Segment Reporting Disclosures (Company's Segment Disclosures) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Earned premiums	$ 2,110,108	$ 2,121,745	$ 6,314,834	$ 6,120,592
Net investment income	235,477	192,207	676,807	521,235
Net investment gains (losses)	917,530	(265,917)	1,689,794	591,173
Products revenues	612,277	601,524	2,060,336	1,964,332
Services and other revenues	735,872	725,627	2,037,991	1,963,779
Operating revenues	4,611,264	3,375,186	12,779,762	11,161,111
Losses and loss adjustment expenses, current accident year	(1,413,526)	(1,434,871)	(4,155,146)	(3,934,969)
Losses and loss adjustment expenses, prior accident year	122,914	30,546	344,212	169,754
Amortization of policy acquisition costs	(431,674)	(437,920)	(1,287,900)	(1,298,771)
Other underwriting expenses	(311,882)	(260,209)	(904,703)	(772,950)
Products expenses	(549,990)	(545,567)	(1,766,930)	(1,712,792)
Services and other expenses	(609,321)	(606,810)	(1,757,222)	(1,678,246)
Amortization of acquired intangible assets	(46,459)	(47,545)	(134,981)	(136,367)
Operating Income	1,371,326	72,810	3,117,092	1,796,770
Interest expense	(53,361)	(44,553)	(151,506)	(141,212)
Net foreign exchange gains (losses)	(111,612)	39,246	(51,401)	(8,658)
Income before income taxes	1,206,353	67,503	2,914,185	1,646,900
Insurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	1,859,934	1,848,267	5,547,398	5,322,377
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
Operating revenues	1,859,934	1,848,267	5,547,398	5,322,377
Losses and loss adjustment expenses, current accident year	(1,215,469)	(1,258,864)	(3,608,979)	(3,415,860)
Losses and loss adjustment expenses, prior accident year	130,431	55,374	374,215	179,564
Amortization of policy acquisition costs	(369,320)	(369,001)	(1,101,256)	(1,096,500)
Other underwriting expenses	(295,992)	(250,684)	(861,305)	(733,334)
Products expenses	0	0	0	0
Services and other expenses	0	0	0	0
Operating Income	109,584	25,092	350,073	256,247
Reinsurance [Member]
Segment Reporting Information [Line Items]
Earned premiums	250,680	274,161	768,649	799,683
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
Operating revenues	250,680	274,161	768,649	799,683
Losses and loss adjustment expenses, current accident year	(198,057)	(176,007)	(546,167)	(519,109)
Losses and loss adjustment expenses, prior accident year	(7,937)	(23,262)	(13,749)	(6,584)
Amortization of policy acquisition costs	(62,354)	(68,919)	(186,644)	(202,271)
Other underwriting expenses	(15,863)	(11,785)	(42,289)	(38,113)
Products expenses	0	0	0	0
Services and other expenses	0	0	0	0
Operating Income	(33,531)	(5,812)	(20,200)	33,606
Investing [Member]
Segment Reporting Information [Line Items]
Earned premiums	0	0	0	0
Net investment income	233,384	191,015	671,042	518,536
Net investment gains (losses)	917,530	(265,917)	1,689,794	591,173
Products revenues	0	0	0	0
Services and other revenues	14,971	(5,033)	45,174	(13,791)
Operating revenues	1,165,885	(79,935)	2,406,010	1,095,918
Losses and loss adjustment expenses, current accident year	0	0	0	0
Losses and loss adjustment expenses, prior accident year	0	0	0	0
Amortization of policy acquisition costs	0	0	0	0
Other underwriting expenses	0	0	0	0
Products expenses	0	0	0	0
Services and other expenses	0	0	0	0
Operating Income	1,165,885	(79,935)	2,406,010	1,095,918
Markel Ventures [Member]
Segment Reporting Information [Line Items]
Earned premiums	0	0	0	0
Net investment income	2,093	1,192	5,765	2,699
Net investment gains (losses)	0	0	0	0
Products revenues	612,277	601,524	2,060,336	1,964,332
Services and other revenues	645,251	644,053	1,787,907	1,770,997
Operating revenues	1,259,621	1,246,769	3,854,008	3,738,028
Losses and loss adjustment expenses, current accident year	0	0	0	0
Losses and loss adjustment expenses, prior accident year	0	0	0	0
Amortization of policy acquisition costs	0	0	0	0
Other underwriting expenses	0	0	0	0
Products expenses	(549,990)	(545,567)	(1,766,930)	(1,712,792)
Services and other expenses	(603,004)	(570,782)	(1,699,038)	(1,632,588)
Operating Income	106,627	130,420	388,040	392,648
Other Insurance Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums	(506)	(683)	(1,213)	(1,468)
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	75,650	86,607	204,910	206,573
Operating revenues	75,144	85,924	203,697	205,105
Losses and loss adjustment expenses, current accident year	0	0	0	0
Losses and loss adjustment expenses, prior accident year	420	(1,566)	(16,254)	(3,226)
Amortization of policy acquisition costs	0	0	0	0
Other underwriting expenses	(27)	2,260	(1,109)	(1,503)
Products expenses	0	0	0	0
Services and other expenses	(6,317)	(36,028)	(58,184)	(45,658)
Operating Income	69,220	50,590	128,150	154,718
Corporate [Member]
Segment Reporting Information [Line Items]
Earned premiums	0	0	0	0
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
Operating revenues	0	0	0	0
Losses and loss adjustment expenses, current accident year	0	0	0	0
Losses and loss adjustment expenses, prior accident year	0	0	0	0
Amortization of policy acquisition costs	0	0	0	0
Other underwriting expenses	0	0	0	0
Products expenses	0	0	0	0
Services and other expenses	0	0	0	0
Amortization of acquired intangible assets	(46,459)	(47,545)	(134,981)	(136,367)
Operating Income	$ (46,459)	$ (47,545)	$ (134,981)	$ (136,367)