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Reserve for Losses and Loss Expenses Reserves for Losses and Loss Expenses (Rollforward of Reserves for Losses and Loss Expenses) (Details) - USD ($) $ in Thousands | 9 Months Ended
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Sep. 30, 2017 | Sep. 30, 2016
Liability for Catastrophe Claims [Line Items] |  |
Gross reserves for losses and loss expenses, at beginning of year | $ 3,691,719 | $ 3,517,728
Less: reinsurance recoverable on unpaid losses and loss expenses, at beginning of year | 611,200 | 551,019
Net reserves for losses and loss expenses, at beginning of year | 3,080,519 | 2,966,709
Incurred losses and loss expenses for claims occurring in the current year | 1,037,079 | 955,347
Total incurred losses and loss expenses | 1,003,618 | 911,881
Paid losses and loss expenses for claims occurring in the current year | 314,686 | 275,623
Paid losses and loss expenses for claims occurring in the prior years | 581,186 | 547,067
Total paid losses and loss expenses | 895,872 | 822,690
Net reserves for losses and loss expenses, at end of period | 3,188,265 | 3,055,900
Add: reinsurance recoverable on unpaid losses and loss expenses, at end of period | 647,535 | 630,686
Gross reserves for losses and loss expenses, at end of period | 3,835,800 | 3,686,586
Change in Historical Claim Rate Experience [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | (33,461) | (43,466)
Change in Historical Claim Rate Experience [Member] | General Liability [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | (48,300) | (33,000)
Change in Historical Claim Rate Experience [Member] | Workers Compensation [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | (29,300) | (36,000)
Change in Historical Claim Rate Experience [Member] | Commercial Automobile [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | 26,000 | 20,000
Change in Historical Claim Rate Experience [Member] | Excess And Surplus Operations [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | 10,000 | $ 3,000
Change in Historical Claim Rate Experience [Member] | Personal Automobile [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | $ 4,000 |

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -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 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&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://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 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 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://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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