Segment Reporting Disclosures (Company's Segment Disclosures) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment		Mar. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable ongoing underwriting segments | segment	2
Gross premium volume	$ 2,322,824		$ 2,255,042
Net written premiums	1,646,478		1,477,093
Earned premiums	1,330,709		1,203,977
Losses and loss adjustment expenses, current accident year	(1,180,365)		(757,970)
Losses and loss adjustment expenses, prior accident year	104,017		70,224
Amortization of policy acquisition costs	(295,811)		(261,827)
Other operating expenses	(199,352)		(193,385)
Underwriting profit (loss)	(240,802)		61,019
Net investment income	88,243		114,182
Net investment gains (losses)	(1,681,441)		612,191
Products revenues	352,161		348,794
Services and other revenues	245,994		193,344
Products expenses	(314,071)		(319,426)
Services and other expenses	(217,556)		(174,606)
Amortization of intangible assets	(37,858)	[1]	(40,668)	[2]
Segment profit (loss)	(1,805,330)		794,830
Interest expense	(45,030)		(40,290)
Net foreign exchange gains (losses)	78,301		(21,864)
Income (Loss) before income taxes	$ (1,772,059)		$ 732,676
U.S. GAAP combined ratio	118.00%	[3]	95.00%	[4]
Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume	$ 1,414,711		$ 1,192,848
Net written premiums	1,195,737		998,358
Earned premiums	1,106,851		973,727
Losses and loss adjustment expenses, current accident year	(1,006,635)		(618,498)
Losses and loss adjustment expenses, prior accident year	116,132		72,574
Amortization of policy acquisition costs	(239,420)		(199,999)
Other operating expenses	(183,302)		(175,721)
Underwriting profit (loss)	(206,374)		52,083
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)	$ (206,374)		$ 52,083
U.S. GAAP combined ratio	119.00%	[3]	95.00%	[4]
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume	$ 513,186		$ 513,377
Net written premiums	452,749		478,967
Earned premiums	225,960		230,510
Losses and loss adjustment expenses, current accident year	(173,730)		(139,472)
Losses and loss adjustment expenses, prior accident year	(13,912)		(11,295)
Amortization of policy acquisition costs	(56,391)		(61,828)
Other operating expenses	(15,886)		(14,559)
Underwriting profit (loss)	(33,959)		3,356
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)	$ (33,959)		$ 3,356
U.S. GAAP combined ratio	115.00%	[3]	99.00%	[4]
Investing [Member]
Segment Reporting Information [Line Items]
Gross premium volume	$ 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 year	0		0
Amortization of policy acquisition costs	0		0
Other operating expenses	0		0
Underwriting profit (loss)	0		0
Net investment income	88,059		113,930
Net investment gains (losses)	(1,681,441)		612,191
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)	(1,593,382)		726,121
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Gross premium volume	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 year	0	[5]	0	[6]
Amortization of policy acquisition costs	0	[5]	0	[6]
Other operating expenses	0	[5]	0	[6]
Underwriting profit (loss)	0	[5]	0	[6]
Net investment income	184	[5]	252	[6]
Net investment gains (losses)	0	[5]	0	[6]
Products revenues	352,161	[5]	348,794	[6]
Services and other revenues	158,876	[5]	105,969	[6]
Products expenses	(314,071)	[5]	(319,426)	[6]
Services and other expenses	(143,552)	[5]	(94,870)	[6]
Amortization of intangible assets	(11,841)	[1],[5]	(10,807)	[2],[6]
Segment profit (loss)	41,757	[5]	29,912	[6]
Depreciation expense	(13,900)	[5]	(14,000)	[6]
Underwriting Segments [Member]
Segment Reporting Information [Line Items]
Amortization of intangible assets	10,500		9,800
Other [Member]
Segment Reporting Information [Line Items]
Gross premium volume	394,927	[7]	548,817	[8]
Net written premiums	(2,008)	[7]	(232)	[8]
Earned premiums	(2,102)	[7]	(260)	[8]
Losses and loss adjustment expenses, current accident year	0	[7]	0	[8]
Losses and loss adjustment expenses, prior accident year	1,797	[7]	8,945	[8]
Amortization of policy acquisition costs	0	[7]	0	[8]
Other operating expenses	(164)	[7]	(3,105)	[8]
Underwriting profit (loss)	(469)	[7]	5,580	[8]
Net investment income	0	[7]	0	[8]
Net investment gains (losses)	0	[7]	0	[8]
Products revenues	0	[7]	0	[8]
Services and other revenues	87,118	[7]	87,375	[8]
Products expenses	0	[7]	0	[8]
Services and other expenses	(74,004)	[7]	(79,736)	[8]
Amortization of intangible assets	(26,017)	[1],[7]	(29,861)	[2],[8]
Segment profit (loss)	$ (13,372)	[7]	$ (16,642)	[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 the Company's Insurance and Reinsurance 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 Insurance and Reinsurance 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 $13.9 million for the three months ended March 31, 2020.

[6] 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.

[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 is not allocated to a reportable segment. Amortization of intangible assets attributable to the Company's underwriting segments was $10.5 million for the three months ended March 31, 2020, however, the Company does not allocate amortization of intangible assets between the Insurance and Reinsurance segments.

[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 is not allocated to a reportable segment. Amortization of intangible assets attributable to the Company's underwriting segments was $9.8 million for the three months ended March 31, 2019, however, the Company does not allocate amortization of intangible assets between the Insurance and Reinsurance segments.

[9] NM - Ratio is not meaningful

[10] NM - Ratio is not meaningful