RESERVE FOR LOSSES AND LOSS EXPENSES (Reserve Roll-Forward) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserve for losses and loss expenses, beginning of period			$ 12,280,769	$ 12,997,553
Less reinsurance recoverable on unpaid losses, beginning of period			(3,501,669)	(3,159,514)
Net reserve for unpaid losses and loss expenses, beginning of period			8,779,100	9,838,039
Net incurred losses and loss expenses related to:
Current year			2,252,424	2,323,028
Prior years	$ (26,727)	$ (45,660)	(65,021)	(160,083)
Net incurred losses and loss expenses			2,187,403	2,162,945
Net paid losses and loss expenses related to:
Current year			(329,519)	(381,158)
Prior years			(1,791,233)	(1,770,667)
Net paid losses and loss expenses			(2,120,752)	(2,151,825)
Foreign exchange and other			(53,037)	(1,040,999)
Net reserve for unpaid losses and loss expenses, end of period	8,792,714	8,808,160	8,792,714	8,808,160
Reinsurance recoverable on unpaid losses, end of period	3,705,793	3,217,787	3,705,793	3,217,787
Gross reserve for losses and loss expenses, end of period	$ 12,498,507	$ 12,025,947	$ 12,498,507	$ 12,025,947