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Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Liability for Catastrophe Claims [Line Items] |  |
Increase in loss and loss expense incurred | $ 57,500 |
Increase in catastrophe losses | 13,800 |
Catastrophe losses | 26,040 |
Increase in non catastrophe property losses | 34,300 |
Non catastrophe property losses | 105,700 |
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 | 388,801 | 331,331
Total incurred loss and loss expense | 384,941 | 317,472
Paid loss and loss expense for claims occurring in the current year | 79,345 | 54,708
Paid loss and loss expense for claims occurring in the prior years | 248,738 | 235,742
Total paid loss and loss expense | 328,083 | 290,450
Net reserve for loss and loss expense, at end of period | 3,242,243 | 3,107,541
Add: reinsurance recoverable on unpaid loss and loss expense, at end of period | 549,837 | 571,930
Gross reserve for loss and loss expense, at end of period | 3,792,080 | 3,679,471
Change in Historical Claim Rate Experience [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | (3,860) | (13,859)
Change in Historical Claim Rate Experience [Member] | Property Insurance Product Line [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | 4,100 |
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | (8,000) |
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Workers Compensation [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | (16,000) |
Change in Historical Claim Rate Experience [Member] | Casualty Insurance Product Line [Member] | Commercial Automobile [Member] |  |
Liability for Catastrophe Claims [Line Items] |  |
Prior Year Claims and Claims Adjustment Expense | $ 8,000 | 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 Claims and Claims Adjustment Expense |  | $ (22,400)

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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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