Markel Corporation (Parent Company Only) Financial Information (Schedule Of Condensed Statements Of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net income to shareholders	$ 197,892	$ 102,519	$ 91,369	$ 190,992	$ 117,595	$ 75,803	$ 40,068	$ 87,716	$ 98,764	$ 65,599	$ 27,756	$ 88,902	$ 582,772	$ 321,182	$ 281,021
Net Cash Provided By Operating Activities													651,151	716,792	745,521
Proceeds from sales of fixed maturities and equity securities													538,978	1,286,871	879,564
Proceeds from maturities, calls and prepayments of fixed maturities													1,503,616	1,420,817	1,475,938
Cost of fixed maturities and equity securities purchased													(1,576,254)	(3,153,055)	(1,651,397)
Net change in short-term investments													(62,124)	(129,164)	(470,423)
Cost of equity method investments													(21,849)	(16,081)	(38,018)
Change in restricted cash and cash equivalents													62,324	264,701	(263,014)
Additions to property and equipment													(79,755)	(82,132)	(47,725)
Other													(797)	(2,368)	1,103
Net Cash Provided (Used) By Investing Activities													125,773	(622,205)	187,387
Additions to senior long-term debt													69,797	89,480	547,214
Repurchases of common stock													(31,491)	(26,053)	(57,388)
Issuance of common stock													4,752	5,691	24,518
Other													(10,488)	(21,357)	(23)
Net Cash Provided (Used) By Financing Activities													(74,211)	(67,124)	175,367
Increase (decrease) in cash and cash equivalents													669,840	(18,357)	1,114,760
Cash and cash equivalents at beginning of year				1,960,169				1,978,526				863,766	1,960,169	1,978,526	863,766
CASH AND CASH EQUIVALENTS AT END OF YEAR	2,630,009				1,960,169				1,978,526				2,630,009	1,960,169	1,978,526
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income to shareholders													582,772	321,182	281,021
Adjustments to reconcile net income to shareholders to net cash provided by operating activities													(464,193)	(218,396)	186,574
Net Cash Provided By Operating Activities													118,579	102,786	467,595
Proceeds from sales of fixed maturities and equity securities													100,633	9,306	142,259
Proceeds from maturities, calls and prepayments of fixed maturities													24,945	15,710	2,819
Cost of fixed maturities and equity securities purchased													(55,656)	(687)	(23,412)
Net change in short-term investments													9,956	(109,728)	10,251
Securities received from subsidiaries as dividends or repayment of notes receivable													0	89,996	249,996
Decrease in notes receivable due from subsidiaries													0	28,506	5,302
Capital contributions to subsidiaries													(228,578)	(74,788)	(67,878)
Acquisitions													0	0	(1,017,988)
Cost of equity method investments													(13,164)	0	(5,291)
Change in restricted cash and cash equivalents													289	51	(348)
Additions to property and equipment													(305)	(342)	(3,653)
Other													(376)	(2,150)	3,207
Net Cash Provided (Used) By Investing Activities													(162,256)	(44,126)	(704,736)
Additions to senior long-term debt													0	0	491,235
Increase in notes payable to subsidiaries													285,000	0	0
Repayment and retirement of senior long-term debt													(2,000)	0	(246,665)
Repurchases of common stock													(31,491)	(26,053)	(57,388)
Issuance of common stock													4,752	5,691	24,518
Other													3,985	(1,948)	(5,023)
Net Cash Provided (Used) By Financing Activities													260,246	(22,310)	206,677
Increase (decrease) in cash and cash equivalents													216,569	36,350	(30,464)
Cash and cash equivalents at beginning of year				$ 243,702				$ 207,352				$ 237,816	243,702	207,352	237,816
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ 460,271				$ 243,702				$ 207,352				$ 460,271	$ 243,702	$ 207,352