SEGMENT INFORMATION - Net premiums earned (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,214,469	$ 1,224,075	$ 1,185,548	$ 1,167,402	$ 1,211,495	$ 1,017,131	$ 981,431	$ 938,703	$ 4,791,495	$ 4,148,760	$ 3,705,625
Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned									4,791,495	4,148,760	3,705,625
Operating Segments | Insurance
Revenue from External Customer [Line Items]
Net premiums earned									2,362,606	1,816,438	1,534,282
Operating Segments | Insurance | Property
Revenue from External Customer [Line Items]
Net premiums earned									796,945	543,342	426,918
Operating Segments | Insurance | Marine
Revenue from External Customer [Line Items]
Net premiums earned									300,944	181,533	150,046
Operating Segments | Insurance | Terrorism
Revenue from External Customer [Line Items]
Net premiums earned									49,150	36,084	33,279
Operating Segments | Insurance | Aviation
Revenue from External Customer [Line Items]
Net premiums earned									74,203	75,107	44,980
Operating Segments | Insurance | Credit and political risk
Revenue from External Customer [Line Items]
Net premiums earned									102,825	56,432	57,964
Operating Segments | Insurance | Professional lines
Revenue from External Customer [Line Items]
Net premiums earned									570,241	519,759	510,806
Operating Segments | Insurance | Liability
Revenue from External Customer [Line Items]
Net premiums earned									229,373	188,770	169,182
Operating Segments | Insurance | Accident and health
Revenue from External Customer [Line Items]
Net premiums earned									207,777	199,121	141,107
Operating Segments | Insurance | Discontinued lines - Novae
Revenue from External Customer [Line Items]
Net premiums earned									31,148	16,290	0
Operating Segments | Reinsurance
Revenue from External Customer [Line Items]
Net premiums earned									2,428,889	2,332,322	2,171,343
Operating Segments | Reinsurance | Catastrophe
Revenue from External Customer [Line Items]
Net premiums earned									250,016	209,470	199,825
Operating Segments | Reinsurance | Property
Revenue from External Customer [Line Items]
Net premiums earned									317,038	304,376	272,403
Operating Segments | Reinsurance | Professional lines
Revenue from External Customer [Line Items]
Net premiums earned									220,687	226,622	289,868
Operating Segments | Reinsurance | Credit and surety
Revenue from External Customer [Line Items]
Net premiums earned									250,276	244,186	252,210
Operating Segments | Reinsurance | Motor
Revenue from External Customer [Line Items]
Net premiums earned									438,693	371,501	318,863
Operating Segments | Reinsurance | Liability
Revenue from External Customer [Line Items]
Net premiums earned									363,292	351,940	332,479
Operating Segments | Reinsurance | Agriculture
Revenue from External Customer [Line Items]
Net premiums earned									176,435	195,391	142,501
Operating Segments | Reinsurance | Engineering
Revenue from External Customer [Line Items]
Net premiums earned									67,932	66,291	62,833
Operating Segments | Reinsurance | Marine and other
Revenue from External Customer [Line Items]
Net premiums earned									35,570	64,449	57,322
Operating Segments | Reinsurance | Accident and health
Revenue from External Customer [Line Items]
Net premiums earned									299,813	289,925	243,039
Operating Segments | Reinsurance | Discontinued lines - Novae
Revenue from External Customer [Line Items]
Net premiums earned									$ 9,137	$ 8,171	$ 0