Segment Reporting Disclosures (Company's Segment Disclosures) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross written premiums		$ 8,779,971		$ 7,864,469		$ 5,506,960
Net written premiums		5,412,071		4,787,578		4,417,787
Earned premiums		5,049,793		4,712,060		4,247,978
Losses and loss adjustment expenses, current accident year		(3,426,441)		(3,371,699)		(3,367,223)
Losses and loss adjustment expenses, prior accident years		535,251		550,984		501,462
Amortization of policy acquisition costs		(1,100,496)		(1,009,303)		(894,353)
Other operating expenses		(777,597)		(768,208)		(695,111)
Underwriting profit (loss)		280,510		113,834		(207,247)
Net investment income		451,888		434,215		405,709
Net investment gains (losses)		1,601,722		(437,596)		(5,303)
Products revenues		1,609,586		1,497,523		951,012
Services and other revenues		813,202		635,083		462,263
Products expenses		(1,455,245)		(1,413,248)		(850,449)
Services and other expenses		(675,679)		(474,924)		(458,621)
Amortization of intangible assets		(148,638)	[1]	(115,930)	[2]	(80,758)	[3]
Goodwill and Intangible Asset Impairment		0		(199,198)		0
Segment profit (loss)		2,477,346		39,759		216,606
Interest expense		(171,687)		(154,212)		(132,451)
Loss on early extinguishment of debt		(17,586)		0		0
Net foreign exchange gains (losses)		(2,265)		106,598		3,140
Income (Loss) Before Income Taxes		$ 2,285,808		$ (7,855)		$ 87,295
U.S. GAAP combined ratio		94.00%	[4]	98.00%	[5]	105.00%	[6]
Insurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums		$ 5,320,253		$ 4,749,166		$ 4,141,201
Net written premiums		4,444,702		3,904,773		3,439,796
Earned premiums		4,144,073		3,783,939		3,314,033
Losses and loss adjustment expenses, current accident year		(2,730,971)		(2,596,057)		(2,442,344)
Losses and loss adjustment expenses, prior accident years		462,124		502,260		500,627
Amortization of policy acquisition costs		(860,917)		(770,183)		(675,470)
Other operating expenses		(704,531)		(691,186)		(611,749)
Underwriting profit (loss)		309,778		228,773		85,097
Net investment income		0		0		0
Net investment gains (losses)		0		0		0
Products revenues		0				0
Services and other revenues		0		0		0
Products expenses		0		0		0
Services and other expenses		0		0		0
Amortization of intangible assets		0	[1]	0	[2]	0	[3]
Goodwill and Intangible Asset Impairment				0
Segment profit (loss)		$ 309,778		$ 228,773		$ 85,097
U.S. GAAP combined ratio		93.00%	[4]	94.00%	[5]	97.00%	[6]
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums		$ 1,114,153		$ 1,050,870		$ 1,112,101
Net written premiums		964,947		882,285		978,160
Earned premiums		903,587		928,574		934,114
Losses and loss adjustment expenses, current accident year		(695,470)		(775,642)		(924,879)
Losses and loss adjustment expenses, prior accident years		64,768		42,982		(7,803)
Amortization of policy acquisition costs		(239,579)		(239,120)		(218,883)
Other operating expenses		(73,305)		(75,081)		(82,567)
Underwriting profit (loss)		(39,999)		(118,287)		(300,018)
Net investment income		0		0		0
Net investment gains (losses)		0		0		0
Products revenues		0				0
Services and other revenues		0		0		0
Products expenses		0		0		0
Services and other expenses		0		0		0
Amortization of intangible assets		0	[1]	0	[2]	0	[3]
Goodwill and Intangible Asset Impairment				0
Segment profit (loss)		$ (39,999)		$ (118,287)		$ (300,018)
U.S. GAAP combined ratio		104.00%	[4]	113.00%	[5]	132.00%	[6]
Investing [Member]
Segment Reporting Information [Line Items]
Gross written premiums		$ 0		$ 0		$ 0
Net written premiums		0		0		0
Earned premiums		0		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		451,152		433,702		405,377
Net investment gains (losses)		1,601,722		(437,596)		(5,303)
