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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years | $ 742,722 | $ 670,176 | $ 641,174
Favorable or adverse loss development | (39,192) | (65,768) | (69,009)
Favorable or unfavorable loss development [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years | 39,200 | 65,800 | 69,000
Favorable or adverse loss development | (39,200) | (65,800) | (69,000)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | 48,600 | 69,000 | 67,000
Favorable or unfavorable loss development [Member] | Property Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | 9,400 | 3,200 | 2,000
Favorable or unfavorable loss development [Member] | General Liability [Member] |  |  |
Favorable or adverse loss development | (48,300) | (45,000) | (51,000)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member] |  |  |
Favorable or adverse loss development | 35,600 | 25,300 | 2,400
Casualty Development | 36,000 | 25,000 |
Favorable or unfavorable loss development [Member] | Workers Compensation [Member] |  |  |
Favorable or adverse loss development | (52,300) | (56,000) | (37,000)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member] |  |  |
Favorable or adverse loss development | 1,900 | 1,800 | 2,200
Favorable or unfavorable loss development [Member] | Commercial Property [Member] |  |  |
Favorable or adverse loss development | 8,700 | 300 | (3,000)
Favorable or unfavorable loss development [Member] | Homeowners [Member] |  |  |
Favorable or adverse loss development | 400 | 1,700 | 1,500
Favorable or unfavorable loss development [Member] | Personal Automobile [Member] |  |  |
Favorable or adverse loss development | 6,700 | 1,000 | 400
Favorable or unfavorable loss development [Member] | Excess And Surplus Operations [Member] |  |  |
Favorable or adverse loss development | 10,000 | 6,000 | 16,000
Casualty Development | 10,000 |  |
Favorable or unfavorable loss development [Member] | Other Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | $ (1,900) | $ (900) | $ (500)

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=65670824&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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 + Details
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