Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Feb. 21, 2019	Dec. 31, 2018
Markel Ventures [Member] | Industrial products [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss	$ 21.4		$ 33.5
Anthony Belisle v. Markel CATCo Investment Management Ltd and Markel Corp. [Member]
Loss Contingencies [Line Items]
Asserted claims		$ 66.0
Alissa Fredricks v. Markel CATCo Investment Management Ltd. and Markel Corp. [Member]
Loss Contingencies [Line Items]
Asserted claims		$ 7.5
Lodgepine Fund Limited [Member]
Loss Contingencies [Line Items]
Other commitment	$ 100.0