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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Jul. 02, 2024 | Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Net losses and loss adjustment expenses | $ 5,508,059 | $ 5,360,559 | $ 4,613,035 |  |
Unpaid losses and loss adjustment expenses | 26,633,094 | 23,483,321 | 20,947,898 |  | $ 18,178,894
Reinsurance recoverables on unpaid losses, end of year | 11,120,367 | 8,820,567 | 7,994,884 |  | $ 6,876,317
Reinsurance Recoverable, Retroactive Reinsurance Transaction | $ 0 | $ (125,067) | 0 |  |
IBNR reserves as a percentage of total unpaid losses and loss adjustment expenses | 71.00% | 72.00% |  |  |
Prior Year Claims and Claims Adjustment Expense | $ 455,300 | $ 38,600 | 167,400 |  |
Insurance [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Unpaid losses and loss adjustment expenses | 16,073,355 | 14,221,258 |  |  |
Prior Year Claims and Claims Adjustment Expense | 451,000 | 104,700 | 142,900 |  |
Insurance [Member] | Accident Years 2014 and Prior [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Prior Year Claims and Claims Adjustment Expense | 50,100 |  |  |  |
Reinsurance [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Unpaid losses and loss adjustment expenses | 3,870,059 | 3,726,472 |  |  |
Prior Year Claims and Claims Adjustment Expense | 12,300 | (57,100) | 26,100 |  |
Reinsurance [Member] | 2023 U.K. Motor Retroactive Transaction |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Reinsurance Recoverable, Retroactive Reinsurance Transaction |  | (125,100) |  |  |
Fronting [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Reinsurance recoverables | 3,500,000 | 2,500,000 | 3,000,000 |  |
Unpaid losses and loss adjustment expenses | 6,515,153 | 5,185,553 |  |  |
Asbestos [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Asbestos-related reserves, gross loss incurred | 122,100 | 132,500 |  |  |
Asbestos-related reserves, net loss incurred | 32,500 | 39,600 |  |  |
Casualty policy portfolio part of retroactive reinsurance contract | Fronting [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Unpaid losses and loss adjustment expenses |  |  |  | $ 302,600 |
Reinsurance recoverables on unpaid losses, end of year |  |  |  | $ 302,600 |
2024 Catastrophes |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Net losses and loss adjustment expenses | 70,600 |  |  |  |
Reinsurance recoverables | $ 60,400 |  |  |  |
2023 Catastrophes | Insurance [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Net losses and loss adjustment expenses |  | 40,100 |  |  |
Reinsurance recoverables |  | $ 9,300 |  |  |
Hurricane Ian |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Net losses and loss adjustment expenses |  |  | 46,200 |  |
Reinsurance recoverables |  |  | 115,300 |  |
Russia-Ukraine conflict |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Net losses and loss adjustment expenses |  |  | 35,700 |  |
Reinsurance recoverables |  |  | $ 44,300 |  |

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