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RESERVE FOR LOSSES AND LOSS EXPENSES (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Feb. 13, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Apr. 16, 2018
Dec. 31, 2017
Apr. 01, 2017
Mar. 20, 2017
Mar. 19, 2017
Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Current year       $ 2,323,028 $ 2,591,135            
Reinsurance recoverable on unpaid losses   $ 3,217,787 $ 2,298,022 3,217,787 2,298,022 $ 108,000 $ 3,159,514       $ 2,276,109
Reinsurance recoverable on unpaid and paid losses   3,439,080   3,439,080     $ 3,338,840        
Net favorable (adverse) prior year reserve development   45,660 47,768 160,083 143,495            
Short Tail Reserving Class Insurance And Reinsurance Business                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Net favorable (adverse) prior year reserve development   12,000 5,000 92,000 41,000            
Reinsurance Credit and Surety                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Net favorable (adverse) prior year reserve development   6,000 17,000 21,000 18,000            
Liability Reinsurance Business                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Net favorable (adverse) prior year reserve development   11,000   19,000 40,000            
Motor Reinsurance Business                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Net favorable (adverse) prior year reserve development   7,000 16,000 15,000 (4,000)            
Catastrophe and Weather-related Events                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Current year       162,000 702,000            
Discontinued Operations, Disposed of by Means Other than Sale                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Liability for unpaid claims and claims adjustment expense $ 223,000                    
Reduction in reinsurance recoverables on unpaid and paid loses by $ 223,000                    
Motor Reserve Class                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Ogden rate                 (0.75%) 2.50%  
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            
Reinsurance | Professional Lines                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Net favorable (adverse) prior year reserve development   10,000 9,000 18,000 36,000            
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            
Insurance | Professional Lines                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Net favorable (adverse) prior year reserve development   10,000   12,000 18,000            
Insurance | Liability Insurance Business                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Net favorable (adverse) prior year reserve development   $ (11,000)   (18,000) (6,000)            
Syndicate 2007                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Policyholder benefits and claims incurred, assumed and ceded       $ 819,000              
Aviabel                      
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]                      
Interests acquired               100.00%      
Reserves of businesses acquired     79,000   79,000            
Reinsurance recoverable on unpaid and paid losses     $ 5,000   $ 5,000