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Reserve for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2022
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net prior year development, unfavorable (favorable) |  |  | $ (6,124) | $ (8,903) |  |
Acquisition costs | $ 102,201 | $ 78,537 | 283,059 | 220,532 |  |
Reserve for losses and loss adjustment expenses | 3,434,800 | 2,948,822 | 3,434,800 | 2,948,822 | $ 3,030,037 | $ 2,856,275
Hurricane Helene |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Reserve for losses and loss adjustment expenses | 33,900 |  | 33,900 |  | 0 |
Baltimore Bridge |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Reserve for losses and loss adjustment expenses | 37,900 |  | 37,900 |  | 0 |
Ukrainian Conflict |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Reserve for losses and loss adjustment expenses | 63,700 |  | 63,700 |  | 64,900 |
Covid-19 |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Reserve for losses and loss adjustment expenses | 15,600 |  | 15,600 |  | $ 14,100 |
Specialty Contracts |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net prior year development, unfavorable (favorable) |  |  | 9,500 | (20,300) |  |
Casualty Lines |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net prior year development, unfavorable (favorable) |  |  | 13,200 | 8,800 |  |
Property Contracts |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net prior year development, unfavorable (favorable) |  |  | (26,100) | 6,600 |  |
Casualty Lines of Business | Reinsurance Contract [Axis]: Loss Portfolio Transfer Agreement |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net prior year development, unfavorable (favorable) |  |  | (2,700) | (4,000) |  |
Amortization of the associated deferred gain |  |  | (1,300) | (2,500) |  |
Net unfavorable (favorable) development in the underlying reserves | $ (1,400) | $ (1,500) | (1,400) | (1,500) |  |
Attritional Losses |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net prior year development, unfavorable (favorable) |  |  | 7,100 | (4,000) |  |
Catastrophe Losses |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net prior year development, unfavorable (favorable) |  |  | $ (13,200) | $ (4,900) |  |

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