RESERVE FOR LOSSES AND LOSS EXPENSES - Reserve Roll-forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Reconciliation of Beginning and Ending Gross Unpaid Losses and Loss Expenses
Gross reserve for losses and loss expenses, beginning of year	$ 16,434,018	$ 15,168,863	$ 14,653,094
Less reinsurance recoverable on unpaid losses and loss expenses, beginning of year	(6,323,083)	(5,831,172)	(5,017,611)
Net reserve for unpaid losses and loss expenses, beginning of year	10,110,935	9,337,691	9,635,483
Net incurred losses and loss expenses related to:
Current year	3,182,810	2,981,220	3,267,943
Prior years	(24,323)	411,882	(25,533)
Net incurred losses and loss expenses	3,158,487	3,393,102	3,242,410
Net paid losses and loss expenses related to:
Current year	(538,709)	(488,016)	(457,857)
Prior years	(2,360,100)	(2,185,588)	(2,397,213)
Net paid losses and loss expenses	(2,898,809)	(2,673,604)	(2,855,070)
Foreign exchange and other	7,419	53,746	(685,132)
Net reserve for unpaid losses and loss expenses, end of year	10,378,032	10,110,935	9,337,691
Reinsurance recoverable on unpaid losses and loss expenses, end of year	6,840,897	6,323,083	5,831,172
Gross reserve for losses and loss expenses, end of year	$ 17,218,929	$ 16,434,018	$ 15,168,863