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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

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