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RESERVE FOR LOSSES AND LOSS EXPENSES - Incurred losses narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 15, 2019
Apr. 16, 2018
Dec. 31, 2017
Additional Information Related to Net Incurred Losses [Line Items]            
Net losses and loss expenses, net of reinstatement premiums $ 3,297,161 $ 3,123,698 $ 3,389,949      
Reinsurance recoverable on unpaid losses 4,496,641 3,877,756 3,501,669 $ 59,000 $ 108,000 $ 3,159,514
Catastrophe and Weather-related Events            
Additional Information Related to Net Incurred Losses [Line Items]            
Net losses and loss expenses, net of reinstatement premiums $ 774,000 $ 336,000 430,000      
Syndicate 2007            
Additional Information Related to Net Incurred Losses [Line Items]            
Ceded reserves for losses and loss expenses     $ 819,000