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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other Revenues and Other Expenses [Line Items]
Other revenues	$ 250,357	$ 194,083	$ 510,415	$ 380,254
Other expenses	242,236	184,942	473,237	367,110
Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	3,937	6,192	11,126	17,829
Other expenses	5,950	15,344	15,608	29,201
Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	246,420	187,891	499,289	362,425
Other expenses	236,286	169,598	457,629	337,909
Managing General Agent Operations [Member] \| Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	3,118	4,774	8,988	13,952
Other expenses	2,602	5,218	4,911	10,099
Life And Annuity [Member] \| Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	350	143	327	186
Other expenses	3,348	8,670	10,697	17,285
Other Insurance [Member] \| Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	469	1,275	1,811	3,691
Other expenses	0	1,456	0	1,817
Manufacturing [Member] \| Non-Insurance Operations [Member] \| Markel Ventures Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	174,141	125,083	351,903	225,694
Other expenses	168,580	110,901	320,380	206,790
Non-Manufacturing [Member] \| Non-Insurance Operations [Member] \| Markel Ventures Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	65,412	58,559	133,093	129,154
Other expenses	63,013	55,409	126,843	120,920
Other Non-Insurance [Member] \| Non-Insurance Operations [Member]
Other Revenues and Other Expenses [Line Items]
Other revenues	6,867	4,249	14,293	7,577
Other expenses	$ 4,693	$ 3,288	$ 10,406	$ 10,199