RESERVE FOR LOSSES AND LOSS EXPENSES (Reserve Roll-Forward) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reconciliation of Beginning and Ending Gross Unpaid Losses and Loss Expenses
Gross reserve for losses and loss expenses, beginning of period			$ 12,997,553	$ 9,697,827
Less reinsurance recoverable on unpaid losses, beginning of period			(3,159,514)	(2,276,109)
Net reserve for unpaid losses and loss expenses, beginning of period			9,838,039	7,421,718
Net incurred losses and loss expenses related to:
Current year			1,482,409	1,308,000
Prior years	$ (60,116)	$ (70,933)	(114,423)	(95,727)
Net incurred losses and loss expenses			1,367,986	1,212,273
Net paid losses and loss expenses related to:
Current year			(186,576)	(140,018)
Prior years			(1,233,793)	(969,153)
Net paid losses and loss expenses			(1,420,369)	(1,109,171)
Foreign exchange and other			(985,628)	242,973
Net reserve for unpaid losses and loss expenses, end of period	8,800,028	7,767,793	8,800,028	7,767,793
Reinsurance recoverable on unpaid losses, end of period	3,152,706	2,110,869	3,152,706	2,110,869
Gross reserve for losses and loss expenses, end of period	$ 11,952,734	$ 9,878,662	$ 11,952,734	$ 9,878,662