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Segment Reporting Disclosures (Company's Segment Disclosures) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Segment Reporting Information [Line Items] |  |  |  |
Earned premiums |  | $ 8,432,412 | $ 8,295,479 | $ 7,587,792
Net investment income |  | 920,496 | 734,532 | 446,755
Net investment gains (losses) |  | 1,807,219 | 1,524,054 | (1,595,733)
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 operating revenues |  | 16,620,763 | 15,803,630 | 11,675,335
Attritional Current Year Claims and Claims Adjustment Expense |  | 5,437,410 | 5,320,487 | 4,531,098
Catastrophe Current Year Claims and Claims Adjustment Expense |  | 70,649 | 40,072 | 81,937
Losses and loss adjustment expenses, prior accident years |  | 455,310 | 38,550 | 167,446
Amortization of policy acquisition costs |  | (1,754,141) | (1,763,670) | (1,655,106)
Other underwriting expenses |  | (1,223,248) | (1,077,064) | (860,477)
Products expenses |  | (2,272,219) | (2,220,676) | (2,241,736)
Services and other expenses |  | (2,424,372) | (2,310,769) | (2,306,985)
Amortization of intangible assets |  | (181,472) | (180,614) | (178,778)
Goodwill, Impairment Loss |  | 0 | 0 | (80,000)
Segment profit (loss) |  | 3,712,562 | 2,928,828 | (93,336)
Interest expense |  | (204,300) | (185,077) | (196,062)
Net foreign exchange gains (losses) |  | 129,438 | (90,045) | 137,832
Income (Loss) Before Income Taxes |  | 3,637,700 | 2,653,706 | (151,566)
Insurance [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Earned premiums |  | 7,407,643 | 7,282,705 | 6,528,263
Net investment income |  | 0 | 0 | 0
Net investment gains (losses) |  | 0 | 0 | 0
Products revenues |  | 0 | 0 | 0
Services and other revenues |  | 0 | 0 | 0
Total operating revenues |  | 7,407,643 | 7,282,705 | 6,528,263
Attritional Current Year Claims and Claims Adjustment Expense |  | 4,704,219 | 4,651,173 | 3,867,195
Catastrophe Current Year Claims and Claims Adjustment Expense |  | 67,169 | 39,572 | 69,230
Losses and loss adjustment expenses, prior accident years |  | 451,016 | 104,743 | 142,924
Amortization of policy acquisition costs |  | (1,501,829) | (1,508,169) | (1,375,539)
Other underwriting expenses |  | (1,163,557) | (1,026,358) | (809,352)
Products expenses |  | 0 | 0 | 0
Services and other expenses |  | 0 | 0 | 0
Segment profit (loss) |  | 421,885 | 162,176 | 549,871
Reinsurance [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Earned premiums |  | 1,028,201 | 1,014,294 | 1,063,347
Net investment income |  | 0 | 0 | 0
Net investment gains (losses) |  | 0 | 0 | 0
Products revenues |  | 0 | 0 | 0
Services and other revenues |  | 0 | 0 | 0
Total operating revenues |  | 1,028,201 | 1,014,294 | 1,063,347
Attritional Current Year Claims and Claims Adjustment Expense |  | 733,191 | 669,314 | 663,903
Catastrophe Current Year Claims and Claims Adjustment Expense |  | 3,480 | 500 | 12,707
Losses and loss adjustment expenses, prior accident years |  | 12,344 | (57,081) | 26,052
Amortization of policy acquisition costs |  | (252,312) | (255,501) | (279,567)
Other underwriting expenses |  | (56,925) | (51,163) | (49,363)
Products expenses |  | 0 | 0 | 0
Services and other expenses |  | 0 | 0 | 0
Segment profit (loss) |  | (5,363) | (19,265) | 83,859
Investing [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Earned premiums |  | 0 | 0 | 0
Net investment income |  | 913,478 | 729,219 | 445,846
Net investment gains (losses) |  | 1,807,219 | 1,524,054 | (1,595,733)
Products revenues |  | 0 | 0 | 0
Services and other revenues |  | 52,253 | (11,854) | (17,661)
Total operating revenues |  | 2,772,950 | 2,241,419 | (1,167,548)
Attritional Current Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
Catastrophe Current Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
Losses and loss adjustment expenses, prior accident years |  | 0 | 0 | 0
Amortization of policy acquisition costs |  | 0 | 0 | 0
Other underwriting expenses |  | 0 | 0 | 0
Products expenses |  | 0 | 0 | 0
Services and other expenses |  | 0 | 0 | 0
Segment profit (loss) |  | 2,772,950 | 2,241,419 | (1,167,548)
Markel Ventures [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Earned premiums |  | 0 | 0 | 0
Net investment income |  | 7,018 | 5,313 | 909
Net investment gains (losses) |  | 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 operating revenues |  | 5,120,096 | 4,985,081 | 4,757,527
Attritional Current Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
Catastrophe Current Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
Losses and loss adjustment expenses, prior accident years |  | 0 | 0 | 0
Amortization of policy acquisition costs |  | 0 | 0 | 0
Other underwriting expenses |  | 0 | 0 | 0
Products expenses |  | (2,272,219) | (2,220,676) | (2,241,736)
Services and other expenses |  | (2,327,795) | (2,244,527) | (2,111,510)
Amortization of intangible assets |  | (81,317) | (82,370) |
Segment profit (loss) |  | 520,082 | 519,878 | 404,281
Other Insurance Operations |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Earned premiums |  | (3,432) | (1,520) | (3,818)
Net investment income |  | 0 | 0 | 0
Net investment gains (losses) |  | 0 | 0 | 0
Products revenues |  | 0 | 0 | 0
Services and other revenues |  | 295,305 | 281,651 | 497,564
Total operating revenues |  | 291,873 | 280,131 | 493,746
Attritional Current Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
Catastrophe Current Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
Losses and loss adjustment expenses, prior accident years |  | (8,050) | (9,112) | (1,530)
Amortization of policy acquisition costs |  | 0 | 0 | 0
Other underwriting expenses |  | (2,766) | 457 | (1,762)
Products expenses |  | 0 | 0 | 0
Services and other expenses |  | (96,577) | (66,242) | (195,475)
Amortization of intangible assets | [1] | (62,879) | (61,168) |
Segment profit (loss) |  | 184,480 | 205,234 | 294,979
Corporate Expense |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Earned premiums |  | 0 | 0 | 0
Net investment income |  | 0 | 0 | 0
Net investment gains (losses) |  | 0 | 0 | 0
Products revenues |  | 0 | 0 | 0
Services and other revenues |  | 0 | 0 | 0
Total operating revenues |  | 0 | 0 | 0
Attritional Current Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
Catastrophe Current Year Claims and Claims Adjustment Expense |  | 0 | 0 | 0
Losses and loss adjustment expenses, prior accident years |  | 0 | 0 | 0
Amortization of policy acquisition costs |  | 0 | 0 | 0
Other underwriting expenses |  | 0 | 0 | 0
Products expenses |  | 0 | 0 | 0
Services and other expenses |  | 0 | 0 | 0
Amortization of intangible assets |  | (181,472) | (180,614) | (178,778)
Goodwill, Impairment Loss |  |  |  | (80,000)
Segment profit (loss) |  | $ (181,472) | $ (180,614) | $ (258,778)
[1] | Amounts included in other insurance operations reflect the Company's operations that are not included in a reportable segment and are primarily related to the Company's program services and insurance-linked securities operations.

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