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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Catastrophe Current Year Claims and Claims Adjustment Expense | $ (4,315) | $ 62,000 | $ 56,562 | $ 62,000
Favorable (adverse) development on prior years' loss reserves |  |  | 360,473 | 344,212
Commutation of ceded reinsurance contracts |  |  | 312,244 | 0
Commutation of ceded reinsurance contracts, cash payments |  |  | 292,600 |
California Wildfires (2025) |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Catastrophe Current Year Claims and Claims Adjustment Expense |  |  | (56,600) |
Hurricane Helene (2024) |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Catastrophe Current Year Claims and Claims Adjustment Expense |  |  |  | (62,000)
Markel Insurance |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Catastrophe Current Year Claims and Claims Adjustment Expense | $ (4,315) | $ 62,000 | 56,562 | 62,000
Markel Insurance | Run-off risk-managed directors and officers product lines |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 142,300 |
Markel Insurance | Marine & Energy, Property, Programs, General Liability, Worker's Compensation product lines |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | $ 314,800 |
Markel Insurance | International Professional Liability, General Liability, Property, and Credit & Surety product lines |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  |  | $ 289,900

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