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RESERVE FOR LOSSES AND LOSS EXPENSES - Prior year development (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Net favorable prior year reserve development | $ 199,662 | $ 200,054 | $ 292,377
Insurance |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Net favorable prior year reserve development | 92,806 | 60,459 | 48,978
Reinsurance |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Net favorable prior year reserve development | $ 106,856 | $ 139,595 | $ 243,399

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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=99382578&loc=d3e14764-158437 + Details
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