RESERVE FOR LOSSES AND LOSS EXPENSES - Reserve Roll-Forward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserve for losses and loss expenses, beginning of period			$ 15,168,863	$ 14,653,094
Less reinsurance recoverable on unpaid losses and loss expenses, beginning of period			(5,831,172)	(5,017,611)
Net reserve for unpaid losses and loss expenses, beginning of period			9,337,691	9,635,483
Net incurred losses and loss expenses related to:
Current year			2,253,958	2,461,828
Prior years	$ (2,762)	$ (4,735)	(13,118)	(17,632)
Net incurred losses and loss expenses	783,940	941,911	2,240,840	2,444,196
Net paid losses and loss expenses related to:
Current year			(272,260)	(269,466)
Prior years			(1,730,640)	(1,900,366)
Net paid losses and loss expenses			(2,002,900)	(2,169,832)
Foreign exchange and other			(51,902)	(501,914)
Net reserve for unpaid losses and loss expenses, end of period	9,523,729	9,407,933	9,523,729	9,407,933
Reinsurance recoverable on unpaid losses and loss expenses, end of period	6,031,527	5,244,263	6,031,527	5,244,263
Gross reserve for losses and loss expenses, end of period	$ 15,555,256	$ 14,652,196	$ 15,555,256	$ 14,652,196