RESERVE FOR LOSSES AND LOSS EXPENSES (Reserve Roll-Forward) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserve for losses and loss expenses, beginning of period			$ 13,926,766	$ 12,752,081
Less reinsurance recoverable on unpaid losses and loss expenses, beginning of period			(4,496,641)	(3,877,756)
Net reserve for unpaid losses and loss expenses, beginning of period			9,430,125	8,874,325
Net incurred losses and loss expenses related to:
Current year			1,393,316	1,593,102
Prior years	$ (6,808)	$ (2,655)	(12,126)	(8,767)
Net incurred losses and loss expenses			1,381,190	1,584,335
Net paid losses and loss expenses related to:
Current year			(109,482)	(89,724)
Prior years			(1,239,814)	(1,271,987)
Net paid losses and loss expenses			(1,349,296)	(1,361,711)
Foreign exchange and other			68,880	(78,304)
Net reserve for unpaid losses and loss expenses, end of period	9,530,899	9,018,645	9,530,899	9,018,645
Reinsurance recoverable on unpaid losses and loss expenses, end of period	4,626,454	4,160,521	4,626,454	4,160,521
Gross reserve for losses and loss expenses, end of period	$ 14,157,353	$ 13,179,166	$ 14,157,353	$ 13,179,166