Segment Reporting Disclosures (Summary Of Segment Revenue By Product) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Product Information [Line Items]
Earned premiums	$ 8,432,412	$ 8,295,479	$ 7,587,792
Products revenues	2,635,659	2,545,053	2,427,096
Services and other revenues	2,824,977	2,704,512	2,809,425
Total products revenues and services and other revenues	5,460,636	5,249,565	5,236,521
Insurance [Member]
Product Information [Line Items]
Earned premiums	7,407,643	7,282,705	6,528,263
Products revenues	0	0	0
Services and other revenues	0	0	0
Insurance [Member] \| General liability [Member]
Product Information [Line Items]
Earned premiums	2,063,827	2,128,317	1,927,721
Insurance [Member] \| Professional liability [Member]
Product Information [Line Items]
Earned premiums	1,655,848	1,798,882	1,739,983
Insurance [Member] \| Marine and energy [Member]
Product Information [Line Items]
Earned premiums	747,004	688,591	585,885
Insurance [Member] \| Personal Lines [Member]
Product Information [Line Items]
Earned premiums	625,542	538,816	489,648
Insurance [Member] \| Property [Member]
Product Information [Line Items]
Earned premiums	609,453	532,337	428,563
Insurance [Member] \| Programs [Member]
Product Information [Line Items]
Earned premiums	576,444	509,866	384,952
Insurance [Member] \| Workers' compensation [Member]
Product Information [Line Items]
Earned premiums	374,371	393,011	385,054
Insurance [Member] \| Credit and surety [Member]
Product Information [Line Items]
Earned premiums	248,001	219,542	193,701
Insurance [Member] \| Other products [Member]
Product Information [Line Items]
Earned premiums	507,153	473,343	392,756
Reinsurance [Member]
Product Information [Line Items]
Earned premiums	1,028,201	1,014,294	1,063,347
Products revenues	0	0	0
Services and other revenues	0	0	0
Reinsurance [Member] \| General liability [Member]
Product Information [Line Items]
Earned premiums	382,151	368,376	382,482
Reinsurance [Member] \| Professional liability [Member]
Product Information [Line Items]
Earned premiums	371,198	388,610	398,839
Reinsurance [Member] \| Specialty [Member]
Product Information [Line Items]
Earned premiums	269,021	245,126	275,033
Reinsurance [Member] \| Property [Member]
Product Information [Line Items]
Earned premiums	5,831	12,182	6,993
Other [Member]
Product Information [Line Items]
Earned premiums	(3,432)	(1,520)	(3,818)
Total products revenues and services and other revenues	347,558	269,797	479,903
Markel Ventures [Member]
Product Information [Line Items]
Earned premiums	0	0	0
Products revenues	2,635,659	2,545,053	2,427,096
Services and other revenues	2,477,419	2,434,715	2,329,522
Total products revenues and services and other revenues	5,113,078	4,979,768	4,756,618
Markel Ventures [Member] \| Consumer and building
Product Information [Line Items]
Products revenues	1,606,353	1,474,499	1,510,130
Markel Ventures [Member] \| Transportation-related
Product Information [Line Items]
Products revenues	584,200	667,040	612,467
Markel Ventures [Member] \| Equipment manufacturing
Product Information [Line Items]
Products revenues	445,106	403,514	304,499
Markel Ventures [Member] \| Construction
Product Information [Line Items]
Services and other revenues	2,089,280	2,042,326	1,910,403
Markel Ventures [Member] \| Consulting
Product Information [Line Items]
Services and other revenues	283,343	297,115	326,549
Markel Ventures [Member] \| Other services
Product Information [Line Items]
Services and other revenues	$ 104,796	$ 95,274	$ 92,570