RESERVE FOR LOSSES AND LOSS EXPENSES - Reconciliation of Development Tables to Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Claims Development [Line Items]
Net outstanding liabilities	$ 10,565,875
Reinsurance recoverable on unpaid claims	6,840,897	$ 6,323,083	$ 5,831,172	$ 5,017,611
Gross outstanding liabilities	17,406,772
Unallocated claims adjustment expenses	212,623
Foreign exchange and other	(10,992)
Ceded reserves related to retroactive transactions	(389,474)
Reserve for losses and loss expenses	17,218,929	$ 16,434,018	$ 15,168,863	$ 14,653,094
Insurance
Claims Development [Line Items]
Net outstanding liabilities	5,738,936
Reinsurance recoverable on unpaid claims	4,796,155
Gross outstanding liabilities	10,535,091
Insurance | Property
Claims Development [Line Items]
Net outstanding liabilities	810,674
Reinsurance recoverable on unpaid claims	476,362
Gross outstanding liabilities	1,287,036
Insurance | Accident and health
Claims Development [Line Items]
Net outstanding liabilities	134,285
Reinsurance recoverable on unpaid claims	9,565
Gross outstanding liabilities	143,850
Insurance | Marine and aviation
Claims Development [Line Items]
Net outstanding liabilities	795,884
Reinsurance recoverable on unpaid claims	348,062
Gross outstanding liabilities	1,143,946
Insurance | Cyber
Claims Development [Line Items]
Net outstanding liabilities	370,549
Reinsurance recoverable on unpaid claims	455,301
Gross outstanding liabilities	825,850
Insurance | Professional lines
Claims Development [Line Items]
Net outstanding liabilities	2,034,530
Reinsurance recoverable on unpaid claims	1,365,043
Gross outstanding liabilities	3,399,573
Insurance | Credit and political risk
Claims Development [Line Items]
Net outstanding liabilities	184,012
Reinsurance recoverable on unpaid claims	35,302
Gross outstanding liabilities	219,314
Insurance | Liability
Claims Development [Line Items]
Net outstanding liabilities	1,409,002
Reinsurance recoverable on unpaid claims	2,106,520
Gross outstanding liabilities	3,515,522
Reinsurance
Claims Development [Line Items]
Net outstanding liabilities	4,826,939
Reinsurance recoverable on unpaid claims	2,044,742
Gross outstanding liabilities	6,871,681
Reinsurance | Accident and health
Claims Development [Line Items]
Net outstanding liabilities	187,670
Reinsurance recoverable on unpaid claims	61,724
Gross outstanding liabilities	249,394
Reinsurance | Marine and aviation
Claims Development [Line Items]
Net outstanding liabilities	149,344
Reinsurance recoverable on unpaid claims	26,273
Gross outstanding liabilities	175,617
Reinsurance | Professional lines
Claims Development [Line Items]
Net outstanding liabilities	943,175
Reinsurance recoverable on unpaid claims	440,572
Gross outstanding liabilities	1,383,747
Reinsurance | Liability
Claims Development [Line Items]
Net outstanding liabilities	1,611,231
Reinsurance recoverable on unpaid claims	813,796
Gross outstanding liabilities	2,425,027
Reinsurance | Agriculture
Claims Development [Line Items]
Net outstanding liabilities	177,971
Reinsurance recoverable on unpaid claims	10,917
Gross outstanding liabilities	188,888
Reinsurance | Credit and surety
Claims Development [Line Items]
Net outstanding liabilities	306,042
Reinsurance recoverable on unpaid claims	132,474
Gross outstanding liabilities	438,516
Reinsurance | Motor
Claims Development [Line Items]
Net outstanding liabilities	934,447
Reinsurance recoverable on unpaid claims	352,438
Gross outstanding liabilities	1,286,885
Reinsurance | Run-off lines
Claims Development [Line Items]
Net outstanding liabilities	517,059
Reinsurance recoverable on unpaid claims	206,548
Gross outstanding liabilities	$ 723,607