Segment Reporting Disclosures (Company's Segment Disclosures) (Details) $ in Thousands		3 Months Ended				9 Months Ended
		Sep. 30, 2019 USD ($)		Sep. 30, 2018 USD ($)		Sep. 30, 2019 USD ($) segment		Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable ongoing underwriting segments | segment						2
Gross premium volume		$ 2,264,492		$ 2,043,771		$ 6,766,681		$ 6,088,088
Net written premiums		1,370,384		1,216,271		4,153,115		3,731,679
Earned premiums		1,300,032		1,185,323		3,703,470		3,484,528
Losses and loss adjustment expenses, current accident year		(900,230)		(860,986)		(2,454,912)		(2,340,868)
Losses and loss adjustment expenses, prior accident years		148,096		124,140		336,912		389,726
Amortization of policy acquisition costs		(280,635)		(241,848)		(805,527)		(733,003)
Other operating expenses		(194,584)		(200,113)		(587,220)		(584,918)
Underwriting profit		72,679		6,516		192,723		215,465
Net investment income		113,382		106,307		339,395		319,710
Net investment gains (losses)		32,144		426,075		1,069,988		408,326
Products revenues		386,708		362,577		1,237,178		1,129,036
Services and other revenues		200,792		155,667		594,631		456,833
Products expenses		(354,404)		(341,718)		(1,098,968)		(1,062,000)
Services and other expenses		(153,358)		(132,024)		(498,760)		(394,311)
Amortization of intangible assets		(35,695)	[1]	(27,795)	[2]	(112,663)	[3]	(86,259)	[4]
Impairment of goodwill and intangible assets		0		0		0		(14,904)
Segment profit (loss)		262,248		555,605		1,723,524		971,896
Interest expense		(47,465)		(37,961)		(129,022)		(114,722)
Net foreign exchange losses (gains)		53,850		1,383		57,001		65,427
Loss on early extinguishment of debt		(6,705)		0		(6,705)		0
Income before income taxes		$ 261,928		$ 519,027		$ 1,644,798		$ 922,601
U.S. GAAP combined ratio		94.00%	[5]	99.00%	[6]	95.00%	[7]	94.00%	[8]
Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 1,418,363		$ 1,248,150		$ 3,979,559		$ 3,575,340
Net written premiums		1,181,919		1,021,684		3,306,447		2,935,789
Earned premiums		1,058,869		961,062		3,023,865		2,782,107
Losses and loss adjustment expenses, current accident year		(722,172)		(674,529)		(1,998,042)		(1,855,190)
Losses and loss adjustment expenses, prior accident years		135,029		111,117		309,324		369,775
Amortization of policy acquisition costs		(218,710)		(182,857)		(627,318)		(554,185)
Other operating expenses		(171,731)		(172,340)		(526,884)		(523,239)
Underwriting profit		81,285		42,453		180,945		219,268
Net investment income		0		0		0		0
Net investment gains (losses)		0		0		0		0
Products revenues		0		0		0		0
Services and other revenues		0		0		0		0
Products expenses		0		0		0		0
Services and other expenses		0		0		0		0
Amortization of intangible assets		0	[1]	0	[2]	0	[3]	0	[4]
Impairment of goodwill and intangible assets								0
Segment profit (loss)		$ 81,285		$ 42,453		$ 180,945		$ 219,268
U.S. GAAP combined ratio		92.00%	[5]	96.00%	[6]	94.00%	[7]	92.00%	[8]
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 226,387		$ 234,360		$ 963,145		$ 935,498
Net written premiums		187,180		195,349		844,949		795,136
Earned premiums		240,144		225,277		678,382		702,854
Losses and loss adjustment expenses, current accident year		(178,058)		(186,505)		(456,870)		(485,678)
Losses and loss adjustment expenses, prior accident years		13,800		12,681		20,695		18,135
Amortization of policy acquisition costs		(61,925)		(58,991)		(178,209)		(178,818)
Other operating expenses		(20,436)		(26,125)		(58,301)		(59,433)
Underwriting profit		(6,475)		(33,663)		5,697		(2,940)
Net investment income		0		0		0		0
Net investment gains (losses)		0		0		0		0
Products revenues		0		0		0		0
Services and other revenues		0		0		0		0
Products expenses		0		0		0		0
Services and other expenses		0		0		0		0
Amortization of intangible assets		0	[1]	0	[2]	0	[3]	0	[4]
Impairment of goodwill and intangible assets								0
Segment profit (loss)		$ (6,475)		$ (33,663)		$ 5,697		$ (2,940)
U.S. GAAP combined ratio		103.00%	[5]	115.00%	[6]	99.00%	[7]	100.00%	[8]
Investing [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 0		$ 0		$ 0		$ 0
Net written premiums		0		0		0		0
Earned premiums		0		0		0		0
Losses and loss adjustment expenses, current accident year		0		0		0		0
Losses and loss adjustment expenses, prior accident years		0		0		0		0
Amortization of policy acquisition costs		0		0		0		0
Other operating expenses		0		0		0		0
Underwriting profit		0		0		0		0
