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Reserve for Loss and Loss Expense (Reserve Loss Development) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years | $ 753,321 | $ 742,722 | $ 670,176
Favorable or adverse loss development | (29,863) | (39,192) | (65,768)
Favorable or unfavorable loss development [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years | 29,900 | 39,200 | 65,800
Favorable or adverse loss development | (29,900) | (39,200) | (65,800)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | 41,500 | 48,600 | 69,000
Favorable or unfavorable loss development [Member] | Property Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | 11,600 | 9,400 | 3,200
Favorable or unfavorable loss development [Member] | General Liability [Member] |  |  |
Favorable or adverse loss development | (9,500) | (48,300) | (45,000)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member] |  |  |
Favorable or adverse loss development | 36,700 | 35,600 | 25,300
Casualty Development | 37,500 | 36,000 | 25,000
Favorable or unfavorable loss development [Member] | Workers Compensation [Member] |  |  |
Favorable or adverse loss development | (83,000) | (52,300) | (56,000)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member] |  |  |
Favorable or adverse loss development | (1,500) | 1,900 | 1,800
Favorable or unfavorable loss development [Member] | Commercial Property [Member] |  |  |
Favorable or adverse loss development | 7,500 | 8,700 | 300
Favorable or unfavorable loss development [Member] | Homeowners [Member] |  |  |
Favorable or adverse loss development | 9,800 | 400 | 1,700
Favorable or unfavorable loss development [Member] | Personal Automobile [Member] |  |  |
Favorable or adverse loss development | 3,000 | 6,700 | 1,000
Favorable or unfavorable loss development [Member] | E&S Casualty Lines [Member] |  |  |
Favorable or adverse loss development | 12,000 | 10,000 | 6,000
Favorable or unfavorable loss development [Member] | E&S Property Lines [Member] |  |  |
Favorable or adverse loss development | (4,800) | 100 | 1,200
Favorable or unfavorable loss development [Member] | Other Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | $ (100) | $ (2,000) | $ (2,100)

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- Definition Amount of provision in the period for casualty claims incurred in prior reporting periods and related claims settlement costs. + References No definition available. + 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 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=99382578&loc=d3e14764-158437 + Details
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