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Reinsurance Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Losses) (Details) - Underwriting [Member] - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
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Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Effects of Reinsurance [Line Items] |  |  |  |
Gross | $ 868,069 | $ 893,276 | $ 2,111,626 | $ 1,715,030
Ceded | (154,307) | (215,389) | (321,487) | (349,412)
Net losses and loss adjustment expenses | $ 713,762 | $ 677,887 | $ 1,790,139 | $ 1,365,618

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- Definition Amount, before effect of policies assumed or ceded, of expense related to provision for policy benefits and costs incurred. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=116637232&loc=SL114874131-224263 + Details
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