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Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Net Incurred And Paid Loss Development Tables, By Segment, To The Liability For Losses And Loss Adjustment Expenses In The Consolidated Balance Sheet) (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | 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] |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | $ 14,284,815 |  |  |
Reinsurance recoverable on unpaid losses | 8,820,567 |  |  |
Unallocated loss adjustment expenses | 377,939 |  |  |
Total gross liability for unpaid losses and loss adjustment expenses | 23,483,321 | $ 20,947,898 | $ 18,178,894 | $ 16,222,376
Market CATCo Re |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 184,967 |  |  |
Insurance [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 10,705,776 |  |  |
Reinsurance recoverable on unpaid losses | 3,168,100 |  |  |
Total gross liability for unpaid losses and loss adjustment expenses | 14,221,258 | 11,616,386 |  |
Reinsurance [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 3,312,405 |  |  |
Reinsurance recoverable on unpaid losses | 387,366 |  |  |
Total gross liability for unpaid losses and loss adjustment expenses | 3,726,472 | 3,581,699 |  |
Other underwriting [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 71,575 |  |  |
Reinsurance recoverable on unpaid losses | 88,701 |  |  |
Total gross liability for unpaid losses and loss adjustment expenses | 165,071 | 197,602 |  |
Program services and other fronting [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liabilities for unpaid losses and loss adjustment expenses, net of reinsurance | 10,092 |  |  |
Reinsurance recoverable on unpaid losses | 5,176,400 |  |  |
Total gross liability for unpaid losses and loss adjustment expenses | $ 5,185,553 | $ 5,204,290 |  |

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