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Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands | 6 Months Ended
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Jun. 30, 2020 | Jun. 30, 2019
Causes of (Increase) Decrease in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Favorable (adverse) development on prior years' loss reserves | $ 268,220 | $ 188,949
Prof liab general liab and workers comp Insurance Segment and prop and general liab Resinsurance Segment [Member] |  |
Causes of (Increase) Decrease in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Favorable (adverse) development on prior years' loss reserves | 206,600 |
Gen liab workers comp and personal lines Insurance segment and aviation whole account and auto Reinsurance segment [Member] |  |
Causes of (Increase) Decrease in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Favorable (adverse) development on prior years' loss reserves |  | $ 168,100
COVID-19 Pandemic [Member] |  |
Causes of (Increase) Decrease in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Net losses and loss adjustment expenses, net of assumed reinstatement premiums | 325,000 |
Reinsurance recoverables | $ 58,000 |

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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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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