Variable Interest Entities Schedule of Variable Interest Entities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Other assets	$ 1,619,079	$ 1,383,823
Other liabilities	2,106,331	1,796,036
Markel Diversified Fund [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Investment in one of the unconsolidated Funds	18,847	27,547
Other assets	1,232	1,082
Variable Interest Entity, Consolidated, Carrying Amount, Assets	20,079	28,629
Other liabilities	200	200
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	25,075	25,075
Other Debt [Member] | Markel Diversified Fund [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Note Payable	$ 24,875	$ 24,875