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Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($) $ in Thousands | 6 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017
Liability for Catastrophe Claims [Line Items] |  |
Increase in current year loss and loss expense incurred | $ 72,000 |
Increase in non catastrophe property losses | 44,000 |
Non catastrophe property losses | 188,700 |
Gross reserve for loss and loss expense, at beginning of year | 3,771,240 | $ 3,691,719
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year | 585,855 | 611,200
Net reserve for loss and loss expense, at beginning of year | 3,185,385 | 3,080,519
Incurred loss and loss expense for claims occurring in the current year | 756,855 | 684,877
Total incurred loss and loss expense | 751,269 | 659,031
Paid loss and loss expense for claims occurring in the current year | 214,169 | 171,724
Paid loss and loss expense for claims occurring in the prior years | 457,441 | 425,521
Total paid loss and loss expense | 671,610 | 597,245
Net reserve for loss and loss expense, at end of period | 3,265,044 | 3,142,305
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period | 539,321 | 588,916
Gross reserve for loss and loss expense, at end of period | 3,804,365 | 3,731,221
Change in Historical Claim Rate Experience [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior year development | (5,586) | (25,846)
Change in Historical Claim Rate Experience [Member] | Property Insurance Product Line [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior year development | 6,400 |
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior year development | (12,000) |
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Workers Compensation [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior year development | (33,000) | (15,300)
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Commercial Automobile [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior year development | 15,000 | 21,000
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Excess And Surplus Operations [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior year development | $ 6,000 |
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | General Liability [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior year development |  | (37,400)
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Personal Automobile [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior year development |  | $ 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 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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