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Reserves For Losses And Loss Expenses (Reconciliation of Net Incurred and Paid Claims Development to Reserves for Loss and Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
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Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | $ 14,665,237 |  |
Total due from reinsurers on unpaid claims | 2,762,344 |  |
Total loss reserve discount | (416,358) |  |
Gross reserves at end of year | 17,011,223 | $ 15,390,888 | $ 13,784,430
Other liability |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 5,536,127 |  |
Workers' compensation |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,921,971 |  |
Professional liability |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,749,386 |  |
Commercial automobile |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,152,717 |  |
Short-tail lines |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 761,628 |  |
Casualty |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,992,711 |  |
Monoline excess |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,146,311 |  |
Property |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 279,800 |  |
Insurance |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 11,246,415 |  |
Total due from reinsurers on unpaid claims | 2,551,563 |  |
Total loss reserve discount | (12,491) |  |
Insurance | Other liability |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 5,536,127 |  |
Total due from reinsurers on unpaid claims | 730,029 |  |
Total loss reserve discount | 0 |  |
Insurance | Workers' compensation |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,921,971 |  |
Total due from reinsurers on unpaid claims | 221,769 |  |
Total loss reserve discount | (12,491) |  |
Insurance | Professional liability |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,749,386 |  |
Total due from reinsurers on unpaid claims | 1,019,810 |  |
Total loss reserve discount | 0 |  |
Insurance | Commercial automobile |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,152,717 |  |
Total due from reinsurers on unpaid claims | 67,895 |  |
Total loss reserve discount | 0 |  |
Insurance | Short-tail lines |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 761,628 |  |
Total due from reinsurers on unpaid claims | 414,549 |  |
Total loss reserve discount | 0 |  |
Insurance | Other |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 124,586 |  |
Total due from reinsurers on unpaid claims | 97,511 |  |
Total loss reserve discount | 0 |  |
Reinsurance & Monoline Excess |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 3,418,822 |  |
Total due from reinsurers on unpaid claims | 210,781 |  |
Total loss reserve discount | (403,867) |  |
Reinsurance & Monoline Excess | Casualty |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,992,711 |  |
Total due from reinsurers on unpaid claims | 108,390 |  |
Total loss reserve discount | (84,668) |  |
Reinsurance & Monoline Excess | Monoline excess |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 1,146,311 |  |
Total due from reinsurers on unpaid claims | 35,926 |  |
Total loss reserve discount | 319,199 |  |
Reinsurance & Monoline Excess | Property |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |
Total undiscounted reserves for loss and loss expenses, net of reinsurance | 279,800 |  |
Total due from reinsurers on unpaid claims | 66,465 |  |
Total loss reserve discount | $ 0 |  |

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