SEGMENT INFORMATION - Net premiums earned (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Information [Line Items]
Net premiums earned	$ 4,709,850	$ 4,371,309	$ 4,587,178
Operating Segments
Segment Information [Line Items]
Net premiums earned	4,709,850	4,371,309	4,587,178
Operating Segments | Insurance
Segment Information [Line Items]
Net premiums earned	2,651,339	2,299,038	2,190,084
Operating Segments | Insurance | Property
Segment Information [Line Items]
Net premiums earned	662,977	605,650	633,550
Operating Segments | Insurance | Marine
Segment Information [Line Items]
Net premiums earned	354,193	293,746	281,764
Operating Segments | Insurance | Terrorism
Segment Information [Line Items]
Net premiums earned	47,995	47,378	47,345
Operating Segments | Insurance | Aviation
Segment Information [Line Items]
Net premiums earned	84,882	70,910	55,028
Operating Segments | Insurance | Credit and political risk
Segment Information [Line Items]
Net premiums earned	96,604	105,869	91,698
Operating Segments | Insurance | Professional lines
Segment Information [Line Items]
Net premiums earned	898,307	715,276	661,250
Operating Segments | Insurance | Liability
Segment Information [Line Items]
Net premiums earned	354,518	313,291	264,667
Operating Segments | Insurance | Accident and health
Segment Information [Line Items]
Net premiums earned	151,134	143,723	144,499
Operating Segments | Insurance | Discontinued lines - Novae
Segment Information [Line Items]
Net premiums earned	729	3,195	10,283
Operating Segments | Reinsurance
Segment Information [Line Items]
Net premiums earned	2,058,511	2,072,271	2,397,094
Operating Segments | Reinsurance | Property
Segment Information [Line Items]
Net premiums earned	231,080	256,244	311,625
Operating Segments | Reinsurance | Catastrophe
Segment Information [Line Items]
Net premiums earned	238,775	244,934	267,591
Operating Segments | Reinsurance | Credit and surety
Segment Information [Line Items]
Net premiums earned	158,549	187,721	208,717
Operating Segments | Reinsurance | Professional lines
Segment Information [Line Items]
Net premiums earned	220,448	207,605	206,328
Operating Segments | Reinsurance | Liability
Segment Information [Line Items]
Net premiums earned	431,012	396,906	373,664
Operating Segments | Reinsurance | Accident and health
Segment Information [Line Items]
Net premiums earned	361,197	333,996	319,619
Operating Segments | Reinsurance | Discontinued lines - Novae
Segment Information [Line Items]
Net premiums earned	698	1,216	(1,048)
Operating Segments | Reinsurance | Motor
Segment Information [Line Items]
Net premiums earned	247,092	255,916	398,565
Operating Segments | Reinsurance | Engineering
Segment Information [Line Items]
Net premiums earned	28,238	60,521	63,899
Operating Segments | Reinsurance | Agriculture
Segment Information [Line Items]
Net premiums earned	82,744	73,696	188,925
Operating Segments | Reinsurance | Marine and aviation
Segment Information [Line Items]
Net premiums earned	$ 58,678	$ 53,516	$ 59,209