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RESERVE FOR LOSSES AND LOSS EXPENSES (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Net loss and loss expenses $ 914,186 $ 678,700    
Reinsurance recoverable on unpaid losses and loss expenses (Allowance for expected credit losses 2020: $18,954) 4,101,579 3,555,341 $ 3,877,756 $ 3,501,669
Net favorable (adverse) prior year reserve development 6,113 14,672    
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 10,000 (33,000)    
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 (17,000) 12,000    
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 13,000 12,000    
Insurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Net favorable (adverse) prior year reserve development 3,832 6,913    
Insurance | 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 13,000 (22,000)    
Insurance | Marine | 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   16,000    
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 (5,000) 6,000    
Reinsurance        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Net favorable (adverse) prior year reserve development 2,281 7,759    
Reinsurance | 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 (4,000) (29,000)    
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 5,000 10,000    
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 5,000      
Catastrophe and Weather-related Events        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Net loss and loss expenses 300,000 $ 11,000    
Pandemic        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Net loss and loss expenses 235,000      
Weather        
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]        
Net loss and loss expenses $ 65,000