v. Markel CATCo Investment Management Ltd and Markel Corp. [Member] |  |  |  |
Loss Contingencies [Line Items] |  |  |  |
Asserted claims |  |  | $ 66.0 |
Lansing Building Products, LLC [Member] |  |  |  |
Loss Contingencies [Line Items] |  |  |  |
Total purchase consideration | $ 556.2 |  |  |
Lodgepine Fund Limited [Member] |  |  |  |
Loss Contingencies [Line Items] |  |  |  |
Other commitment |  | $ 100.0 |  |

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- Definition Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions. + References No definition available. + Details
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