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Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | --- | --- | ---
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | $ 4,566,037 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 618,601 | $ 757,513 | $ 578,641 | $ 554,269
Unallocated loss expenses | 152,273 |  |  |
Gross liability for unpaid loss and loss expenses | 5,336,911 | $ 5,144,821 | $ 4,580,903 | $ 4,260,355
Standard Commercial Lines [Member] | General Liability [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 1,874,944 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 255,493 |  |  |
Standard Commercial Lines [Member] | Workers Compensation [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 767,784 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 203,982 |  |  |
Standard Commercial Lines [Member] | Commercial Automobile [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 939,644 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 15,683 |  |  |
Standard Commercial Lines [Member] | Business Owners' Policies [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 101,601 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 5,252 |  |  |
Standard Commercial Lines [Member] | Commercial Property [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 133,412 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 39,152 |  |  |
Standard Commercial Lines [Member] | Other Commercial [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 18,637 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 10,241 |  |  |
Standard Commercial Lines [Member] | Total Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 3,836,022 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 529,803 |  |  |
Standard Personal Lines | Personal Automobile [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 125,940 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 38,273 |  |  |
Standard Personal Lines | Homeowners [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 58,334 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 1,647 |  |  |
Standard Personal Lines | Other Personal [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 14,178 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 32,736 |  |  |
Standard Personal Lines | Standard Personal Lines |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 198,452 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 72,656 |  |  |
E&S Lines [Member] | Total E&S Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 531,563 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 16,142 |  |  |
E&S Lines [Member] | Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 456 |  |  |
E&S Lines [Member] | Property Insurance [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 13,575 |  |  |
E&S Lines [Member] | Casualty Insurance Product Line [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 517,988 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | $ 15,686 |  |  |

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (cc) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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