Segment Reporting Disclosures (Company's Segment Disclosures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross premium volume	$ 7,864,469		$ 5,506,960		$ 4,796,645
Net written premiums	4,787,578		4,417,787		4,001,020
Earned premiums	4,712,060		4,247,978		3,865,870
Losses and loss adjustment expenses, current accident year	(3,371,699)		(3,367,223)		(2,555,902)
Losses and loss adjustment expenses, prior accident years	550,984		501,462		505,158
Amortization of policy acquisition costs	(1,009,303)		(894,353)		(782,221)
Other operating expenses	(768,208)		(695,111)		(714,904)
Underwriting profit (loss)	113,834		(207,247)		318,001
Net investment income	434,215		405,709		373,230
Net investment gains (losses)	(437,596)	[1]	(5,303)	[2]	65,147	[3]
Products Revenues	1,497,523		951,012		885,473
Services and other revenues	635,083		462,263		422,306
Products expenses	(1,413,248)	[4]	(850,449)	[5]	(755,591)	[6]
Services and other expenses	(474,924)	[4]	(458,621)	[5]	(416,141)	[6]
Amortization of intangible assets	(115,930)	[7]	(80,758)	[8]	(68,533)	[9]
Impairment of Goodwill and Intangible Assets	(199,198)		0		(18,723)
Segment profit (loss)	39,759		216,606		805,169
Interest expense	(154,212)		(132,451)		(129,896)
Loss on early extinguishment of debt	0		0		(44,100)
Net foreign exchange gains (losses)	106,598		3,140		(1,253)
Income (Loss) Before Income Taxes	$ (7,855)		$ 87,295		$ 629,920
U.S. GAAP combined ratio	98.00%	[10]	105.00%	[11]	92.00%	[12]
Depreciation expense	$ (80,900)		$ (71,600)		$ (64,800)
Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume	4,749,166		4,141,201		3,755,081
Net written premiums	3,904,773		3,439,796		3,101,657
Earned premiums	3,783,939		3,314,033		3,028,844
Losses and loss adjustment expenses, current accident year	(2,596,057)		(2,442,344)		(2,009,426)
Losses and loss adjustment expenses, prior accident years	502,260		500,627		369,594
Amortization of policy acquisition costs	(770,183)		(675,470)		(592,766)
Other operating expenses	(691,186)		(611,749)		(597,201)
Underwriting profit (loss)	228,773		85,097		199,045
Net investment income	0		0		0
Net investment gains (losses)	0	[1]	0	[2]	0	[3]
Products Revenues	0				0
Services and other revenues	0		0		0
Products expenses	0	[4]	0	[5]	0	[6]
Services and other expenses	0	[4]	0	[5]	0	[6]
Amortization of intangible assets	0	[7]	0	[8]	0	[9]
Impairment of Goodwill and Intangible Assets	0				0
Segment profit (loss)	$ 228,773		$ 85,097		$ 199,045
U.S. GAAP combined ratio	94.00%	[10]	97.00%	[11]	93.00%	[12]
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume	$ 1,050,870		$ 1,112,101		$ 1,041,055
Net written premiums	882,285		978,160		898,728
Earned premiums	928,574		934,114		836,264
Losses and loss adjustment expenses, current accident year	(775,642)		(924,879)		(546,476)
Losses and loss adjustment expenses, prior accident years	42,982		(7,803)		125,514
Amortization of policy acquisition costs	(239,120)		(218,883)		(189,455)
Other operating expenses	(75,081)		(82,567)		(116,642)
Underwriting profit (loss)	(118,287)		(300,018)		109,205
Net investment income	0		0		0
Net investment gains (losses)	0	[1]	0	[2]	0	[3]
Products Revenues	0				0
Services and other revenues	0		0		0
Products expenses	0	[4]	0	[5]	0	[6]
Services and other expenses	0	[4]	0	[5]	0	[6]
Amortization of intangible assets	0	[7]	0	[8]	0	[9]
Impairment of Goodwill and Intangible Assets	0				0
Segment profit (loss)	$ (118,287)		$ (300,018)		$ 109,205
U.S. GAAP combined ratio	113.00%	[10]	132.00%	[11]	87.00%	[12]
Investing [Member]
Segment Reporting Information [Line Items]
Gross premium volume	$ 0		$ 0		$ 0
Net written premiums	0		0		0
Earned premiums	0				0
Losses and loss adjustment expenses, current accident year	0		0		0
Losses and loss adjustment expenses, prior accident years	0		0		0
Amortization of policy acquisition costs	0		0		0
Other operating expenses	0		0		0
Underwriting profit (loss)	0		0		0
Net investment income	433,702		405,377		373,121
