Commitments And Contingencies (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Feb. 21, 2019	Dec. 31, 2018	Dec. 31, 2017
Industrial Products [Member] | Markel Ventures [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	$ 20.2		$ 33.5
Markel CATCo IM [Member]
Loss Contingencies [Line Items]
Accrued incentive and retention compensation				$ 34.9
Alissa Fredricks v. Markel CATCo Investment Management Ltd. and Markel Corp. [Member]
Loss Contingencies [Line Items]
Asserted claims		$ 7.5
Anthony Belisle v. Markel CATCo Investment Management Ltd and Markel Corp. [Member]
Loss Contingencies [Line Items]
Asserted claims		$ 66.0
Unconsolidated Markel CATCo Funds [Member]
Loss Contingencies [Line Items]
Other Commitment	90.0
Lodgepine Fund Limited [Member]
Loss Contingencies [Line Items]
Other Commitment	$ 100.0