SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Information [Line Items]
Net premiums earned	$ 1,322,564	$ 1,284,866	$ 3,818,508	$ 3,820,163
Other insurance related income (loss)	10,344	1,092	16,444	9,998
Net losses and loss expenses	(783,940)	(941,911)	(2,240,840)	(2,444,196)
Acquisition costs	(263,389)	(240,511)	(747,027)	(746,443)
Underwriting-related general and administrative expenses and corporate expenses	(179,283)	(158,245)	(514,596)	(492,872)
Net investment income	154,201	88,177	424,802	271,744
Foreign exchange gains	50,570	135,660	11,755	236,934
Interest expense and financing costs	(16,445)	(15,915)	(50,077)	(46,720)
Reorganization expenses	(28,997)	(6,213)	(28,997)	(21,941)
Amortization of intangible assets	(2,729)	(2,729)	(8,188)	(8,188)
Income (loss) before income taxes and interest in income (loss) of equity method investments	209,782	(2,187)	584,113	164,248
Income tax (expense) benefit	(24,624)	363	(68,078)	5,304
Interest in income (loss) of equity method investments	2,940	(7,560)	2,835	5,040
Net income (loss)	188,098	(9,384)	518,870	174,592
Preferred share dividends	7,563	7,563	22,688	22,688
Net income (loss) available (attributable) to common shareholders	180,535	(16,947)	496,182	151,904
Operating Segments
Segment Information [Line Items]
Gross premiums written	1,905,878	1,707,808	6,572,232	6,455,899
Net premiums written	975,357	1,036,784	4,030,070	4,166,502
Net premiums earned	1,322,564	1,284,866	3,818,508	3,820,163
Other insurance related income (loss)	10,344	1,092	16,444	9,998
Net losses and loss expenses	(783,940)	(941,911)	(2,240,840)	(2,444,196)
Acquisition costs	(263,389)	(240,511)	(747,027)	(746,443)
Underwriting-related general and administrative expenses and corporate expenses	(138,601)	(132,570)	(412,251)	(413,069)
Underwriting income (loss)	$ 146,978	$ (29,034)	$ 434,834	$ 226,453
Net losses and loss expenses ratio (as a percent)	59.30%	73.30%	58.70%	64.00%
Acquisition cost ratio (as a percent)	19.90%	18.70%	19.60%	19.50%
General and administrative expense ratio (as a percent)	13.50%	12.30%	13.40%	12.90%
Combined ratio (as a percent)	92.70%	104.30%	91.70%	96.40%
Goodwill and intangible assets	$ 290,413	$ 301,330	$ 290,413	$ 301,330
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	1,457,624	1,317,890	4,557,386	4,114,776
Net premiums written	885,252	777,789	2,788,849	2,491,120
Net premiums earned	885,714	782,101	2,544,920	2,303,640
Other insurance related income (loss)	(22)	151	90	470
Net losses and loss expenses	(491,368)	(519,006)	(1,398,486)	(1,346,585)
Acquisition costs	(169,384)	(139,436)	(473,413)	(422,979)
Underwriting-related general and administrative expenses and corporate expenses	(120,330)	(108,072)	(350,494)	(330,598)
Underwriting income (loss)	$ 104,610	$ 15,738	$ 322,617	$ 203,948
Net losses and loss expenses ratio (as a percent)	55.50%	66.40%	55.00%	58.50%
Acquisition cost ratio (as a percent)	19.10%	17.80%	18.60%	18.40%
General and administrative expense ratio (as a percent)	13.60%	13.80%	13.70%	14.30%
Combined ratio (as a percent)	88.20%	98.00%	87.30%	91.20%
Goodwill and intangible assets	$ 290,413	$ 301,330	$ 290,413	$ 301,330
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	448,254	389,918	2,014,846	2,341,123
Net premiums written	90,105	258,995	1,241,221	1,675,382
Net premiums earned	436,850	502,765	1,273,588	1,516,523
Other insurance related income (loss)	10,366	941	16,354	9,528
Net losses and loss expenses	(292,572)	(422,905)	(842,354)	(1,097,611)
Acquisition costs	(94,005)	(101,075)	(273,614)	(323,464)
Underwriting-related general and administrative expenses and corporate expenses	(18,271)	(24,498)	(61,757)	(82,471)
Underwriting income (loss)	$ 42,368	$ (44,772)	$ 112,217	$ 22,505
Net losses and loss expenses ratio (as a percent)	67.00%	84.10%	66.10%	72.40%
Acquisition cost ratio (as a percent)	21.50%	20.10%	21.50%	21.30%
General and administrative expense ratio (as a percent)	4.20%	4.90%	4.90%	5.40%
Combined ratio (as a percent)	92.70%	109.10%	92.50%	99.10%
Goodwill and intangible assets	$ 0	$ 0	$ 0	$ 0
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	154,201	88,177	424,802	271,744
Net investment gains (losses)	(53,114)	(146,458)	(97,671)	(414,231)
Foreign exchange gains	50,570	135,660	11,755	236,934
Interest expense and financing costs	(16,445)	(15,915)	(50,077)	(46,720)
Reorganization expenses	(28,997)	(6,213)	(28,997)	(21,941)
Amortization of intangible assets	(2,729)	(2,729)	(8,188)	(8,188)
Corporate expenses
Segment Information [Line Items]
Underwriting-related general and administrative expenses and corporate expenses	$ (40,682)	$ (25,675)	$ (102,345)	$ (79,803)