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Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | 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 |  |  | $ 4,067,163 |  | $ 3,893,868 |  |  |
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year |  |  | 547,066 | [1] | 537,388 |  |  |
Net reserve for loss and loss expense, at beginning of year |  |  | 3,520,097 |  | 3,356,480 |  |  |
Incurred loss and loss expense for claims occurring in the current year |  |  | 1,295,285 |  | 1,200,878 |  |  |
Total incurred loss and loss expense |  |  | 1,252,075 |  | 1,166,238 |  |  |
Paid loss and loss expense for claims occurring in the current year |  |  | 421,981 |  | 384,436 |  |  |
Paid loss and loss expense for claims occurring in the prior years |  |  | 646,974 |  | 631,589 |  |  |
Total paid loss and loss expense |  |  | 1,068,955 |  | 1,016,025 |  |  |
Net reserve for loss and loss expense, at end of period | $ 3,703,217 | $ 3,506,693 | 3,703,217 |  | 3,506,693 |  |  |
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period | 590,059 | 548,938 | 590,059 |  | 548,938 |  |  |
Gross reserve for loss and loss expense, at end of period | 4,293,276 | 4,055,631 | 4,293,276 |  | 4,055,631 |  |  |
Loss Reserve for COVID-19 |  |  | 10,000 |  |  |  |  |
Loss from Catastrophes | 79,500 | $ 24,200 | 195,900 |  | 74,500 |  |  |
Allowance for credit losses | $ (1,831) |  | (1,831) |  |  | $ (2,396) | $ (4,400) |
Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Prior year development |  |  | (43,210) |  | (34,640) |  |  |
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Prior year development |  |  | (50,000) |  | (41,000) |  |  |
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Prior year development |  |  | 6,800 |  | 6,400 |  |  |
Workers' Compensation Insurance [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Prior year development |  |  | (40,000) |  | (33,000) |  |  |
General Liability [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Prior year development |  |  | (20,000) |  | (10,000) |  |  |
Personal Automobile [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Prior year development |  |  |  |  | $ 2,000 |  |  |
Commercial Automobile [Member] | Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Prior year development |  |  | $ 10,000 |  |  |  |  |
Reinsurance Recoverable [Domain] |  |  |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |  |  |
Allowance for credit losses |  |  |  |  |  | (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 | $ 2,903 | [1],[2]
[1] | Includes an adjustment of $2.9 million related to our adoption of ASU 2016-13. Refer to Note 2. "Adoption of Accounting Pronouncements" for additional information.
[2] | See Note 2. "Adoption of Accounting Pronouncements" for additional information regarding our adoption of ASU 2016-13.

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