RESERVE FOR LOSSES AND LOSS EXPENSES - Reserve Roll-Forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Beginning and Ending Gross Unpaid Losses and Loss Expenses
Gross reserve for losses and loss expenses, beginning of year	$ 13,926,766	$ 12,752,081	$ 12,280,769
Less reinsurance recoverable on unpaid losses, beginning of year	(4,496,641)	(3,877,756)	(3,501,669)
Net reserve for unpaid losses and loss expenses, beginning of year	9,430,125	8,874,325	8,779,100
Net incurred losses and loss expenses related to:
Current year	3,041,193	3,297,161	3,123,698
Prior years	(32,410)	(15,909)	(78,900)
Net incurred losses and loss expenses	3,008,783	3,281,252	3,044,798
Net paid losses and loss expenses related to:
Current year	(490,011)	(571,442)	(598,988)
Prior years	(2,274,240)	(2,365,959)	(2,371,637)
Net paid losses and loss expenses	(2,764,251)	(2,937,401)	(2,970,625)
Foreign exchange and other	(39,174)	211,949	21,052
Net reserve for unpaid losses and loss expenses, end of year	9,635,483	9,430,125	8,874,325
Reinsurance recoverable on unpaid losses, end of year	5,017,611	4,496,641	3,877,756
Gross reserve for losses and loss expenses, end of year	$ 14,653,094	$ 13,926,766	$ 12,752,081