RESERVE FOR LOSSES AND LOSS EXPENSES (Reserve Roll-Forward) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserve for losses and loss expenses, beginning of period			$ 12,752,081	$ 12,280,769
Less reinsurance recoverable on unpaid losses and loss expenses, beginning of period			(3,877,756)	(3,501,669)
Net reserve for unpaid losses and loss expenses, beginning of period			8,874,325	8,779,100
Net incurred losses and loss expenses related to:
Current year			2,473,363	2,252,424
Prior years	$ (584)	$ (26,727)	(9,351)	(65,021)
Net incurred losses and loss expenses			2,464,012	2,187,403
Net paid losses and loss expenses related to:
Current year			(227,332)	(329,519)
Prior years			(1,831,192)	(1,791,233)
Net paid losses and loss expenses			(2,058,524)	(2,120,752)
Foreign exchange and other			35,992	(53,037)
Net reserve for unpaid losses and loss expenses, end of period	9,315,805	8,792,714	9,315,805	8,792,714
Reinsurance recoverable on unpaid losses and loss expenses, end of period	4,337,683	3,705,793	4,337,683	3,705,793
Gross reserve for losses and loss expenses, end of period	$ 13,653,488	$ 12,498,507	$ 13,653,488	$ 12,498,507