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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022 | Aug. 31, 2023
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Incurred losses and loss adjustment expenses, current accident years | $ 1,434,871 | $ 1,266,996 | $ 3,934,969 | $ 3,420,906 |
Reinsurance Recoverable, Allowance for Credit Loss, Period Increase (Decrease) |  |  | 25,000 |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 169,754 | 204,411 |
Reinsurance recoverable for retroactive reinsurance transaction | (125,067) | 0 | (125,067) | 0 |
Vesttoo LOC 1 |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Letters of Credit, Collateral for Reinsurance Recoverable |  |  |  |  | $ 50,000
Vesttoo LOC 2 |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Letters of Credit, Collateral for Reinsurance Recoverable |  |  |  |  | $ 77,800
Hawaiian wildfires and Hurricane Idalia |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Incurred losses and loss adjustment expenses, current accident years |  |  | 46,200 |  |
Hurricane Ian (2022) |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Incurred losses and loss adjustment expenses, current accident years |  |  |  | 70,000 |
Russian-Ukraine Conflict |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Incurred losses and loss adjustment expenses, current accident years |  |  |  | 35,000 |
Insurance [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Incurred losses and loss adjustment expenses, current accident years | 1,258,864 | 1,100,511 | 3,415,860 | 2,906,031 |
Insurance [Member] | Professional Liability, Property, Marine and Energy, Workers Comp and Personal lines [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 239,800 |  |
Insurance [Member] | General Liability [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 70,500 |  |
Insurance [Member] | Workers' Compensation, Property, Marine and Energy and Programs [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  |  | 155,000 |
Reinsurance [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Incurred losses and loss adjustment expenses, current accident years | 176,007 | $ 166,485 | 519,109 | 514,875 |
Reinsurance recoverable for retroactive reinsurance transaction | $ (125,100) |  | (125,100) |  |
Reinsurance [Member] | Professional liability and Property [Member] |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | 34,100 |  |
Reinsurance [Member] | Public Entity and General Liability |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  | $ (60,900) |  |
Reinsurance [Member] | Property and Credit and Surety |  |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |
Favorable (adverse) development on prior years' loss reserves |  |  |  | $ 41,700 |

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