Products revenues		0				0
Services and other revenues		0		0		0
Products expenses		0		0		0
Services and other expenses		0		0		0
Amortization of intangible assets		0	[1]	0	[2]	0	[3]
Goodwill and Intangible Asset Impairment				0
Segment profit (loss)		2,052,874		(3,894)		400,074
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Gross written premiums		0	[7]	0	[8]	0	[9]
Net written premiums		0	[7]	0	[8]	0	[9]
Earned premiums		0	[7]	0	[8]	0	[9]
Losses and loss adjustment expenses, current accident year		0	[7]	0	[8]	0	[9]
Losses and loss adjustment expenses, prior accident years		0	[7]	0	[8]	0	[9]
Amortization of policy acquisition costs		0	[7]	0	[8]	0	[9]
Other operating expenses		0	[7]	0	[8]	0	[9]
Underwriting profit (loss)		0	[7]	0	[8]	0	[9]
Net investment income		736	[7]	513	[8]	332	[9]
Net investment gains (losses)		0	[7]	0	[8]	0	[9]
Products revenues		1,609,586	[7]	1,497,523	[8]	951,012	[9]
Services and other revenues		444,698	[7]	414,542	[8]	382,268	[9]
Products expenses		(1,455,245)	[7]	(1,413,248)	[8]	(850,449)	[9]
Services and other expenses		(389,385)	[7]	(366,739)	[8]	(336,484)	[9]
Amortization of intangible assets		(41,973)	[1],[7]	(40,208)	[2],[8]	(31,429)	[3],[9]
Goodwill and Intangible Asset Impairment	[7]			(14,904)
Segment profit (loss)		168,417	[7]	77,479	[8]	115,250	[9]
Depreciation expense		(53,600)	[7]	(52,200)	[8]	(41,700)	[9]
Other [Member]
Segment Reporting Information [Line Items]
Gross written premiums		2,345,565	[10]	2,064,433	[11]	253,658	[12]
Net written premiums		2,422	[10]	520	[11]	(169)	[12]
Earned premiums		2,133	[10]	(453)	[11]	(169)	[12]
Losses and loss adjustment expenses, current accident year		0	[10]	0	[11]	0	[12]
Losses and loss adjustment expenses, prior accident years		8,359	[10]	5,742	[11]	8,638	[12]
Amortization of policy acquisition costs		0	[10]	0	[11]	0	[12]
Other operating expenses		239	[10]	(1,941)	[11]	(795)	[12]
Underwriting profit (loss)		10,731	[10]	3,348	[11]	7,674	[12]
Net investment income		0	[10]	0	[11]	0	[12]
Net investment gains (losses)		0	[10]	0	[11]	0	[12]
Products revenues		0	[10],[11]			0	[12]
Services and other revenues		368,504	[10]	220,541	[11]	79,995	[9]
Products expenses		0	[10]	0	[11]	0	[12]
Services and other expenses		(286,294)	[10]	(108,185)	[11]	(122,137)	[12]
Amortization of intangible assets		(106,665)	[1],[10]	(75,722)	[2],[11]	(49,329)	[3],[12]
Goodwill and Intangible Asset Impairment	[11]			(184,294)
Segment profit (loss)		$ (13,724)	[10]	$ (144,312)	[11]	$ (83,797)	[12]
U.S. GAAP combined ratio - not meaniningful		NM	[4],[10],[13]	NM	[5],[11],[14]	NM	[6],[12],[15]

[1] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to the Company's underwriting segments.

[2] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to the Company's underwriting segments.

[3] Segment profit for the Markel Ventures segment includes amortization of intangible assets attributable to Markel Ventures. Amortization of intangible assets is not allocated to the Company's underwriting segments.

[4] 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.

[5] 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.

[6] 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.

[7] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $53.6 million for the year ended December 31, 2019.

[8] 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

[9] 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.

[10] 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.

[11] 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.

[12] 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.

[13] NM - Ratio is not meaningful

[14] NM - Ratio is not meaningful

[15] NM - Ratio is not meaningful