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Unpaid 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 | Jul. 02, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Net losses and loss adjustment expenses | $ 1,413,526 | $ 1,434,871 | $ 4,155,146 | $ 3,934,969 |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 344,212 | 169,754 |  |  |
Unpaid losses and loss adjustment expenses | 25,877,478 | 22,144,366 | 25,877,478 | 22,144,366 |  | $ 23,483,321 | $ 20,947,898
Reinsurance recoverable on unpaid losses | 10,293,842 | 8,119,634 | 10,293,842 | 8,119,634 |  | $ 8,820,567 | $ 7,994,884
Reinsurance recoverable for retroactive reinsurance transaction |  |  | 0 | (125,067) |  |  |
Hurricane Helene (2024) |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Net losses and loss adjustment expenses |  |  | 62,000 |  |  |  |
2023 Catastrophes |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Net losses and loss adjustment expenses |  |  |  | 46,200 |  |  |
Insurance [Member] |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Net losses and loss adjustment expenses | 1,215,469 | 1,258,864 | 3,608,979 | 3,415,860 |  |  |
Insurance [Member] | International professional liability product lines, general liability, property and credit and surety product lines [Member] |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 322,100 |  |  |  |
Insurance [Member] | General liability product lines |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  |  | (70,500) |  |  |
Reinsurance [Member] |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Net losses and loss adjustment expenses | $ 198,057 | $ 176,007 | $ 546,167 | 519,109 |  |  |
Reinsurance recoverable for retroactive reinsurance transaction |  |  |  | 125,100 |  |  |
Reinsurance [Member] | Public entity and general liability product lines |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  |  | (60,900) |  |  |
Program services and other fronting [Member] | Casualty policy portfolio part of retroactive reinsurance contract [Member] |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Unpaid losses and loss adjustment expenses |  |  |  |  | $ 302,600 |  |
Reinsurance recoverable on unpaid losses |  |  |  |  | $ 302,600 |  |
Insurance and Reinsurance | International professional liability product lines and property, marine and energy, workers' compensation and personal lines product lines [Member] |  |  |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  |  | $ 276,600 |  |  |

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