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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Catastrophe Current Year Claims and Claims Adjustment Expense | $ (5,187) | $ 0 | $ 60,877 | $ 0
Favorable (adverse) development on prior years' loss reserves |  |  | 228,839 | 221,298
California Wildfires (2025) |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Catastrophe Current Year Claims and Claims Adjustment Expense |  |  | (60,900) |
Markel Insurance |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Catastrophe Current Year Claims and Claims Adjustment Expense | $ (5,187) | $ 0 | 60,877 | 0
Markel Insurance | Marine & Energy, Propety, Professional Liability, and Workers' Compensation product lines |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 177,900 |
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 |  |  | $ (127,900) |
Markel Insurance | International Professional Liability, International Marine & Energy, and Property product lines [Member] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  |  | $ 165,700

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