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Reserve for Loss and Loss Expense (Reserve Loss Development) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Favorable or adverse loss development | $ 72,932 | $ 50,289 | $ 29,863
Favorable or unfavorable loss development [Member] |  |  |
Favorable or adverse loss development | (72,900) | (50,300) | (29,900)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | (85,000) | (61,000) | (41,500)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member] | Commercial Automobile [Member] |  |  |
Favorable or adverse loss development | 10,000 |  | 37,500
Favorable or unfavorable loss development [Member] | Property Insurance [Member] |  |  |
Favorable or adverse loss development | 12,100 | 10,700 | 11,600
Favorable or unfavorable loss development [Member] | Property Insurance [Member] | Commercial Automobile [Member] |  |  |
Favorable or adverse loss development | 7,100 |  |
Favorable or unfavorable loss development [Member] | General Liability [Member] |  |  |
Favorable or adverse loss development | (35,000) | (5,000) | (9,500)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member] |  |  |
Favorable or adverse loss development | 7,100 | 700 | 36,700
Favorable or unfavorable loss development [Member] | Workers Compensation [Member] |  |  |
Favorable or adverse loss development | (60,000) | (68,000) | (83,000)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member] |  |  |
Favorable or adverse loss development | 3,900 | 1,900 | (1,500)
Favorable or unfavorable loss development [Member] | Commercial Property [Member] |  |  |
Favorable or adverse loss development | 9,200 | 5,100 | 7,500
Favorable or unfavorable loss development [Member] | Homeowners [Member] |  |  |
Favorable or adverse loss development | 7,700 | 7,500 | 9,800
Favorable or unfavorable loss development [Member] | Personal Automobile [Member] |  |  |
Favorable or adverse loss development | (1,800) | 4,400 | 3,000
Favorable or unfavorable loss development [Member] | E&S Casualty Lines [Member] |  |  |
Favorable or adverse loss development | 0 | 2,000 | 12,000
Favorable or unfavorable loss development [Member] | E&S Property Lines [Member] |  |  |
Favorable or adverse loss development | (4,000) | 1,000 | (4,800)
Favorable or unfavorable loss development [Member] | Insurance, Other [Member] |  |  |
Favorable or adverse loss development | $ 0 | $ 100 | $ (100)

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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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