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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, 2020 | Mar. 31, 2019 | Dec. 31, 2019 | Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Cumulative effect of new accounting principle in period of adoption |  |  |  |  | $ 1,435 |
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 |  | 407,276 |  | 397,253 |  |
Total incurred loss and loss expense |  | 400,324 |  | 386,579 |  |
Paid loss and loss expense for claims occurring in the current year |  | 70,610 |  | 73,964 |  |
Paid loss and loss expense for claims occurring in the prior years |  | 281,736 |  | 246,168 |  |
Total paid loss and loss expense |  | 352,346 |  | 320,132 |  |
Net reserve for loss and loss expense, at end of period |  | 3,568,075 |  | 3,422,927 |  |
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period |  | 535,560 |  | 543,582 |  |
Gross reserve for loss and loss expense, at end of period |  | 4,103,635 |  | 3,966,509 |  |
Loss Reserve for COVID-19 |  | 10,000 |  |  |  |
Change in Historical Claim Rate Experience [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Prior year development |  | (6,952) |  | (10,674) |  |
Workers' Compensation Insurance [Member] | Change in Historical Claim Rate Experience [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 |  | $ 3,000 |  |  |  |
Workers Compensation [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 |  |  |  | (8,000) |  |
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 |  |  |  | $ (2,000) |  |
Accounting Standards Update 2016-13 [Member] | Retained Earnings [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Cumulative effect of new accounting principle in period of adoption |  |  |  |  | 1,435 | $ 0
Reinsurance Recoverable [Domain] | Accounting Standards Update 2016-13 [Member] | Retained Earnings [Member] |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Cumulative effect of new accounting principle in period of adoption | [2] |  |  |  | $ (2,903) |
[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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- 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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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(13)(a)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 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 Reference 3: 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 + Details
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