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, 2019		Sep. 30, 2019	[1]	Jun. 30, 2019	[1]	Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) to Shareholders		$ 511,104	[1]	$ 205,637		$ 497,298		$ 576,427	[1]	$ (751,543)	[1]	$ 409,438		$ 278,231		$ (64,306)	[1]	$ 434,881	[1]	$ (259,141)		$ 149,660		$ 69,869	[1]	$ 1,790,466	[2]	$ (128,180)	[2]	$ 395,269	[2]
Net Cash Provided By Operating Activities																										1,274,120		892,857		858,529
Proceeds from sales of fixed maturities and equity securities																										353,918		419,199		577,650
Proceeds from maturities, calls and prepayments of fixed maturities																										772,336		551,808		1,129,895
Cost of fixed maturities and equity securities purchased																										(955,970)		(1,545,913)		(1,176,281)
Net change in short-term investments																										(95,867)		1,101,636		234,743
Acquisitions, net of cash acquired																										(245,332)		(1,175,211)		(1,431,712)
Cost of equity method investments																										(257,663)		(8,864)		(13,023)
Additions to property and equipment																										(123,376)		(106,593)		(74,652)
Other																										16,795		(33,301)		8,923
Net Cash Used By Investing Activities																										(535,159)		(797,239)		(744,457)
Additions to senior long-term debt																										1,645,182		206,949		664,657
Repayment and retirement of senior long-term debt																										(1,103,674)		(289,199)		(259,972)
Premiums and fees related to early extinguishment of debt																										(27,073)		0		0
Repurchases of common stock																										(116,307)		(54,007)		(110,838)
Other																										(5,952)		(4,127)		(6,281)
Net Cash Provided (Used) By Financing Activities																										359,317		(178,985)		256,315
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents																										1,103,921		(104,414)		415,682
Cash, cash equivalents, restricted cash and restricted cash equivalents at beginning of year								2,396,432								2,500,846								2,085,164		2,396,432		2,500,846		2,085,164
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR		3,500,353								2,396,432								2,500,846								3,500,353		2,396,432		2,500,846
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) to Shareholders	[3]																									1,790,466		(128,180)		395,269
Adjustments to reconcile net income (loss) to shareholders to net cash provided (used) by operating activities																										(1,530,940)		3,637		(166,132)
Net Cash Provided By Operating Activities																										259,526		(124,543)		229,137
Proceeds from sales of fixed maturities and equity securities																										326,564		204,478		20,562
Proceeds from maturities, calls and prepayments of fixed maturities																										41,673		34,560		64,705
Cost of fixed maturities and equity securities purchased																										(82,332)		(26,336)		(72,910)
Net change in short-term investments																										(236,251)		930,876		649,181
Return of capital from subsidiaries																										14,865		12,712		45,225
Decrease (increase) in notes receivable due from subsidiaries																										100,000		(20,000)		(58)
Capital contributions to subsidiaries																										(413,148)		(103,133)		(270,623)
Acquisitions, net of cash acquired																										0		(972,619)		(1,153,683)
Cost of equity method investments																										(213,100)		(4,917)		0
Other																										6,719		(8,652)		(3,661)
Net Cash Used By Investing Activities																										(455,010)		46,969		(721,262)
Additions to senior long-term debt																										1,384,182		0		592,923
IncreaseDecreaseInNotesPayableFromSubsidiaries																										(99,839)		47,105		0
Repayment and retirement of senior long-term debt																										(484,811)		0		0
Premiums and fees related to early extinguishment of debt																										(13,248)		0		0
Repurchases of common stock																										(116,307)		(54,007)		(110,838)
Other																										(2,564)		(70)		(9,848)
Net Cash Provided (Used) By Financing Activities																										667,413		(6,972)		472,237
Increase (decrease) in cash, cash equivalents, restricted cash and restricted cash equivalents																										471,929		(84,546)		(19,888)
Cash, cash equivalents, restricted cash and restricted cash equivalents at beginning of year								$ 266,220								$ 350,766								$ 370,654		266,220		350,766		370,654
CASH, CASH EQUIVALENTS, RESTRICTED CASH AND RESTRICTED CASH EQUIVALENTS AT END OF YEAR		$ 738,149								$ 266,220								$ 350,766								$ 738,149		$ 266,220		$ 350,766

[1] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, the change in fair value of equity securities is no longer recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net income. Prior periods have not been restated to conform to the current presentation.

[2] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, the change in fair value of equity securities is no longer recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net income. Prior periods have not been restated to conform to the current presentation.

[3] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, equity securities are no longer classified as available-for-sale with unrealized gains and losses recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net income. Prior periods have not been restated to conform to the current presentation.