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Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($) $ in Thousands | 6 Months Ended
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Jun. 30, 2019 | Jun. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Gross reserve for loss and loss expense, at beginning of year | $ 3,893,868 | $ 3,771,240
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year | 537,388 | 585,855
Net reserve for loss and loss expense, at beginning of year | 3,356,480 | 3,185,385
Incurred loss and loss expense for claims occurring in the current year | 783,938 | 756,855
Total incurred loss and loss expense | 767,563 | 751,269
Paid loss and loss expense for claims occurring in the current year | 210,374 | 214,169
Paid loss and loss expense for claims occurring in the prior years | 442,820 | 457,441
Total paid loss and loss expense | 653,194 | 671,610
Net reserve for loss and loss expense, at end of period | 3,470,849 | 3,265,044
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period | 556,203 | 539,321
Gross reserve for loss and loss expense, at end of period | 4,027,052 | 3,804,365
Change in Historical Claim Rate Experience [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | (16,375) | (5,586)
Casualty Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development |  | (12,000)
Property Insurance Product Line [Member] | Change in Historical Claim Rate Experience [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development | 10,600 | 6,400
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 | (20,000) | (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 | $ (7,000) |
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 |  | 15,000
Excess And Surplus Operations [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 |  | $ 6,000

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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=99382578&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=99382578&loc=d3e14764-158437 + Details
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- Definition Amount, after effects of reinsurance, of payments to settle claims incurred in the current period and related claims settlement costs. + 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 (c) -URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437 + Details
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- Definition Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs. + 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 (c) -URI http://asc.fasb.org/extlink&oid=99382578&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=6879938&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=99382578&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=99382578&loc=d3e14764-158437 + Details
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