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RESERVE FOR LOSSES AND LOSS EXPENSES - Reserve for losses and loss expenses (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Insurance Loss Reserves [Abstract]        
Reserve for reported losses and loss expenses $ 4,860,916 $ 4,626,204    
Reserve for losses incurred but not reported 7,891,165 7,654,565    
Reserve for losses and loss expenses $ 12,752,081 $ 12,280,769 $ 12,997,553 $ 9,697,827