SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) reportable_segment	Sep. 30, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | reportable_segment			2
Segment Information [Line Items]
Net premiums earned	$ 1,091,312	$ 1,157,307	$ 3,283,941	$ 3,415,126
Other insurance related income	1,440	1,533	(5,270)	11,385
Net losses and loss expenses	(879,677)	(850,913)	(2,464,012)	(2,187,403)
Acquisition costs	(230,564)	(260,026)	(697,716)	(762,807)
Expenses	(138,823)	(155,522)	(436,538)	(496,008)
Net investment income	101,956	115,763	240,098	361,014
Net investment gains	55,609	14,527	45,777	48,522
Foreign exchange (losses) gains	(60,734)	59,543	(8,760)	64,868
Interest expense and financing costs	(15,574)	(18,042)	(59,641)	(49,545)
Reorganization expenses	(1,413)	(11,215)	(822)	(29,310)
Amortization of value of business acquired	(1,028)	(4,368)	(4,111)	(24,666)
Amortization of intangible assets	(2,838)	(2,831)	(8,564)	(8,744)
Income (loss) before income taxes and interest in income (loss) of equity method investments	(80,334)	45,756	(115,618)	342,432
Operating Segments
Segment Information [Line Items]
Gross premiums written	1,331,178	1,406,506	5,478,519	5,637,491
Net premiums written	815,982	856,081	3,550,960	3,703,460
Net premiums earned	1,091,312	1,157,307	3,283,941	3,415,126
Other insurance related income	1,440	1,533	(5,270)	11,385
Net losses and loss expenses	(879,677)	(850,913)	(2,464,012)	(2,187,403)
Acquisition costs	(230,564)	(260,026)	(697,716)	(762,807)
Expenses	(117,835)	(126,619)	(361,623)	(398,540)
Underwriting loss	$ (135,324)	$ (78,718)	$ (244,680)	$ 77,761
Net losses and loss expenses ratio	80.60%	73.50%	75.00%	64.10%
Acquisition cost ratio	21.10%	22.50%	21.20%	22.30%
General and administrative expense ratio	12.80%	13.40%	13.40%	14.50%
Combined ratio	114.50%	109.40%	109.60%	100.90%
Goodwill and intangible assets	$ 324,365	$ 346,356	$ 324,365	$ 346,356
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	935,817	894,902	2,914,100	2,714,322
Net premiums written	544,857	517,050	1,729,268	1,638,197
Net premiums earned	570,184	536,451	1,709,268	1,630,473
Other insurance related income	688	733	2,091	1,779
Net losses and loss expenses	(443,389)	(338,966)	(1,252,569)	(961,444)
Acquisition costs	(114,569)	(115,551)	(343,579)	(344,981)
Expenses	(94,379)	(100,559)	(284,909)	(311,491)
Underwriting loss	$ (81,465)	$ (17,892)	$ (169,698)	$ 14,336
Net losses and loss expenses ratio	77.80%	63.20%	73.30%	59.00%
Acquisition cost ratio	20.10%	21.50%	20.10%	21.20%
General and administrative expense ratio	16.50%	18.80%	16.70%	19.00%
Combined ratio	114.40%	103.50%	110.10%	99.20%
Goodwill and intangible assets	$ 324,365	$ 346,356	$ 324,365	$ 346,356
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	395,361	511,604	2,564,419	2,923,169
Net premiums written	271,125	339,031	1,821,692	2,065,263
Net premiums earned	521,128	620,856	1,574,673	1,784,653
Other insurance related income	752	800	(7,361)	9,606
Net losses and loss expenses	(436,288)	(511,947)	(1,211,443)	(1,225,959)
Acquisition costs	(115,995)	(144,475)	(354,137)	(417,826)
Expenses	(23,456)	(26,060)	(76,714)	(87,049)
Underwriting loss	$ (53,859)	$ (60,826)	$ (74,982)	$ 63,425
Net losses and loss expenses ratio	83.70%	82.50%	76.90%	68.70%
Acquisition cost ratio	22.30%	23.30%	22.50%	23.40%
General and administrative expense ratio	4.50%	4.10%	4.90%	4.90%
Combined ratio	110.50%	109.90%	104.30%	97.00%
Goodwill and intangible assets	$ 0	$ 0	$ 0	$ 0
Corporate Expenses
Segment Information [Line Items]
Expenses	(20,988)	(28,903)	(74,915)	(97,468)
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	101,956	115,763	240,098	361,014
Net investment gains	55,609	14,527	45,777	48,522
Foreign exchange (losses) gains	(60,734)	59,543	(8,760)	64,868
Interest expense and financing costs	(15,574)	(18,042)	(59,641)	(49,545)
Reorganization expenses	(1,413)	(11,215)	(822)	(29,310)
Amortization of value of business acquired	(1,028)	(4,368)	(4,111)	(24,666)
Amortization of intangible assets	$ (2,838)	$ (2,831)	$ (8,564)	$ (8,744)