Variable Interest Entities Schedule of Variable Interest Entities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Other assets	$ 1,879,695	$ 1,383,823
Other liabilities	2,247,179	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	16,866	27,547
Other assets	198	1,082
Variable interest entity, consolidated, carrying amount, Assets	17,064	28,629
Other liabilities	65	200
Variable interest entity, consolidated, carrying amount, Liabilities	24,940	25,075
Variable interest entity, consolidated, carrying amount, Equity	(7,876)	3,554
Variable interest entity, consolidated, carrying amount, Liabilities and Equity	17,064	28,629
Other debt [Member] | Markel Diversified Fund [Member]
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, Note payable	$ 24,875	$ 24,875