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RESERVE FOR LOSSES AND LOSS EXPENSES (Prior year development) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net favorable (adverse) prior year reserve development | $ 45,660 | $ 47,768 | $ 160,083 | $ 143,495
Insurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net favorable (adverse) prior year reserve development | 13,478 | 7,926 | 60,547 | 35,579
Reinsurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net favorable (adverse) prior year reserve development | $ 32,182 | $ 39,842 | $ 99,536 | $ 107,916

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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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