RESERVE FOR LOSSES AND LOSS EXPENSES (Reserve Roll-Forward) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reconciliation of Beginning and Ending Gross Unpaid Losses and Loss Expenses
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			1,374,784	1,482,409
Prior years	$ (23,621)	$ (60,116)	(38,293)	(114,423)
Net incurred losses and loss expenses			1,336,491	1,367,986
Net paid losses and loss expenses related to:
Current year			(132,111)	(186,576)
Prior years			(1,304,413)	(1,233,793)
Net paid losses and loss expenses			(1,436,524)	(1,420,369)
Foreign exchange and other			10,832	(985,628)
Net reserve for unpaid losses and loss expenses, end of period	8,689,899	8,800,028	8,689,899	8,800,028
Reinsurance recoverable on unpaid losses, end of period	3,564,812	3,152,706	3,564,812	3,152,706
Gross reserve for losses and loss expenses, end of period	$ 12,254,711	$ 11,952,734	$ 12,254,711	$ 11,952,734