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Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions	Apr. 30, 2020	Jun. 30, 2020	Feb. 21, 2019	Dec. 31, 2018
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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