Segment Reporting Disclosures (Company's Segment Disclosures) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) segment		Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable ongoing underwriting segments | segment	2
Premiums Written, Gross	$ 2,255,042		$ 2,046,884
Net written premiums	1,477,093		1,334,802
Earned premiums	1,203,977		1,151,021
Losses and loss adjustment expenses, current accident year	(757,970)		(723,208)
Losses and loss adjustment expenses, prior accident years	70,224		108,090
Amortization of policy acquisition costs	(261,827)		(241,905)
Other operating expenses	(193,385)		(182,485)
Underwriting profit	61,019		111,513
Net investment income	114,182		108,016
Net investment gains (losses)	612,191		(122,998)
Products Revenues	348,794		294,136
Services and other revenues	193,344		145,296
Products expenses	(319,426)		(269,697)
Services and other expenses	(174,606)		(132,433)
Amortization of intangible assets	(40,668)	[1]	(28,823)	[2]
Segment profit (loss)	794,830		105,010
Interest expense	(40,290)		(40,059)
Net foreign exchange losses	(21,864)		(22,114)
Income (Loss) Before Income Taxes	$ 732,676		$ 42,837
U.S. GAAP combined ratio	95.00%	[3]	90.00%	[4]
Insurance [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	$ 1,192,848		$ 1,093,362
Net written premiums	998,358		912,979
Earned premiums	973,727		902,851
Losses and loss adjustment expenses, current accident year	(618,498)		(570,027)
Losses and loss adjustment expenses, prior accident years	72,574		119,173
Amortization of policy acquisition costs	(199,999)		(179,485)
Other operating expenses	(175,721)		(169,971)
Underwriting profit	52,083		102,541
Net investment income	0		0
Net investment gains (losses)	0		0
Products Revenues	0		0
Services and other revenues	0		0
Products expenses	0		0
Services and other expenses	0		0
Amortization of intangible assets	0	[1]	0	[2]
Segment profit (loss)	$ 52,083		$ 102,541
U.S. GAAP combined ratio	95.00%	[3]	89.00%	[4]
Reinsurance [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	$ 513,377		$ 492,333
Net written premiums	478,967		421,058
Earned premiums	230,510		247,964
Losses and loss adjustment expenses, current accident year	(139,472)		(153,181)
Losses and loss adjustment expenses, prior accident years	(11,295)		(13,071)
Amortization of policy acquisition costs	(61,828)		(62,420)
Other operating expenses	(14,559)		(12,130)
Underwriting profit	3,356		7,162
Net investment income	0		0
Net investment gains (losses)	0		0
Products Revenues	0		0
Services and other revenues	0		0
Products expenses	0		0
Services and other expenses	0		0
Amortization of intangible assets	0	[1]	0	[2]
Segment profit (loss)	$ 3,356		$ 7,162
U.S. GAAP combined ratio	99.00%	[3]	97.00%	[4]
Investing [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	$ 0		$ 0
Net written premiums	0		0
Earned premiums	0		0
Losses and loss adjustment expenses, current accident year	0		0
Losses and loss adjustment expenses, prior accident years	0		0
Amortization of policy acquisition costs	0		0
Other operating expenses	0		0
Underwriting profit	0		0
Net investment income	113,930		107,894
Net investment gains (losses)	612,191		(122,998)
Products Revenues	0		0
Services and other revenues	0		0
Products expenses	0		0
Services and other expenses	0		0
Amortization of intangible assets	0	[1]	0	[2]
Segment profit (loss)	726,121		(15,104)
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	0	[5]	0	[6]
Net written premiums	0	[5]	0	[6]
Earned premiums	0	[5]	0	[6]
Losses and loss adjustment expenses, current accident year	0	[5]	0	[6]
Losses and loss adjustment expenses, prior accident years	0	[5]	0	[6]
Amortization of policy acquisition costs	0	[5]	0	[6]
Other operating expenses	0	[5]	0	[6]
Underwriting profit	0	[5]	0	[6]
Net investment income	252	[5]	122	[6]
Net investment gains (losses)	0	[5]	0	[6]
Products Revenues	348,794	[5]	294,136	[6]
Services and other revenues	105,969	[5]	97,921	[6]
Products expenses	(319,426)	[5]	(269,697)	[6]
Services and other expenses	(94,870)	[5]	(88,608)	[6]
Amortization of intangible assets	(10,807)	[1],[5]	(10,097)	[2],[6]
Segment profit (loss)	29,912	[5]	23,777	[6]
Depreciation expense	(14,000)		(12,700)
Other [Member]
Segment Reporting Information [Line Items]
Premiums Written, Gross	548,817	[7]	461,189	[8]
Net written premiums	(232)	[7]	765	[8]
Earned premiums	(260)	[7]	206	[8]
Losses and loss adjustment expenses, current accident year	0	[7]	0	[8]
Losses and loss adjustment expenses, prior accident years	8,945	[7]	1,988	[8]
Amortization of policy acquisition costs	0	[7]	0	[8]
Other operating expenses	(3,105)	[7]	(384)	[8]
Underwriting profit	5,580	[7]	1,810	[8]
Net investment income	0	[5]	0	[8]
Net investment gains (losses)	0	[7]	0	[8]
Products Revenues	0	[7]	0	[8]
Services and other revenues	87,375	[5]	47,375	[8]
Products expenses	0	[7]	0	[8]
Services and other expenses	(79,736)	[7]	(43,825)	[8]
Amortization of intangible assets	(29,861)	[1],[7]	(18,726)	[2],[8]
Segment profit (loss)	$ (16,642)	[7]	$ (13,366)	[6]
U.S. GAAP combined ratio - not meaniningful	NM	[3],[7],[9]	NM	[4],[8],[10]

[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] 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.

[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] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $14.0 million for the three months ended March 31, 2019.

[6] Products expenses and services and other expenses for the Markel Ventures segment include depreciation expense of $12.7 million for the three months ended March 31, 2018.

[7] 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.

[8] 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.

[9] NM - Ratio is not meaningful

[10] NM - Ratio is not meaningful