Variable Interest Entities Schedule of Variable interest Entities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Other assets	$ 1,383,823	$ 1,223,365
Other liabilities	1,796,036	1,748,460
Markel Diversified Fund [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Investment in one of the unconsolidated funds	27,547	168,192
Other assets	1,082	2,059
Variable Interest Entity, Consolidated, Carrying Amount, Assets	28,629	170,251
Other liabilities	200	168
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	25,075	62,668
Other Debt [Member] | Markel Diversified Fund [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Note Payable	$ 24,875	$ 62,500