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, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net investment income																										$ 434,215		$ 405,709		$ 373,230
Change in fair value of equity securities	[1]																									(425,622)		(44,888)		16,791
Net investment gains (losses)																										(437,596)	[2]	(5,303)	[3]	65,147	[4]
Total Operating Revenues		$ 1,042,852		$ 2,235,949		$ 1,987,013		$ 1,575,471		$ 1,662,267		$ 1,506,148		$ 1,481,493		$ 1,411,751		$ 1,428,625		$ 1,431,282		$ 1,375,937		$ 1,376,182		6,841,285		6,061,659		5,612,026
Services and other expenses																										474,924	[5]	458,621	[6]	416,141	[7]
Interest expense																										154,212		132,451		129,896
Net foreign exchange gains (losses)																										106,598		3,140		(1,253)
Loss on early extinguishment of debt																										0		0		44,100
Total operating expenses																										6,801,526		5,845,053		4,806,857
Income tax benefit																										122,498		(313,463)		169,477
Net Income (Loss) to Shareholders		(751,543)	[8]	409,438	[8]	278,231	[8]	(64,306)	[8]	434,881	[8]	(259,141)	[8]	149,660	[8]	69,869	[8]	132,726	[8]	83,796	[8]	78,797	[8]	160,370	[8]	(128,180)	[9]	395,269	[9]	455,689	[9]
Net holding gains (losses) arising during the period																										(241,325)		787,339		275,661
Reclassification adjustments for net gains included in net income to shareholders																										7,849		(24,296)		(33,528)
Change in net unrealized gains on available-for-sale investments, net of taxes																										(233,476)		763,043		242,168
Change in foreign currency translation adjustments, net of taxes																										(16,495)		10,449		(11,704)
Total Other Comprehensive Income (Loss)																										(247,630)		779,751		211,364
Comprehensive Income (Loss) to Shareholders		$ (680,373)		$ 315,106		$ 164,336		$ (174,839)		$ 629,247		$ (19,869)		$ 342,357		$ 223,239		$ (29,050)		$ 89,161		$ 209,942		$ 396,994		(375,770)		1,174,974		667,047
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net investment income																										32,631		21,076		9,561
Dividends on common stock of consolidated subsidiaries																										749,171		895,920		349,622
Marketable Securities, Realized Gain (Loss)																										(3,341)		3,383		1,068
Change in fair value of equity securities	[10]																									(110,356)		0		0
Net investment gains (losses)																										(113,697)		3,383		1,068
Total Operating Revenues																										668,105		920,379		360,251
Services and other expenses																										6,532		11,708		13,076
Interest expense																										145,681		122,151		116,013
Net foreign exchange gains (losses)																										(3,391)		0		0
Loss on early extinguishment of debt																										0		0		44,100
Total operating expenses																										148,822		133,859		173,189
Income before equity in undistributed earnings of consolidated subsidiaries and income taxes																										519,283		786,520		187,062
Equity in undistributed earnings of consolidated subsidiaries																										(696,045)		(469,365)		196,615
Income tax benefit																										(48,582)		(78,114)		(72,012)
Net Income (Loss) to Shareholders																										(128,180)		395,269		455,689
Net holding gains (losses) arising during the period																										(1,492)		52,277		37,045
Consolidated subsidiaries' net holding gains (losses) arising during the period																										(239,833)		735,062		238,616
Reclassification adjustments for net gains included in net income to shareholders																										2,564		(1,513)		(523)
Consolidated subsidiaries' reclassification adjustments for net gains included in net income to shareholders																										5,285		(22,783)		(32,970)
Change in net unrealized gains on available-for-sale investments, net of taxes																										(233,476)		763,043		242,168
Change in foreign currency translation adjustments, net of taxes																										0		(2,260)		(1,326)
Consolidated subsidiaries' change in foreign currency translation adjustments, net of taxes																										(16,455)		12,663		(10,384)
Consolidated subsidiaries' change in net actuarial pension loss, net of taxes																										2,341		6,259		(19,100)
Total Other Comprehensive Income (Loss)																										(247,590)		779,705		211,358
Comprehensive Income (Loss) to Shareholders																										$ (375,770)		$ 1,174,974		$ 667,047

[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. See note 1. 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 investment losses within net income. Prior periods have not been restated to conform to the current presentation. See note 1.

[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 investment losses within net income. Prior periods have not been restated to conform to the current presentation. See note 1.

[4] 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 investment gains within net income. Prior periods have not been restated to conform to the current presentation. See note 1.

[5] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $52.2 million for the year ended December 31, 2018.

[6] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $41.7 million for the year ended December 31, 2017.

[7] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $35.5 million for the year ended December 31, 2016.

[8] 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. See note 1.

[9] Effective January 1, 2018, the Company adopted ASU No. 2016-01 and equity securities are no longer classified as available-for-sale with unrealized gains and losses recognized in other comprehensive income, rather, 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. See note 1.

[10] 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. See note 1.