Net investment income		113,220		106,164		338,783		319,253
Net investment gains (losses)		32,144		426,075		1,069,988		408,326
Products revenues		0		0		0		0
Services and other revenues		0		0		0		0
Products expenses		0		0		0		0
Services and other expenses		0		0		0		0
Amortization of intangible assets		0	[1]	0	[2]	0	[3]	0	[4]
Impairment of goodwill and intangible assets								0
Segment profit (loss)		145,364		532,239		1,408,771		727,579
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Gross premium volume		0	[9]	0	[10]	0	[11]	0	[12]
Net written premiums		0	[9]	0	[10]	0	[11]	0	[12]
Earned premiums		0	[9]	0	[10]	0	[11]	0	[12]
Losses and loss adjustment expenses, current accident year		0	[9]	0	[10]	0	[11]	0	[12]
Losses and loss adjustment expenses, prior accident years		0	[9]	0	[10]	0	[11]	0	[12]
Amortization of policy acquisition costs		0	[9]	0	[10]	0	[11]	0	[12]
Other operating expenses		0	[9]	0	[10]	0	[11]	0	[12]
Underwriting profit		0	[9]	0	[10]	0	[11]	0	[12]
Net investment income		162	[9]	143	[10]	612	[11]	457	[12]
Net investment gains (losses)		0	[9]	0	[10]	0	[11]	0	[12]
Products revenues		386,708	[9]	362,577	[10]	1,237,178	[11]	1,129,036	[12]
Services and other revenues		109,373	[9]	106,694	[10]	330,653	[11]	311,048	[12]
Products expenses		(354,404)	[9]	(341,718)	[10]	(1,098,968)	[11]	(1,062,000)	[12]
Services and other expenses		(96,015)	[9]	(94,924)	[10]	(290,745)	[11]	(273,876)	[12]
Amortization of intangible assets		(10,357)	[1],[9]	(9,139)	[2],[10]	(31,674)	[3],[11]	(29,332)	[4],[12]
Impairment of goodwill and intangible assets								(14,904)
Segment profit (loss)		35,467	[9]	23,633	[10]	147,056	[11]	60,429	[12]
Depreciation expense		(12,900)		(13,000)		(40,400)		(38,500)
Other [Member]
Segment Reporting Information [Line Items]
Gross premium volume		619,742	[13]	561,261	[14]	1,823,977	[15]	1,577,250	[16]
Net written premiums		1,285	[13]	(762)	[14]	1,719	[15]	754	[16]
Earned premiums		1,019	[13]	(1,016)	[14]	1,223	[15]	(433)	[16]
Losses and loss adjustment expenses, current accident year		0	[13]	48	[14]	0	[15]	0	[16]
Losses and loss adjustment expenses, prior accident years		(733)	[13]	342	[14]	6,893	[15]	1,816	[16]
Amortization of policy acquisition costs		0	[13]	0	[14]	0	[15]	0	[16]
Other operating expenses		(2,417)	[13]	(1,648)	[14]	(2,035)	[15]	(2,246)	[16]
Underwriting profit		(2,131)	[13]	(2,274)	[14]	6,081	[15]	(863)	[16]
Net investment income		0	[13]	0	[14]	0	[15]	0	[16]
Net investment gains (losses)		0	[13]	0	[14]	0	[15]	0	[16]
Products revenues		0	[13]	0	[14]	0	[15]	0	[16]
Services and other revenues		91,419	[13]	48,973	[14]	263,978	[15]	145,785	[16]
Products expenses		0	[13]	0	[14]	0	[15]	0	[16]
Services and other expenses		(57,343)	[13]	(37,100)	[14]	(208,015)	[15]	(120,435)	[16]
Amortization of intangible assets		(25,338)	[1],[13]	(18,656)	[2],[14]	(80,989)	[3],[15]	(56,927)	[4],[16]
Impairment of goodwill and intangible assets	[16]							0
Segment profit (loss)		$ 6,607	[13]	$ (9,057)	[14]	$ (18,945)	[15]	$ (32,440)	[12]
U.S. GAAP combined ratio - not meaniningful		NM	[5],[13],[17]	NM	[6],[14],[18]	NM	[7],[15],[19]	NM	[8],[16],[20]

[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 any other reportable 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 any other reportable 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 any other reportable segments.

[4] 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.

[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] 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.

[8] 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.

[9] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $12.9 million for the quarter ended September 30, 2019.

[10] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $13.0 million for the quarter ended September 30, 2018.

[11] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $40.4 million for the nine months ended September 30, 2019.

[12] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $38.5 million for the nine months ended September 30, 2018.

[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 that are not allocated to a reportable segment.

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

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

[17] NM - Ratio is not meaningful

[18] NM - Ratio is not meaningful

[19] NM - Ratio is not meaningful

[20] NM - Ratio is not meaningful