RESERVE FOR LOSSES AND LOSS EXPENSES (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Apr. 01, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 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				1,308,000	1,279,369
Prior years		$ (70,933)	$ (77,719)	(95,727)	(148,113)
Net incurred losses and loss expenses				1,212,273	1,131,256
Net paid losses and loss expenses related to:
Current year				(140,018)	(112,730)
Prior years				(969,153)	(966,589)
Net paid losses and loss expenses				(1,109,171)	(1,079,319)
Foreign exchange and other				242,973	(106,629)
Net reserve for unpaid losses and loss expenses, end of period		7,767,793	7,560,284	7,767,793	7,560,284
Reinsurance recoverable on unpaid losses, end of period		2,110,869	2,222,020	2,110,869	2,222,020
Gross reserve for losses and loss expenses, end of period		9,878,662	$ 9,782,304	9,878,662	$ 9,782,304
Reinsurance recoverable on unpaid and paid losses		$ 2,184,934		$ 2,184,934		$ 2,334,922
Compagnie Belge d'Assurances Aviation NV/SA [Member]
Net paid losses and loss expenses related to:
Increase (decrease) of reserves for losses and loss expenses	$ 79,000
Interests acquired	100.00%
Reinsurance recoverable on unpaid and paid losses	$ 5,000