SEGMENT INFORMATION - Segment results (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) reportable_segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Information [Line Items]
Number of reportable segments | reportable_segment									2
Gross premiums written									$ 6,910,065	$ 5,556,273	$ 4,970,208
Net premiums written									4,658,962	4,027,143	3,752,974
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
Other insurance related income (losses)									10,622	(1,240)	7,222
Net losses and loss expenses									(3,190,287)	(3,287,772)	(2,204,197)
Acquisition costs									(968,835)	(823,591)	(746,876)
General and administrative expenses									(627,389)	(579,428)	(602,717)
Underwriting income (loss)	(194,664)	59,026	115,726	143,737	26,130	(512,853)	57,012	16,385
Net investment income	113,128	114,421	109,960	100,999	100,908	95,169	106,063	98,664	438,507	400,805	353,335
Net investment losses	(72,667)	(17,628)	(45,093)	(14,830)	43,038	$ 14,632	$ (4,392)	$ (25,050)	(150,218)	28,226	(60,525)
Foreign exchange gains									29,165	(134,737)	121,295
Interest expense and financing costs									(67,432)	(54,811)	(51,360)
Bargain purchase gain									0	15,044	0
Transaction and reorganization expenses									(66,940)	(26,718)	0
Amortization of value of business acquired	(23,000)	$ (39,000)	$ (53,000)	$ (57,000)	(50,000)				(172,332)	(50,104)	0
Amortization of intangible assets									(13,814)	(2,543)	0
Income (loss) before income taxes and interest in income (loss) of equity method investments									12,542	(368,109)	521,802
Total intangible assets	379,285				566,828				379,285	566,828	85,049	$ 86,858
Operating Segments
Segment Information [Line Items]
Gross premiums written									6,910,065	5,556,273	4,970,208
Net premiums written									4,658,962	4,027,143	3,752,974
Net premiums earned									4,791,495	4,148,760	3,705,625
Other insurance related income (losses)									10,622	(1,240)	7,222
Net losses and loss expenses									(3,190,287)	(3,287,772)	(2,204,197)
Acquisition costs									(968,835)	(823,591)	(746,876)
General and administrative expenses									(519,168)	(449,483)	(482,701)
Underwriting income (loss)									$ 123,827	$ (413,326)	$ 279,073
Net loss and loss expense ratio									66.60%	79.20%	59.50%
Acquisition cost ratio									20.20%	19.90%	20.20%
General and administrative expense ratio									13.10%	14.00%	16.20%
Combined ratio									99.90%	113.10%	95.90%
Total intangible assets	379,285				566,828				$ 379,285	$ 566,828	$ 85,049
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written									3,797,592	2,814,918	2,432,475
Net premiums written									2,324,747	1,775,825	1,519,559
Net premiums earned									2,362,606	1,816,438	1,534,282
Other insurance related income (losses)									3,460	2,944	89
Net losses and loss expenses									(1,494,323)	(1,465,427)	(977,771)
Acquisition costs									(399,193)	(270,229)	(206,619)
General and administrative expenses									(395,252)	(325,368)	(327,351)
Underwriting income (loss)									$ 77,298	$ (241,642)	$ 22,630
Net loss and loss expense ratio									63.20%	80.70%	63.70%
Acquisition cost ratio									16.90%	14.90%	13.50%
General and administrative expense ratio									16.80%	17.90%	21.30%
Combined ratio									96.90%	113.50%	98.50%
Total intangible assets	379,285				566,828				$ 379,285	$ 566,828	$ 85,049
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written									3,112,473	2,741,355	2,537,733
Net premiums written									2,334,215	2,251,318	2,233,415
Net premiums earned									2,428,889	2,332,322	2,171,343
Other insurance related income (losses)									7,162	(4,184)	7,133
Net losses and loss expenses									(1,695,964)	(1,822,345)	(1,226,426)
Acquisition costs									(569,642)	(553,362)	(540,257)
General and administrative expenses									(123,916)	(124,115)	(155,350)
Underwriting income (loss)									$ 46,529	$ (171,684)	$ 256,443
Net loss and loss expense ratio									69.80%	78.10%	56.50%
Acquisition cost ratio									23.50%	23.70%	24.90%
General and administrative expense ratio									5.10%	5.30%	7.10%
Combined ratio									98.40%	107.10%	88.50%
Total intangible assets	$ 0				$ 0				$ 0	$ 0	$ 0
Corporate
Segment Information [Line Items]
General and administrative expenses									(108,221)	(129,945)	(120,016)
Significant Reconciling Items
Segment Information [Line Items]
Net investment income									438,507	400,805	353,335
Net investment losses									(150,218)	28,226	(60,525)
Foreign exchange gains									29,165	(134,737)	121,295
Interest expense and financing costs									(67,432)	(54,811)	$ (51,360)
Bargain purchase gain										15,044
Transaction and reorganization expenses									(66,940)	(26,718)
Amortization of value of business acquired									(172,332)	(50,104)
Amortization of intangible assets									$ (13,814)	$ (2,543)