Net investment gains (losses)	(437,596)	[1]	(5,303)	[2]	65,147	[3]
Products Revenues	0				0
Services and other revenues	0		0		0
Products expenses	0	[4]	0	[5]	0	[6]
Services and other expenses	0	[4]	0	[5]	0	[6]
Amortization of intangible assets	0	[7]	0	[8]	0	[9]
Impairment of Goodwill and Intangible Assets	0				0
Segment profit (loss)	(3,894)		400,074		438,268
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Gross premium volume	0		0		0
Net written premiums	0		0		0
Earned premiums	0				0
Losses and loss adjustment expenses, current accident year	0		0		0
Losses and loss adjustment expenses, prior accident years	0		0		0
Amortization of policy acquisition costs	0		0		0
Other operating expenses	0		0		0
Underwriting profit (loss)	0		0		0
Net investment income	513		332		109
Net investment gains (losses)	0	[1]	0	[2]	0	[3]
Products Revenues	1,497,523		951,012		885,473
Services and other revenues	414,542		382,268		328,976
Products expenses	(1,413,248)	[4]	(850,449)	[5]	(755,591)	[6]
Services and other expenses	(366,739)	[4]	(336,484)	[5]	(297,841)	[6]
Amortization of intangible assets	(40,208)	[7]	(31,429)	[8]	(29,105)	[9]
Impairment of Goodwill and Intangible Assets	(14,904)				(18,723)
Segment profit (loss)	77,479		115,250		113,298
Depreciation expense	(52,200)	[4]	(41,700)	[5]	(35,500)	[6]
Other [Member]
Segment Reporting Information [Line Items]
Gross premium volume	2,064,433	[13]	253,658	[14]	509	[15]
Net written premiums	520	[13]	(169)	[14]	635	[15]
Earned premiums	(453)	[13]	(169)	[14]	762	[15]
Losses and loss adjustment expenses, current accident year	0	[13]	0	[14]	0	[15]
Losses and loss adjustment expenses, prior accident years	5,742	[13]	8,638	[14]	10,050	[15]
Amortization of policy acquisition costs	0	[13]	0	[14]	0	[15]
Other operating expenses	(1,941)	[13]	(795)	[14]	(1,061)	[15]
Underwriting profit (loss)	3,348	[13]	7,674	[14]	9,751	[15]
Net investment income	0	[13]		[14]		[15]
Net investment gains (losses)	0	[1],[13]		[2],[14]		[3],[15]
Products Revenues	0	[13],[14]			0	[15]
Services and other revenues	220,541	[13]	79,995	[14]	93,330	[15]
Products expenses	0	[4],[13]	0	[5],[14]	0	[6],[15]
Services and other expenses	(108,185)	[4],[13]	(122,137)	[5],[14]	(118,300)	[6],[15]
Amortization of intangible assets	(75,722)	[7],[13]	(49,329)	[8],[14]	(39,428)	[9],[15]
Impairment of Goodwill and Intangible Assets	(184,294)	[13]			0	[15]
Segment profit (loss)	$ (144,312)	[13]	$ (83,797)	[14]	$ (54,647)	[15]
U.S. GAAP combined ratio - not meaniningful	NM	[10],[13],[16]	NM	[11],[14],[17]	NM	[12],[15],[18]

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

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

[4] 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.

[5] 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.

[6] 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.

[7] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to any other reportable segments.

[8] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to any other reportable segments.

[9] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to any other reportable segments.

[10] The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.

[11] The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.

[12] The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.

[13] Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment as well as any amortization of intangible assets and impairment of goodwill and intangible assets that are not allocated to a reportable segment.

[14] Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment as well as any amortization of intangible assets that is not allocated to a reportable segment.

[15] Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment as well as any amortization of intangible assets and impairment of goodwill and intangible assets that are not allocated to a reportable segment.

[16] NM - Ratio is not meaningful

[17] NM - Ratio is not meaningful

[18] NM - Ratio is not meaningful