RESERVE FOR LOSSES AND LOSS EXPENSES (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reconciliation of Beginning and Ending Gross Unpaid Losses and Loss Expenses
Gross reserve for losses and loss expenses, beginning of period			$ 9,697,827	$ 9,646,285
Less reinsurance recoverable on unpaid losses, beginning of period			(2,276,109)	(2,031,309)
Net reserve for unpaid losses and loss expenses, beginning of period			7,421,718	7,614,976
Net incurred losses and loss expenses related to:
Current year			2,591,135	1,887,715
Prior years	$ (47,768)	$ (76,019)	(143,495)	(224,131)
Net incurred losses and loss expenses			2,447,640	1,663,584
Net paid losses and loss expenses related to:
Current year			(328,751)	(233,124)
Prior years			(1,384,510)	(1,334,772)
Net paid losses and loss expenses			(1,713,261)	(1,567,896)
Foreign exchange and other			333,456	(112,649)
Net reserve for unpaid losses and loss expenses, end of period	8,489,553	7,598,015	8,489,553	7,598,015
Reinsurance recoverable on unpaid losses, end of period	2,298,022	2,276,792	2,298,022	2,276,792
Gross reserve for losses and loss expenses, end of period	$ 10,787,575	$ 9,874,807	$ 10,787,575	$ 9,874,807