RESERVE FOR LOSSES AND LOSS EXPENSES (Reserve Roll-Forward) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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			1,593,102	1,374,784
Prior years	$ (2,655)	$ (23,621)	(8,767)	(38,293)
Net incurred losses and loss expenses			1,584,335	1,336,491
Net paid losses and loss expenses related to:
Current year			(89,724)	(132,111)
Prior years			(1,271,987)	(1,304,413)
Net paid losses and loss expenses			(1,361,711)	(1,436,524)
Foreign exchange and other			(78,304)	10,832
Net reserve for unpaid losses and loss expenses, end of period	9,018,645	8,689,899	9,018,645	8,689,899
Reinsurance recoverable on unpaid losses and loss expenses, end of period	4,160,521	3,564,812	4,160,521	3,564,812
Gross reserve for losses and loss expenses, end of period	$ 13,179,166	$ 12,254,711	$ 13,179,166	$ 12,254,711