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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)

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