SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) reportable_segment	Jun. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | reportable_segment			2
Segment Information [Line Items]
Net premiums earned	$ 1,104,003	$ 1,123,607	$ 2,192,628	$ 2,257,819
Other insurance related income (loss)	1,996	2,925	(6,710)	9,852
Net losses and loss expenses	(676,261)	(672,463)	(1,584,335)	(1,336,491)
Acquisition costs	(228,502)	(242,363)	(467,152)	(502,781)
Expenses	(140,652)	(165,395)	(297,712)	(340,486)
Net investment income	45,040	137,949	138,140	245,254
Net investment gains	53,043	21,225	(9,831)	33,996
Foreign exchange (losses) gains	(9,709)	12,381	51,974	5,325
Interest expense and financing costs	(20,595)	(15,607)	(44,067)	(31,502)
Reorganization expenses	(392)	(3,276)	591	(18,096)
Amortization of value of business acquired	(1,285)	(7,194)	(3,083)	(20,298)
Amortization of intangible assets	(2,855)	(2,912)	(5,725)	(5,914)
Income (loss) before income taxes and interest in income (loss) of equity method investments	123,831	188,877	(35,282)	296,678
Operating Segments
Segment Information [Line Items]
Gross premiums written	1,716,183	1,647,760	4,147,341	4,230,986
Net premiums written	1,055,934	1,070,321	2,734,978	2,847,381
Net premiums earned	1,104,003	1,123,607	2,192,628	2,257,819
Other insurance related income (loss)	1,996	2,925	(6,710)	9,852
Net losses and loss expenses	(676,261)	(672,463)	(1,584,335)	(1,336,491)
Acquisition costs	(228,502)	(242,363)	(467,152)	(502,781)
Expenses	(113,824)	(133,047)	(243,786)	(271,920)
Underwriting income	$ 87,412	$ 78,659	$ (109,355)	$ 156,479
Net losses and loss expenses ratio	61.30%	59.80%	72.30%	59.20%
Acquisition cost ratio	20.70%	21.60%	21.30%	22.30%
General and administrative expense ratio	12.70%	14.70%	13.50%	15.00%
Combined ratio	94.70%	96.10%	107.10%	96.50%
Goodwill and intangible assets	$ 327,095	$ 353,428	$ 327,095	$ 353,428
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	1,037,568	968,325	1,978,283	1,819,421
Net premiums written	602,761	591,909	1,184,411	1,121,149
Net premiums earned	577,019	537,260	1,139,083	1,094,022
Other insurance related income (loss)	755	(695)	1,403	1,046
Net losses and loss expenses	(337,367)	(308,703)	(809,180)	(622,479)
Acquisition costs	(116,259)	(111,655)	(229,010)	(229,430)
Expenses	(89,751)	(104,898)	(190,529)	(210,932)
Underwriting income	$ 34,397	$ 11,309	$ (88,233)	$ 32,227
Net losses and loss expenses ratio	58.50%	57.50%	71.00%	56.90%
Acquisition cost ratio	20.10%	20.80%	20.10%	21.00%
General and administrative expense ratio	15.60%	19.50%	16.80%	19.20%
Combined ratio	94.20%	97.80%	107.90%	97.10%
Goodwill and intangible assets	$ 327,095	$ 353,428	$ 327,095	$ 353,428
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	678,615	679,435	2,169,058	2,411,565
Net premiums written	453,173	478,412	1,550,567	1,726,232
Net premiums earned	526,984	586,347	1,053,545	1,163,797
Other insurance related income (loss)	1,241	3,620	(8,113)	8,806
Net losses and loss expenses	(338,894)	(363,760)	(775,155)	(714,012)
Acquisition costs	(112,243)	(130,708)	(238,142)	(273,351)
Expenses	(24,073)	(28,149)	(53,257)	(60,988)
Underwriting income	$ 53,015	$ 67,350	$ (21,122)	$ 124,252
Net losses and loss expenses ratio	64.30%	62.00%	73.60%	61.40%
Acquisition cost ratio	21.30%	22.30%	22.60%	23.50%
General and administrative expense ratio	4.60%	4.80%	5.00%	5.20%
Combined ratio	90.20%	89.10%	101.20%	90.10%
Goodwill and intangible assets	$ 0	$ 0	$ 0	$ 0
Corporate Expenses
Segment Information [Line Items]
Expenses	(26,828)	(32,348)	(53,926)	(68,566)
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	45,040	137,949	138,140	245,254
Net investment gains	53,043	21,225	(9,831)	33,996
Foreign exchange (losses) gains	(9,709)	12,381	51,974	5,325
Interest expense and financing costs	(20,595)	(15,607)	(44,067)	(31,502)
Reorganization expenses	(392)	(3,276)	591	(18,096)
Amortization of value of business acquired	(1,285)	(7,194)	(3,083)	(20,298)
Amortization of intangible assets	$ (2,855)	$ (2,912)	$ (5,725)	$ (5,914)