RESERVE FOR LOSSES AND LOSS EXPENSES (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 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	631,735	569,356
Prior years	(24,793)	(70,394)
Net incurred losses and loss expenses	606,942	498,962
Net paid losses and loss expenses related to:
Current year	(31,047)	(21,740)
Prior years	(521,478)	(534,532)
Net paid losses and loss expenses	(552,525)	(556,272)
Foreign exchange and other	36,797	87,420
Net reserve for unpaid losses and loss expenses, end of period	7,512,932	7,645,086
Reinsurance recoverable on unpaid losses, end of period	2,029,031	2,071,401
Gross reserve for losses and loss expenses, end of period	$ 9,541,963	$ 9,716,487