SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($) segment	Jun. 30, 2022 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Information [Line Items]
Net premiums earned	$ 1,265,745	$ 1,277,052	$ 2,495,944	$ 2,535,297
Other insurance related income	5,524	2,213	6,100	8,906
Net losses and loss expenses	(736,257)	(769,587)	(1,456,899)	(1,502,285)
Acquisition costs	(253,265)	(257,582)	(483,638)	(505,932)
Underwriting-related general and administrative expenses and corporate expenses	(168,503)	(165,586)	(335,314)	(334,627)
Net investment income	136,829	92,214	270,601	183,569
Foreign exchange (losses) gains	(30,104)	57,000	(38,814)	101,274
Interest expense and financing costs	(16,738)	(15,241)	(33,632)	(30,805)
Reorganization expenses	0	(15,728)	0	(15,728)
Amortization of intangible assets	(2,729)	(2,729)	(5,458)	(5,458)
Income before income taxes and interest in income (loss) of equity method investments	176,132	28,763	374,332	166,440
Income tax (expense) benefit	(27,558)	4,965	(43,454)	4,942
Interest in income of equity method investments	2,100	1,050	(105)	12,600
Net income	150,674	34,778	330,773	183,982
Preferred share dividends	7,563	7,563	15,125	15,125
Net income available to common shareholders	143,111	27,215	315,648	168,857
Operating Segments
Segment Information [Line Items]
Gross premiums written	2,284,378	2,113,483	4,666,354	4,748,091
Net premiums written	1,446,357	1,316,847	3,054,713	3,129,719
Net premiums earned	1,265,745	1,277,052	2,495,944	2,535,297
Other insurance related income	5,524	2,213	6,100	8,906
Net losses and loss expenses	(736,257)	(769,587)	(1,456,899)	(1,502,285)
Acquisition costs	(253,265)	(257,582)	(483,638)	(505,932)
Underwriting-related general and administrative expenses and corporate expenses	(133,255)	(135,403)	(273,650)	(280,499)
Underwriting income	$ 148,492	$ 116,693	$ 287,857	$ 255,487
Net losses and loss expenses ratio (as a percent)	58.20%	60.30%	58.40%	59.30%
Acquisition cost ratio (as a percent)	20.00%	20.20%	19.40%	20.00%
General and administrative expense ratio (as a percent)	13.30%	12.90%	13.40%	13.10%
Combined ratio (as a percent)	91.50%	93.40%	91.20%	92.40%
Goodwill and intangible assets	$ 293,143	$ 304,060	$ 293,143	$ 304,060
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	1,684,150	1,469,622	3,099,762	2,796,886
Net premiums written	1,021,021	869,419	1,903,597	1,713,332
Net premiums earned	842,751	768,724	1,659,206	1,521,539
Other insurance related income	58	237	112	319
Net losses and loss expenses	(457,650)	(421,836)	(907,117)	(827,579)
Acquisition costs	(156,972)	(144,732)	(304,030)	(283,543)
Underwriting-related general and administrative expenses and corporate expenses	(113,534)	(108,577)	(230,164)	(222,527)
Underwriting income	$ 114,653	$ 93,816	$ 218,007	$ 188,209
Net losses and loss expenses ratio (as a percent)	54.30%	54.90%	54.70%	54.40%
Acquisition cost ratio (as a percent)	18.60%	18.80%	18.30%	18.60%
General and administrative expense ratio (as a percent)	13.50%	14.10%	13.90%	14.70%
Combined ratio (as a percent)	86.40%	87.80%	86.90%	87.70%
Goodwill and intangible assets	$ 293,143	$ 304,060	$ 293,143	$ 304,060
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	600,228	643,861	1,566,592	1,951,205
Net premiums written	425,336	447,428	1,151,116	1,416,387
Net premiums earned	422,994	508,328	836,738	1,013,758
Other insurance related income	5,466	1,976	5,988	8,587
Net losses and loss expenses	(278,607)	(347,751)	(549,782)	(674,706)
Acquisition costs	(96,293)	(112,850)	(179,608)	(222,389)
Underwriting-related general and administrative expenses and corporate expenses	(19,721)	(26,826)	(43,486)	(57,972)
Underwriting income	$ 33,839	$ 22,877	$ 69,850	$ 67,278
Net losses and loss expenses ratio (as a percent)	65.90%	68.40%	65.70%	66.60%
Acquisition cost ratio (as a percent)	22.80%	22.20%	21.50%	21.90%
General and administrative expense ratio (as a percent)	4.60%	5.30%	5.20%	5.70%
Combined ratio (as a percent)	93.30%	95.90%	92.40%	94.20%
Goodwill and intangible assets	$ 0	$ 0	$ 0	$ 0
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	136,829	92,214	270,601	183,569
Net investment gains (losses)	(24,370)	(173,263)	(44,558)	(267,771)
Foreign exchange (losses) gains	(30,104)	57,000	(38,814)	101,274
Interest expense and financing costs	(16,738)	(15,241)	(33,632)	(30,805)
Reorganization expenses	0	(15,728)	0	(15,728)
Amortization of intangible assets	(2,729)	(2,729)	(5,458)	(5,458)
Corporate Expenses
Segment Information [Line Items]
Underwriting-related general and administrative expenses and corporate expenses	$ (35,248)	$ (30,183)	$ (61,664)	$ (54,128)