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Reserve for Loss and Loss Expense - Narrative (Details) - USD ($) $ in Thousands | 9 Months Ended
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Sep. 30, 2024 | Sep. 30, 2023
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | $ 182,748 | $ (5,782)
Casualty reserve |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | 211,000 | (16,500)
Casualty reserve | Commercial automobile |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year |  | 4,000
Casualty reserve | Workers' compensation |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year |  | (24,500)
Casualty reserve | E&S |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year |  | (5,000)
Casualty reserve | Personal automobile |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year |  | 9,000
Casualty reserve | Standard Commercial Lines | General liability |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | 216,000 |
Casualty reserve | Standard Commercial Lines | Commercial automobile |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | 20,000 |
Casualty reserve | Standard Commercial Lines | Workers' compensation |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | (20,000) |
Casualty reserve | Standard Commercial Lines | Surety Lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | (5,000) |
Casualty reserve | Standard Personal Lines | Homeowners |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | (5,000) |
Casualty reserve | Standard Personal Lines | Personal automobile |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | 5,000 |
Property reserve |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Incurred loss and loss expense for claims occurring in the prior year | $ (28,300) | $ 10,700

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