Variable Interest Entities Schedule of Variable Interest Entities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Other assets	$ 1,798,797	$ 1,383,823
Other liabilities	2,261,343	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	11,320	27,547
Other assets	503	1,082
Variable Interest Entity, Consolidated, Carrying Amount, Assets	11,823	28,629
Other liabilities	99	200
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	24,974	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