RESERVE FOR LOSSES AND LOSS EXPENSES - Reserve Roll-Forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Beginning and Ending Gross Unpaid Losses and Loss Expenses
Gross reserve for losses and loss expenses, beginning of year	$ 12,997,553	$ 9,697,827	$ 9,646,285
Less reinsurance recoverable on unpaid losses, beginning of year	(3,159,514)	(2,276,109)	(2,031,309)
Net reserve for unpaid losses and loss expenses, beginning of year	9,838,039	7,421,718	7,614,976
Net incurred losses and loss expenses related to:
Current year	3,389,949	3,487,826	2,496,574
Prior years	(199,662)	(200,054)	(292,377)
Net incurred losses and loss expenses	3,190,287	3,287,772	2,204,197
Net paid losses and loss expenses related to:
Current year	(724,199)	(703,796)	(428,153)
Prior years	(2,368,615)	(1,880,882)	(1,763,696)
Net paid losses and loss expenses	(3,092,814)	(2,584,678)	(2,191,849)
Foreign exchange and other	(1,156,412)	1,713,227	(205,606)
Net reserve for unpaid losses and loss expenses, end of year	8,779,100	9,838,039	7,421,718
Reinsurance recoverable on unpaid losses, end of year	3,501,669	3,159,514	2,276,109
Gross reserve for losses and loss expenses, end of year	$ 12,280,769	$ 12,997,553	$ 9,697,827