Markel Corporation (Parent Company Only) Financial Information (Schedule Of Condensed Statements Of Income And Comprehensive Income) (Details) - USD ($) $ in Thousands		3 Months Ended																								12 Months Ended
		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net investment income																										$ 451,888		$ 434,215		$ 405,709
Change in fair value of equity securities	[1]																									1,603,204		(425,622)		(44,888)
Net investment gains (losses)																										1,601,722		(437,596)		(5,303)
Total Operating Revenues		$ 2,581,529		$ 2,033,058		$ 2,439,116		$ 2,472,488		$ 1,042,852		$ 2,235,949		$ 1,987,013		$ 1,575,471		$ 1,662,267		$ 1,506,148		$ 1,481,493		$ 1,411,751		9,526,191		6,841,285		6,061,659
Services and other expenses																										675,679		474,924		458,621
Interest expense																										171,687		154,212		132,451
Net foreign exchange gains (losses)																										(2,265)		106,598		3,140
Loss on early extinguishment of debt																										17,586		0		0
Total operating expenses																										7,048,845		6,801,526		5,845,053
Income tax (expense) benefit																										(486,346)		(122,498)		313,463
Net Income (Loss) to Shareholders		511,104	[2]	205,637	[2]	497,298	[2]	576,427	[2]	(751,543)	[2]	409,438	[2]	278,231	[2]	(64,306)	[2]	434,881	[2]	(259,141)	[2]	149,660	[2]	69,869	[2]	1,790,466	[3]	(128,180)	[3]	395,269	[3]
Net holding gains (losses) arising during the period																										299,125		(241,325)		787,339
Reclassification adjustments for net gains (losses) included in net income to shareholders																										(1,148)		7,849		(24,296)
Change in net unrealized gains on available-for-sale investments, net of taxes																										297,977		(233,476)		763,043
Change in foreign currency translation adjustments, net of taxes																										382		(16,495)		10,449
Total Other Comprehensive Income (Loss)																										303,401		(247,630)		779,751
Comprehensive Income (Loss) to Shareholders		$ 488,244		$ 250,069		$ 623,330		$ 732,245		$ (680,373)		$ 315,106		$ 164,336		$ (174,839)		$ 629,247		$ (19,869)		$ 342,357		$ 223,239		2,093,888		(375,770)		1,174,974
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net investment income																										48,845		32,631		21,076
Dividends on common stock of consolidated subsidiaries																										863,335		749,171		895,920
Marketable Securities, Realized Gain (Loss)																										3,848		(3,341)		3,383
Change in fair value of equity securities	[4]																									293,296		(110,356)		0
Net investment gains (losses)																										297,144		(113,697)		3,383
Total Operating Revenues																										1,209,324		668,105		920,379
Services and other expenses																										6,436		6,532		11,708
Interest expense																										219,082		145,681		122,151
Net foreign exchange gains (losses)																										3,973		(3,391)		0
Loss on early extinguishment of debt																										13,656		0		0
Total operating expenses																										243,147		148,822		133,859
Income before equity in undistributed earnings of consolidated subsidiaries and income taxes																										966,177		519,283		786,520
Equity in undistributed earnings of consolidated subsidiaries																										851,337		(696,045)		(469,365)
Income tax (expense) benefit																										(27,048)		48,582		78,114
Net Income (Loss) to Shareholders	[4]																									1,790,466		(128,180)		395,269
Net holding gains (losses) arising during the period																										14,016		(1,492)		52,277
Consolidated subsidiaries' net holding gains (losses) arising during the period																										285,109		(239,833)		735,062
Reclassification adjustments for net gains (losses) included in net income to shareholders																										(4,591)		2,564		(1,513)
Consolidated subsidiaries' reclassification adjustments for net gains (losses) included in net income to shareholders																										3,443		5,285		(22,783)
Change in net unrealized gains on available-for-sale investments, net of taxes																										297,977		(233,476)		763,043
Change in foreign currency translation adjustments, net of taxes																										0		0		(2,260)
Consolidated subsidiaries' change in foreign currency translation adjustments, net of taxes																										403		(16,455)		12,663
Consolidated subsidiaries' change in net actuarial pension loss, net of taxes																										5,042		2,341		6,259
Total Other Comprehensive Income (Loss)																										303,422		(247,590)		779,705
Comprehensive Income (Loss) to Shareholders																										$ 2,093,888		$ (375,770)		$ 1,174,974

[1] 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. Prior to adopting ASU No. 2016-01, the Company recorded certain investments in equity securities at estimated fair value with changes in fair value recorded in net income.

[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, 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.

[4] 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.