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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Net favorable prior year reserve development | $ 200,054 | $ 292,377 | $ 243,048
Insurance |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Net favorable prior year reserve development | 48,969 | 55,905 | 23,447
Reinsurance |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Net favorable prior year reserve development | $ 151,085 | $ 236,472 | $ 219,601

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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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -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 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 + Details
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