RESERVE FOR LOSSES AND LOSS EXPENSES - Reserve Roll-Forward (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserve for losses and loss expenses, beginning of period	$ 17,218,929	$ 16,434,018
Less reinsurance recoverable on unpaid losses and loss expenses, beginning of period	(6,840,897)	(6,323,083)
Net reserve for unpaid losses and loss expenses, beginning of period	10,378,032	10,110,935
Net incurred losses and loss expenses related to:
Current year	803,862	728,671
Prior years	(17,937)	0
Net incurred losses and loss expenses	785,925	728,671
Net paid losses and loss expenses related to:
Current year	(45,288)	(50,724)
Prior years	(723,513)	(612,571)
Net paid losses and loss expenses	(768,801)	(663,295)
Foreign exchange and other	149,785	(48,602)
Net reserve for unpaid losses and loss expenses, end of period	10,544,941	10,127,709
Reinsurance recoverable on unpaid losses and loss expenses, end of period	6,944,518	6,503,188
Gross reserve for losses and loss expenses, end of period	$ 17,489,459	$ 16,630,897