SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) reportable_segment	Jun. 30, 2020 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | reportable_segment			2
Segment Information [Line Items]
Net premiums earned	$ 1,156,941	$ 1,104,003	$ 2,260,663	$ 2,192,628
Other insurance related income	5,817	1,996	8,598	(6,710)
Net losses and loss expenses	(666,473)	(676,261)	(1,381,190)	(1,584,335)
Acquisition costs	(219,070)	(228,502)	(437,941)	(467,152)
Expenses	(162,452)	(140,652)	(320,860)	(297,712)
Net investment income	104,672	45,040	218,836	138,140
Net investment gains	73,293	53,043	102,936	(9,831)
Foreign exchange losses	(19,602)	(9,709)	(23,716)	51,974
Interest expense and financing costs	(15,235)	(20,595)	(30,806)	(44,067)
Reorganization expenses	0	(392)	0	591
Amortization of value of business acquired	(1,028)	(1,285)	(2,056)	(3,083)
Amortization of intangible assets	(3,324)	(2,855)	(6,013)	(5,725)
Income before income taxes and interest in income of equity method investments	253,539	123,831	388,451	(35,282)
Operating Segments
Segment Information [Line Items]
Gross premiums written	1,941,186	1,716,183	4,476,667	4,147,341
Net premiums written	1,203,858	1,055,934	2,982,744	2,734,978
Net premiums earned	1,156,941	1,104,003	2,260,663	2,192,628
Other insurance related income	5,817	1,996	8,598	(6,710)
Net losses and loss expenses	(666,473)	(676,261)	(1,381,190)	(1,584,335)
Acquisition costs	(219,070)	(228,502)	(437,941)	(467,152)
Expenses	(128,961)	(113,824)	(261,629)	(243,786)
Underwriting income	$ 148,254	$ 87,412	$ 188,501	$ (109,355)
Net losses and loss expenses ratio	57.60%	61.30%	61.10%	72.30%
Acquisition cost ratio	18.90%	20.70%	19.40%	21.30%
General and administrative expense ratio	14.10%	12.70%	14.20%	13.50%
Combined ratio	90.60%	94.70%	94.70%	107.10%
Goodwill and intangible assets	$ 315,087	$ 327,095	$ 315,087	$ 327,095
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	1,268,472	1,037,568	2,371,670	1,978,283
Net premiums written	712,885	602,761	1,420,699	1,184,411
Net premiums earned	631,675	577,019	1,247,962	1,139,083
Other insurance related income	552	755	967	1,403
Net losses and loss expenses	(332,175)	(337,367)	(689,072)	(809,180)
Acquisition costs	(106,963)	(116,259)	(224,642)	(229,010)
Expenses	(99,569)	(89,751)	(202,872)	(190,529)
Underwriting income	$ 93,520	$ 34,397	$ 132,343	$ (88,233)
Net losses and loss expenses ratio	52.60%	58.50%	55.20%	71.00%
Acquisition cost ratio	16.90%	20.10%	18.00%	20.10%
General and administrative expense ratio	15.80%	15.60%	16.30%	16.80%
Combined ratio	85.30%	94.20%	89.50%	107.90%
Goodwill and intangible assets	$ 315,087	$ 327,095	$ 315,087	$ 327,095
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	672,714	678,615	2,104,997	2,169,058
Net premiums written	490,973	453,173	1,562,045	1,550,567
Net premiums earned	525,266	526,984	1,012,701	1,053,545
Other insurance related income	5,265	1,241	7,631	(8,113)
Net losses and loss expenses	(334,298)	(338,894)	(692,118)	(775,155)
Acquisition costs	(112,107)	(112,243)	(213,299)	(238,142)
Expenses	(29,392)	(24,073)	(58,757)	(53,257)
Underwriting income	$ 54,734	$ 53,015	$ 56,158	$ (21,122)
Net losses and loss expenses ratio	63.60%	64.30%	68.30%	73.60%
Acquisition cost ratio	21.30%	21.30%	21.10%	22.60%
General and administrative expense ratio	5.70%	4.60%	5.80%	5.00%
Combined ratio	90.60%	90.20%	95.20%	101.20%
Goodwill and intangible assets	$ 0	$ 0	$ 0	$ 0
Corporate Expenses
Segment Information [Line Items]
Expenses	(33,491)	(26,828)	(59,231)	(53,926)
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	104,672	45,040	218,836	138,140
Net investment gains	73,293	53,043	102,936	(9,831)
Foreign exchange losses	(19,602)	(9,709)	(23,716)	51,974
Interest expense and financing costs	(15,235)	(20,595)	(30,806)	(44,067)
Reorganization expenses	0	(392)	0	591
Amortization of value of business acquired	(1,028)	(1,285)	(2,056)	(3,083)
Amortization of intangible assets	$ (3,324)	$ (2,855)	$ (6,013)	$ (5,725)