RESERVE FOR LOSSES AND LOSS EXPENSES - Prior year reserve development by segment, reserving class and accident year (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	$ 32,410	$ 15,909	$ 78,900
Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	18,360	8,937	53,302
Insurance | Property and other
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	86,876	46,791	11,042
Insurance | Marine
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	28,430	16,780	33,260
Insurance | Aviation
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	14,106	6,416	3,741
Insurance | Credit and political risk
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	10,363	(745)	18,810
Insurance | Professional lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	(78,588)	(35,661)	11,721
Insurance | Liability
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	(42,827)	(24,644)	(25,272)
Reinsurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	14,049	6,972	25,598
Reinsurance | Property and other
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	8,282	(5,935)	(133,448)
Reinsurance | Professional lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	(23,718)	(15,352)	3,668
Reinsurance | Liability
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	(17,919)	(29,656)	31,283
Reinsurance | Credit and surety
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	3,436	36,829	53,223
Reinsurance | Motor
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year favorable (unfavorable) adjustments	$ 43,968	$ 21,086	$ 70,872