Segment Reporting Disclosures (Company's Segment Disclosures) (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019 USD ($)		Jun. 30, 2018 USD ($)		Jun. 30, 2019 USD ($) segment		Jun. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable ongoing underwriting segments | segment					2
Premiums Written, Gross	$ 2,247,147		$ 1,997,433		$ 4,502,189		$ 4,044,317
Net written premiums	1,305,638		1,180,606		2,782,731		2,515,408
Earned premiums	1,199,461		1,148,184		2,403,438		2,299,205
Losses and loss adjustment expenses, current accident year	(796,712)		(756,674)		(1,554,682)		(1,479,882)
Losses and loss adjustment expenses, prior accident years	118,592		157,496		188,816		265,586
Amortization of policy acquisition costs	(263,065)		(249,250)		(524,892)		(491,155)
Other operating expenses	(199,251)		(202,320)		(392,636)		(384,805)
Underwriting profit	59,025		97,436		120,044		208,949
Net investment income	111,831		105,387		226,013		213,403
Net investment gains (losses)	425,653		105,249		1,037,844		(17,749)
Products revenues	501,676		472,323		850,470		766,459
Services and other revenues	200,495		155,870		393,839		301,166
Products expenses	(425,138)		(450,585)		(744,564)		(720,282)
Services and other expenses	170,796		129,854		345,402		262,287
Amortization of intangible assets	(36,300)	[1]	(29,641)	[2]	(76,968)	[3]	(58,464)	[4]
Impairment of goodwill and intangible assets	0		(14,904)		0		(14,904)
Segment profit (loss)	666,446		311,281		1,461,276		416,291
Interest expense	(41,267)		(36,702)		(81,557)		(76,761)
Net foreign exchange losses (gains)	25,015		86,158		3,151		64,044
Income before income taxes	$ 650,194		$ 360,737		$ 1,382,870		$ 403,574
U.S. GAAP combined ratio	95.00%	[5]	92.00%	[6]	95.00%	[7]	91.00%	[8]
Insurance [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	$ 1,368,348		$ 1,233,828		$ 2,561,196		$ 2,327,190
Net written premiums	1,126,170		1,001,126		2,124,528		1,914,105
Earned premiums	991,269		918,194		1,964,996		1,821,045
Losses and loss adjustment expenses, current accident year	(657,372)		(610,634)		(1,275,870)		(1,180,661)
Losses and loss adjustment expenses, prior accident years	101,721		139,485		174,295		258,658
Amortization of policy acquisition costs	(208,609)		(191,843)		(408,608)		(371,328)
Other operating expenses	(179,432)		(180,928)		(355,153)		(350,899)
Underwriting profit	47,577		74,274		99,660		176,815
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				0
Segment profit (loss)	$ 47,577		$ 74,274		$ 99,660		$ 176,815
U.S. GAAP combined ratio	95.00%	[5]	92.00%	[6]	95.00%	[7]	90.00%	[8]
Reinsurance [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	$ 223,381		$ 208,805		$ 736,758		$ 701,138
Net written premiums	178,802		178,729		657,769		599,787
Earned premiums	207,728		229,613		438,238		477,577
Losses and loss adjustment expenses, current accident year	(139,340)		(145,992)		(278,812)		(299,173)
Losses and loss adjustment expenses, prior accident years	18,190		18,525		6,895		5,454
Amortization of policy acquisition costs	(54,456)		(57,407)		(116,284)		(119,827)
Other operating expenses	(23,306)		(21,178)		(37,865)		(33,308)
Underwriting profit	8,816		23,561		12,172		30,723
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				0
Segment profit (loss)	$ 8,816		$ 23,561		$ 12,172		$ 30,723
U.S. GAAP combined ratio	96.00%	[5]	90.00%	[6]	97.00%	[7]	94.00%	[8]
Investing [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	$ 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	111,633		105,195		225,563		213,089
Net investment gains (losses)	425,653		105,249		1,037,844		(17,749)
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				0
Segment profit (loss)	537,286		210,444		1,263,407		195,340
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	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	198	[9]	192	[10]	450	[11]	314	[12]
Net investment gains (losses)	0	[9]	0	[10]	0	[11]	0	[12]
Products revenues	501,676	[9]	472,323	[10]	850,470	[11]	766,459	[12]
Services and other revenues	115,311	[9]	106,433	[10]	221,280	[11]	204,354	[12]
Products expenses	(425,138)	[9]	(450,585)	[10]	(744,564)	[11]	(720,282)	[12]
Services and other expenses	99,860	[9]	90,344	[10]	194,730	[11]	178,952	[12]
Amortization of intangible assets	(10,510)	[1],[9]	(10,096)	[2],[10]	(21,317)	[3],[11]	(20,193)	[4],[12]
Impairment of goodwill and intangible assets			(14,904)	[10]			(14,904)
Segment profit (loss)	81,677	[9]	13,019	[10]	111,589	[11]	36,796	[12]
Depreciation expense	(13,500)		(12,800)		(27,500)		(25,500)
Other [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	655,418	[13]	554,800	[14]	1,204,235	[15]	1,015,989	[16]
Net written premiums	666	[13]	751	[14]	434	[15]	1,516	[16]
Earned premiums	464	[13]	377	[14]	204	[15]	583	[16]
Losses and loss adjustment expenses, current accident year	0	[13]	(48)	[14]	0	[15]	(48)	[16]
Losses and loss adjustment expenses, prior accident years	(1,319)	[13]	(514)	[14]	7,626	[15]	1,474	[16]
Amortization of policy acquisition costs	0	[13]	0	[14]	0	[15]	0	[16]
Other operating expenses	3,487	[13]	(214)	[14]	382	[15]	(598)	[16]
Underwriting profit	2,632	[13]	(399)	[14]	8,212	[15]	1,411	[16]
Net investment income	0	[13]	0	[14]	0	[11]	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	85,184	[13]	49,437	[14]	172,559	[15]	96,812	[16]
Products expenses	0	[13]	0	[14]	0	[15]	0	[16]
Services and other expenses	70,936	[13]	39,510	[14]	150,672	[15]	83,335	[16]
Amortization of intangible assets	(25,790)	[1],[13]	(19,545)	[2],[14]	(55,651)	[3],[15]	(38,271)	[4],[16]
Impairment of goodwill and intangible assets			0	[14]			0	[16]
Segment profit (loss)	$ (8,910)	[13]	$ (10,017)	[14]	$ (25,552)	[15]	$ (23,383)	[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 $13.5 million for the quarter ended June 30, 2019.

[10] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $12.8 million for the quarter ended June 30, 2018.

[11] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $27.5 million for the six months ended June 30, 2019.

[12] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $25.5 million for the six months ended June 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 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.

[17] NM - Ratio is not meaningful

[18] NM - Ratio is not meaningful

[19] NM - Ratio is not meaningful

[20] NM - Ratio is not meaningful