Other Revenues And Other Expenses (Summary Of Other Revenues And Other Expenses By Component) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Other Revenues and Other Expenses [Line Items]
Other revenues	$ 330,993	$ 312,841	$ 638,909	$ 618,864
Other expenses	299,112	277,909	581,697	553,002
Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	2,388	2,013	7,898	8,048
Other expenses	9,982	10,020	20,150	20,299
Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	328,605	310,828	631,011	610,816
Other expenses	289,130	267,889	561,547	532,703
Managing General Agent Operations [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	1,617	1,567	6,275	7,107
Other expenses	1,493	2,821	3,346	5,099
Life And Annuity [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	771	446	1,207	941
Other expenses	7,169	7,199	14,233	15,200
Other Insurance [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	0	0	416	0
Other expenses	1,320	0	2,571	0
Manufacturing [Member] | Non-Insurance Operations [Member] | Markel Ventures Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	184,021	193,152	361,156	385,843
Other expenses	156,897	159,227	310,550	319,593
Non-Manufacturing [Member] | Non-Insurance Operations [Member] | Markel Ventures Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	129,576	104,602	239,376	198,430
Other expenses	114,504	91,685	212,115	180,118
Investment Management [Member] | Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	9,277	7,350	18,636	14,523
Other expenses	11,195	10,836	26,130	20,766
Other Non-Insurance [Member] | Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	5,731	5,724	11,843	12,020
Other expenses	$ 6,534	$ 6,141	$ 12,752	$ 12,226