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Reserve for Loss and Loss Expense - Schedule of Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability (Details) - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
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Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | $ 5,397,736 |  |  |
Total reinsurance recoverable on unpaid loss | 1,022,245 | $ 618,601 | $ 757,513 | $ 578,641
Unallocated loss expenses | 169,820 |  |  |
Total gross liability for unpaid loss and loss expenses | 6,589,801 | $ 5,336,911 | $ 5,144,821 | $ 4,580,903
Standard Commercial Lines | Total Standard Commercial Lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 4,551,605 |  |  |
Total reinsurance recoverable on unpaid loss | 692,000 |  |  |
Standard Commercial Lines | General liability |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 2,432,883 |  |  |
Total reinsurance recoverable on unpaid loss | 396,702 |  |  |
Standard Commercial Lines | Workers Compensation |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 757,202 |  |  |
Total reinsurance recoverable on unpaid loss | 238,995 |  |  |
Standard Commercial Lines | Commercial automobile |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 1,073,194 |  |  |
Total reinsurance recoverable on unpaid loss | 14,774 |  |  |
Standard Commercial Lines | Businessowners’ policies |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 117,985 |  |  |
Total reinsurance recoverable on unpaid loss | 2,604 |  |  |
Standard Commercial Lines | Commercial property |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 151,963 |  |  |
Total reinsurance recoverable on unpaid loss | 36,313 |  |  |
Standard Commercial Lines | Other |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 18,378 |  |  |
Total reinsurance recoverable on unpaid loss | 2,612 |  |  |
Standard Personal Lines | Standard Personal Lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 224,434 |  |  |
Total reinsurance recoverable on unpaid loss | 310,361 |  |  |
Standard Personal Lines | Personal automobile |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 154,177 |  |  |
Total reinsurance recoverable on unpaid loss | 35,386 |  |  |
Standard Personal Lines | Homeowners |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 53,380 |  |  |
Total reinsurance recoverable on unpaid loss | 1,962 |  |  |
Standard Personal Lines | Other |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 16,877 |  |  |
Total reinsurance recoverable on unpaid loss | 273,013 |  |  |
E&S Lines | Total E&S Lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 621,697 |  |  |
Total reinsurance recoverable on unpaid loss | 19,884 |  |  |
E&S Lines | Casualty lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 601,439 |  |  |
Total reinsurance recoverable on unpaid loss | 18,903 |  |  |
E&S Lines | Property lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Total liabilities for unpaid loss and loss expenses, net of reinsurance | 20,258 |  |  |
Total reinsurance recoverable on unpaid loss | $ 981 |  |  |

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