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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Other Revenues and Other Expenses [Line Items]
Other revenues	$ 312,841	$ 250,357	$ 618,864	$ 510,415
Other expenses	277,909	242,236	553,002	473,237
Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	2,013	3,937	8,048	11,126
Other expenses	10,020	5,950	20,299	15,608
Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	310,828	246,420	610,816	499,289
Other expenses	267,889	236,286	532,703	457,629
Managing General Agent Operations [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	1,567	3,118	7,107	8,988
Other expenses	2,821	2,602	5,099	4,911
Life And Annuity [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	446	350	941	327
Other expenses	7,199	3,348	15,200	10,697
Other Insurance [Member] | Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	0	469	0	1,811
Other expenses	0	0	0	0
Manufacturing [Member] | Non-Insurance Operations [Member] | Markel Ventures Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	193,152	174,141	385,843	351,903
Other expenses	159,227	168,580	319,593	320,380
Non-Manufacturing [Member] | Non-Insurance Operations [Member] | Markel Ventures Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	104,602	65,412	198,430	133,093
Other expenses	91,685	63,013	180,118	126,843
Investment Management [Member] | Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	7,350	0	14,523	0
Other expenses	10,836	0	20,766	0
Other Non-Insurance [Member] | Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	5,724	6,867	12,020	14,293
Other expenses	$ 6,141	$ 4,693	$ 12,226	$ 10,406