RESERVE FOR LOSSES AND LOSS EXPENSES - Prior year reserve development by segment, reserving class and accident year (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	$ 6,808	$ 2,655	$ 12,126	$ 8,767
Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	6,427	420	7,932	4,251
Insurance | Property and other
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	35,290	25,117	50,681	37,607
Insurance | Marine
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	7,348	(1,262)	19,221	(3,464)
Insurance | Aviation
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	4,808	2,002	5,844	5,993
Insurance | Credit and political risk
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	4,055	(178)	(1,316)	(1,094)
Insurance | Professional lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	(33,054)	(8,519)	(47,646)	(13,606)
Insurance | Liability
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	(12,020)	(16,740)	(18,852)	(21,185)
Reinsurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	381	2,235	4,194	4,516
Reinsurance | Property and other
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	12,310	(4,162)	13,278	(8,065)
Reinsurance | Professional lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	(8,600)	(4,879)	(10,804)	283
Reinsurance | Liability
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	(11,358)	(3,655)	(18,512)	(20,570)
Reinsurance | Credit and surety
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	(4,917)	10,483	(9,854)	15,335
Reinsurance | Motor
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior years	$ 12,946	$ 4,448	$ 30,086	$ 17,533