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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 |  |
--- | --- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 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 | [1] |  |  |
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 | 447,170 | 407,276 |  |  |  |
Total incurred loss and loss expense | 413,401 | 400,324 |  |  |  |
Paid loss and loss expense for claims occurring in the current year | 80,158 | 70,610 |  |  |  |
Paid loss and loss expense for claims occurring in the prior years | 243,687 | 281,736 |  |  |  |
Total paid loss and loss expense | 323,845 | 352,346 |  |  |  |
Net reserve for loss and loss expense, at end of period | 3,795,642 | 3,568,075 |  |  |  |
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period | 564,546 | 535,560 |  |  |  |
Gross reserve for loss and loss expense, at end of period | 4,360,188 | 4,103,635 |  |  |  |
Allowance for credit losses | (1,840) | (1,502) |  | $ (1,777) | $ (4,400) |
Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Prior year development | (33,769) | (6,952) |  |  |  |
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Prior year development | (35,000) | (10,000) |  |  |  |
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 | (15,000) |  |  |  |  |
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 | (15,000) |  |  |  |  |
Property Insurance [Member] | Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Prior year development | 1,200 | $ 3,000 |  |  |  |
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 | $ (5,000) |  |  |  |  |
Reinsurance Recoverable [Domain] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (1,777) | (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]
[1] | includes an adjustment of $2.9 million related to our adoption of ASU 2016-13, Financial Instruments - Credit Losses.

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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 -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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