RESERVE FOR LOSSES AND LOSS EXPENSES - Prior Year Reserve Development By Segment and Line of Business (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) cedant claim	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) cedant	Sep. 30, 2022 USD ($) cedant
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	$ 2,762	$ 4,735	$ 13,118	$ 17,632
Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	1,609	2,558	5,433	12,396
Insurance | Property
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	$ (10,068)	12,888	(4,376)	23,668
Number of claims | claim	2
Insurance | Accident and health
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	$ (5,213)	(7,677)	(5,765)	(5,379)
Insurance | Marine and aviation
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	19,472	20,550	35,374	38,128
Insurance | Cyber
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	10,869	(712)	19,997	6,385
Insurance | Professional lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	(4,383)	(2,036)	(20,475)	(10,322)
Insurance | Credit and political risk
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	3,374	8,106	16,323	13,288
Insurance | Liability
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	(12,442)	(28,561)	(35,645)	(53,372)
Reinsurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	1,153	2,177	7,685	5,236
Reinsurance | Non Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	(5,796)	(3,748)	(49,235)	(22,361)
Reinsurance | Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	6,949	5,925	56,920	27,597
Reinsurance | Property | Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	3,421	9,145	12,568	33,738
Reinsurance | Accident and health | Non Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	5,842	6,799	20,115	8,486
Reinsurance | Marine and aviation | Non Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	3,576	(785)	8,523	(1,185)
Reinsurance | Professional lines | Non Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	$ (9,044)	(11,431)	$ (22,772)	$ (41,954)
Number of cedants | cedant	2		2	1
Reinsurance | Liability | Non Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	$ (15,222)	(11,940)	$ (55,092)	$ (22,868)
Reinsurance | Agriculture | Non Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	729	855	14,741	9,228
Reinsurance | Credit and surety | Non Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	9,625	5,905	8,498	16,953
Reinsurance | Motor | Non Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	(1,302)	6,849	(23,248)	8,979
Reinsurance | Catastrophe | Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	1,436	(1,452)	39,982	(2,391)
Reinsurance | Engineering | Run-Off Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net favorable (adverse) prior year reserve development	$ 2,092	$ (1,768)	$ 4,370	$ (3,750)