SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) reportable_segment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Information [Line Items]
Number of reportable segments | reportable_segment			2
Net premiums earned	$ 981,431	$ 946,990	$ 1,920,133	$ 1,849,331
Other insurance related income (losses)	2,560	(892)	(1,222)	(1,094)
Net losses and loss expenses	(605,332)	(632,294)	(1,212,273)	(1,131,256)
Acquisition costs	(204,361)	(189,125)	(394,153)	(369,761)
Expenses	(147,816)	(146,746)	(309,075)	(296,648)
Net investment income	106,063	91,730	204,728	140,896
Net realized investment gains (losses)	(4,392)	21,010	(29,443)	(45,500)
Foreign exchange (losses) gains	(36,118)	56,602	(57,583)	55,986
Interest expense and financing costs	(12,751)	(12,914)	(25,543)	(25,747)
Bargain purchase gain	15,044	0	15,044	0
Income before income taxes and interest in income (loss) of equity method investments	$ 94,328	$ 134,361	$ 110,613	$ 176,207
Net loss and loss expense ratio	61.70%	66.80%	63.10%	61.20%
Acquisition cost ratio	20.80%	20.00%	20.50%	20.00%
General and administrative expense ratio	15.10%	15.40%	16.20%	16.00%
Combined ratio	97.60%	102.20%	99.80%	97.20%
Goodwill and intangible assets	$ 86,220	$ 85,954	$ 86,220	$ 85,954	$ 85,049
Operating Segments [Member]
Segment Information [Line Items]
Gross premiums written	1,362,327	1,320,434	3,274,199	3,279,595
Net premiums written	956,017	1,007,350	2,464,976	2,693,156
Net premiums earned	981,431	946,990	1,920,133	1,849,331
Other insurance related income (losses)	2,560	(892)	(1,222)	(1,094)
Net losses and loss expenses	(605,332)	(632,294)	(1,212,273)	(1,131,256)
Acquisition costs	(204,361)	(189,125)	(394,153)	(369,761)
Expenses	(117,286)	(114,819)	(239,086)	(238,409)
Underwriting income (loss)	57,012	9,860	73,399	108,811
Operating Segments [Member] | Insurance [Member]
Segment Information [Line Items]
Gross premiums written	796,023	784,017	1,490,030	1,437,365
Net premiums written	527,678	526,764	1,033,008	999,926
Net premiums earned	493,836	439,279	952,265	877,958
Other insurance related income (losses)	508	(234)	551	(96)
Net losses and loss expenses	(325,728)	(306,141)	(612,630)	(580,546)
Acquisition costs	(81,276)	(61,829)	(149,433)	(123,227)
Expenses	(87,822)	(82,487)	(178,270)	(168,064)
Underwriting income (loss)	$ (482)	$ (11,412)	$ 12,483	$ 6,025
Net loss and loss expense ratio	66.00%	69.70%	64.30%	66.10%
Acquisition cost ratio	16.50%	14.10%	15.70%	14.00%
General and administrative expense ratio	17.70%	18.70%	18.70%	19.20%
Combined ratio	100.20%	102.50%	98.70%	99.30%
Goodwill and intangible assets	$ 86,220	$ 85,954	$ 86,220	$ 85,954
Operating Segments [Member] | Reinsurance [Member]
Segment Information [Line Items]
Gross premiums written	566,304	536,417	1,784,169	1,842,230
Net premiums written	428,339	480,586	1,431,968	1,693,230
Net premiums earned	487,595	507,711	967,868	971,373
Other insurance related income (losses)	2,052	(658)	(1,773)	(998)
Net losses and loss expenses	(279,604)	(326,153)	(599,643)	(550,710)
Acquisition costs	(123,085)	(127,296)	(244,720)	(246,534)
Expenses	(29,464)	(32,332)	(60,816)	(70,345)
Underwriting income (loss)	$ 57,494	$ 21,272	$ 60,916	$ 102,786
Net loss and loss expense ratio	57.30%	64.20%	62.00%	56.70%
Acquisition cost ratio	25.20%	25.10%	25.30%	25.40%
General and administrative expense ratio	6.10%	6.40%	6.20%	7.20%
Combined ratio	88.60%	95.70%	93.50%	89.30%
Goodwill and intangible assets	$ 0	$ 0	$ 0	$ 0
Corporate, Non-Segment [Member]
Segment Information [Line Items]
Expenses	(30,530)	(31,927)	(69,989)	(58,239)
Significant Reconciling Items [Member]
Segment Information [Line Items]
Net investment income	106,063	91,730	204,728	140,896
Net realized investment gains (losses)	(4,392)	21,010	(29,443)	(45,500)
Foreign exchange (losses) gains	(36,118)	56,602	(57,583)	55,986
Interest expense and financing costs	(12,751)	(12,914)	(25,543)	(25,747)
Bargain purchase gain	$ 15,044	$ 0	$ 15,044	$ 0