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Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($) $ in Thousands | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Jun. 30, 2021 | Dec. 31, 2020 | Jun. 30, 2020 | Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Gross reserve for loss and loss expense, at beginning of year | $ 4,260,355 | $ 4,067,163 |  |  |  |  |
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year | 554,269 | 547,066 |  |  |  |  |
Net reserve for loss and loss expense, at beginning of year | 3,706,086 | 3,520,097 |  |  |  |  |
Incurred loss and loss expense for claims occurring in the current year | 1,408,240 | 1,295,285 |  |  |  |  |
Total incurred loss and loss expense | 1,340,293 | 1,252,075 |  |  |  |  |
Paid loss and loss expense for claims occurring in the current year | 430,288 | 421,981 |  |  |  |  |
Paid loss and loss expense for claims occurring in the prior years | 675,782 | 646,974 |  |  |  |  |
Total paid loss and loss expense | 1,106,070 | 1,068,955 |  |  |  |  |
Net reserve for loss and loss expense, at end of period | 3,940,309 | 3,703,217 |  |  |  |  |
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period | 670,031 | 590,059 |  |  |  |  |
Gross reserve for loss and loss expense, at end of period | 4,610,340 | 4,293,276 |  |  |  |  |
Allowance for credit losses | (1,595) | (1,831) | $ (1,777) | $ (1,777) | $ (2,396) | $ (4,400) |
Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Prior year development | (67,947) | (43,210) |  |  |  |  |
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Prior year development | (66,000) | (50,000) |  |  |  |  |
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | Workers' Compensation Insurance [Member] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Prior year development | (28,000) | (40,000) |  |  |  |  |
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | General Liability [Member] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Prior year development | (29,000) | (20,000) |  |  |  |  |
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] | Commercial Automobile [Member] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Prior year development |  | 10,000 |  |  |  |  |
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Prior year development |  | $ 6,800 |  |  |  |  |
E&S Lines | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Prior year development | $ (7,000) |  |  |  |  |  |
Reinsurance Recoverable [Domain] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Allowance for credit losses |  |  | (1,777) | (1,777) | (2,396) | (1,497) |
Reinsurance Recoverable [Domain] | Retained Earnings [Member] | Accounting Standards Update 2016-13 [Member] |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |
Allowance for credit losses |  |  | $ 0 | $ 0 | $ 0 | $ 2,903 | [1]
[1] | Represents the impact of our adoption of ASU 2016-13, Financial Instruments - Credit Losses.

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