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Segment Reporting Disclosures (Summary Of Segment Revenue By Product) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | $ 8,295,479 |  | $ 7,587,792 |  | $ 6,503,029 |
Products revenues | 2,545,053 |  | 2,427,096 |  | 1,712,120 |
Services and other revenues | 2,704,512 |  | 2,809,425 |  | 2,285,325 |
Total products revenues and services and other revenues | 5,249,565 |  | 5,236,521 |  | 3,997,445 |
Insurance [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 7,282,705 |  | 6,528,263 |  | 5,465,284 |
Products revenues | 0 |  |  |  | 0 |
Services and other revenues | 0 |  | 0 |  | 0 |
Insurance [Member] | General liability [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 2,128,317 |  | 1,927,721 |  | 1,564,221 |
Insurance [Member] | Professional liability [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 1,798,882 |  | 1,739,983 |  | 1,523,536 |
Insurance [Member] | Property [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 532,337 |  | 428,563 |  | 362,637 |
Insurance [Member] | Marine and energy [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 688,591 |  | 585,885 |  | 495,897 |
Insurance [Member] | Personal Lines [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 538,816 |  | 489,648 |  | 451,095 |
Insurance [Member] | Programs [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 509,866 |  | 384,952 |  | 222,410 |
Insurance [Member] | Workers' compensation [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 393,011 |  | 385,054 |  | 354,337 |
Insurance [Member] | Credit and surety [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 219,542 |  | 193,701 |  | 161,155 |
Insurance [Member] | Other products [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 473,343 |  | 392,756 |  | 329,996 |
Reinsurance [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 1,014,294 |  | 1,063,347 |  | 1,042,048 |
Products revenues | 0 |  |  |  | 0 |
Services and other revenues | 0 |  | 0 |  | 0 |
Reinsurance [Member] | General liability [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 368,376 |  | 382,482 |  | 314,699 |
Reinsurance [Member] | Professional liability [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 388,610 |  | 398,839 |  | 320,646 |
Reinsurance [Member] | Property [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 12,182 |  | 6,993 |  | 129,760 |
Reinsurance [Member] | Specialty [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 245,126 |  | 275,033 |  | 276,943 |
Other [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | (1,520) | [1] | (3,818) | [2] | (4,303) | [3]
Products revenues | 0 | [1] |  |  | 0 | [3]
Services and other revenues | 281,651 | [1] | 497,564 | [2] | 346,445 | [3]
Total products revenues and services and other revenues | 269,797 |  | 479,903 |  | 353,629 |
Markel Ventures [Member] |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Earned premiums | 0 |  | 0 |  | 0 |
Products revenues | 2,545,053 |  | 2,427,096 |  | 1,712,120 |
Services and other revenues | 2,434,715 |  | 2,329,522 |  | 1,931,696 |
Total products revenues and services and other revenues | 4,979,768 |  | 4,756,618 |  | 3,643,816 |
Markel Ventures [Member] | Consumer and building |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Products revenues | 1,474,499 |  | 1,510,130 |  | 911,422 |
Markel Ventures [Member] | Transportation-related |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Products revenues | 667,040 |  | 612,467 |  | 474,839 |
Markel Ventures [Member] | Equipment manufacturing |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Products revenues | 403,514 |  | 304,499 |  | 325,859 |
Markel Ventures [Member] | Construction |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Services and other revenues | 2,042,326 |  | 1,910,403 |  | 1,554,592 |
Markel Ventures [Member] | Consulting |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Services and other revenues | 297,115 |  | 326,549 |  | 277,902 |
Markel Ventures [Member] | Other services |  |  |  |  |  |
Product Information [Line Items] |  |  |  |  |  |
Services and other revenues | $ 95,274 |  | $ 92,570 |  | $ 99,202 |
[1] | Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment, as well as amortization of intangible assets attributable to the underwriting segments, which is not allocated between the Insurance and Reinsurance segments.
[2] | Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment, as well as amortization of intangible assets attributable to the underwriting segments, which is not allocated between the Insurance and Reinsurance segments.
[3] | Other represents the total profit (loss) attributable to the Company's operations that are not included in a reportable segment, as well as amortization of intangible assets attributable to the underwriting segments, which is not allocated between the Insurance and Reinsurance segments.

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