SEGMENT INFORMATION - Net premiums earned (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,211,495	$ 1,017,131	$ 981,431	$ 938,703	$ 921,879	$ 934,415	$ 946,990	$ 902,340	$ 4,148,760	$ 3,705,625	$ 3,686,417
Insurance
Revenue from External Customer [Line Items]
Net premiums earned									2,106,363	1,777,321	1,798,191
Insurance | Property
Revenue from External Customer [Line Items]
Net premiums earned									543,342	426,918	432,587
Insurance | Marine
Revenue from External Customer [Line Items]
Net premiums earned									181,533	150,046	183,696
Insurance | Terrorism
Revenue from External Customer [Line Items]
Net premiums earned									36,084	33,279	36,818
Insurance | Aviation
Revenue from External Customer [Line Items]
Net premiums earned									75,107	44,980	45,659
Insurance | Credit and Political Risk
Revenue from External Customer [Line Items]
Net premiums earned									56,432	57,964	63,583
Insurance | Professional Lines
Revenue from External Customer [Line Items]
Net premiums earned									519,759	510,806	596,430
Insurance | Liability
Revenue from External Customer [Line Items]
Net premiums earned									188,770	169,182	161,614
Insurance | Accident and Health
Revenue from External Customer [Line Items]
Net premiums earned									489,046	384,146	277,804
Insurance | Discontinued lines - Novae
Revenue from External Customer [Line Items]
Net premiums earned									16,290	0	0
Reinsurance
Revenue from External Customer [Line Items]
Net premiums earned									2,042,397	1,928,304	1,888,226
Reinsurance | Property
Revenue from External Customer [Line Items]
Net premiums earned									304,376	272,403	306,083
Reinsurance | Catastrophe
Revenue from External Customer [Line Items]
Net premiums earned									209,470	199,825	216,020
Reinsurance | Professional Lines
Revenue from External Customer [Line Items]
Net premiums earned									226,622	289,868	310,915
Reinsurance | Credit and Surety
Revenue from External Customer [Line Items]
Net premiums earned									244,186	252,210	250,208
Reinsurance | Motor
Revenue from External Customer [Line Items]
Net premiums earned									371,501	318,863	299,883
Reinsurance | Liability
Revenue from External Customer [Line Items]
Net premiums earned									351,940	332,479	297,000
Reinsurance | Agriculture
Revenue from External Customer [Line Items]
Net premiums earned									195,391	142,501	129,346
Reinsurance | Discontinued lines - Novae
Revenue from External Customer [Line Items]
Net premiums earned									8,171	0	0
Reinsurance | Engineering
Revenue from External Customer [Line Items]
Net premiums earned									66,291	62,833	61,043
Reinsurance | Marine and Other
Revenue from External Customer [Line Items]
Net premiums earned									$ 64,449	$ 57,322	$ 17,728