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Reserve for Loss and Loss Expense (Reserve Loss Development) (Details) - USD ($) $ in Thousands | 9 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | $ (5,782) | $ (44,010)
Change in Historical Claim Rate Experience [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | (5,800) | (44,000)
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | (16,500) | (48,000)
Change in Historical Claim Rate Experience [Member] | Property Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | 10,700 | 4,000
Change in Historical Claim Rate Experience [Member] | Workers' Compensation Insurance [Member] | Casualty Insurance Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | (24,500) | (40,000)
Change in Historical Claim Rate Experience [Member] | E&S Lines | Casualty Insurance Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | (5,000) |
Change in Historical Claim Rate Experience [Member] | Personal Automobile [Member] | Casualty Insurance Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | 9,000 |
Change in Historical Claim Rate Experience [Member] | General Liability [Member] | Casualty Insurance Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development |  | (5,000)
Change in Historical Claim Rate Experience [Member] | Surety Lines | Casualty Insurance Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development |  | (10,000)
Change in Historical Claim Rate Experience [Member] | Commercial Automobile [Member] | Casualty Insurance Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | $ (4,000) | (15,000)
Change in Historical Claim Rate Experience [Member] | Businessowners' Policies [Member] | Casualty Insurance Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development |  | $ (8,000)

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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