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Reserve for Losses and Loss Expenses (Reserve Loss Development) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years | $ 670,176 | $ 641,174 | $ 592,062
Favorable or adverse loss development | (65,768) | (69,009) | (59,269)
Favorable or unfavorable loss development [Member] |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years | 65,800 | 69,000 | 59,300
Favorable or adverse loss development | (65,800) | (69,000) | (59,300)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | 69,000 | 67,000 | 48,200
Favorable or unfavorable loss development [Member] | Property Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | 3,200 | 2,000 | 11,100
Favorable or unfavorable loss development [Member] | General Liability [Member] |  |  |
Favorable or adverse loss development | (45,000) | (51,000) | (43,900)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member] |  |  |
Favorable or adverse loss development | 25,300 | 2,400 | (4,100)
Favorable or unfavorable loss development [Member] | Workers Compensation [Member] |  |  |
Favorable or adverse loss development | (56,000) | (37,000) | 0
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member] |  |  |
Favorable or adverse loss development | 1,800 | 2,200 | 1,900
Favorable or unfavorable loss development [Member] | Commercial Property [Member] |  |  |
Favorable or adverse loss development | 300 | (3,000) | (2,100)
Favorable or unfavorable loss development [Member] | Homeowners [Member] |  |  |
Favorable or adverse loss development | 1,700 | 1,500 | (4,000)
Favorable or unfavorable loss development [Member] | Personal Automobile [Member] |  |  |
Favorable or adverse loss development | 1,000 | 400 | (10,800)
Favorable or unfavorable loss development [Member] | Excess And Surplus Operations [Member] |  |  |
Favorable or adverse loss development | 7,100 | 15,500 | 3,700
Casualty Development | 6,000 |  |
Favorable or unfavorable loss development [Member] | Other Insurance Product Line [Member] |  |  |
Favorable or adverse loss development | $ (2,000) | $ 0 | $ 0

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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://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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