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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reinsurance Recoverable, Retroactive Reinsurance Transaction | $ (125,067) | $ 0 | $ 0
Net losses and loss adjustment expenses | $ 5,360,559 | $ 4,613,035 | 4,061,000
IBNR reserves as a percentage of total unpaid losses and loss adjustment expenses | 72.00% | 70.00% |
Prior Year Claims and Claims Adjustment Expense | $ 38,600 | $ 167,400 | 479,800
Ceded Premiums Written | (5,604,662) | (4,998,292) | (4,318,951)
Vesttoo LOCs |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease) | 65,000 |  |
Insurance [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Net losses and loss adjustment expenses | 4,690,745 | 3,936,425 | 3,311,185
Prior Year Claims and Claims Adjustment Expense | 104,700 | 142,900 | 506,300
Insurance [Member] | 2013 and Prior Accident Years [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior Year Claims and Claims Adjustment Expense | 25,900 |  |
Reinsurance [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reinsurance Recoverable, Retroactive Reinsurance Transaction | (125,100) |  |
Net losses and loss adjustment expenses | 669,814 | 676,610 | 749,815
Prior Year Claims and Claims Adjustment Expense | (57,100) | 26,100 | (19,900)
Reinsurance [Member] | 2013 and Prior Accident Years [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior Year Claims and Claims Adjustment Expense | 69,400 |  |
Reinsurance [Member] | 2013 and Prior Accident Years [Member] | 2023 U.K. Motor Retroactive Transaction |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Prior Year Claims and Claims Adjustment Expense | 28,100 |  |
Program services and other fronting [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reinsurance recoverables | 2,500,000 | 3,000,000 |
Ceded Premiums Written | (3,725,818) | (3,358,127) | (2,958,188)
Asbestos [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Asbestos-related reserves, gross loss incurred | 132,500 | 153,200 |
Asbestos-related reserves, net loss incurred | 39,600 | 54,500 |
2023 Catastrophes |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Net losses and loss adjustment expenses | 40,100 |  |
Reinsurance recoverables | $ 9,300 |  |
Hurricane Ian | Insurance [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Net losses and loss adjustment expenses |  | 46,200 |
Reinsurance recoverables |  | 115,300 |
Hurricane Ian | Program services and other fronting [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Net losses and loss adjustment expenses |  | 850,000 |
Ceded Premiums Written |  | 850,000 |
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 |
2021 Catastrophes [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Net losses and loss adjustment expenses |  |  | 195,000
Reinsurance recoverables |  |  | $ 221,700

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