SEGMENT INFORMATION - Summary of Underwriting Results of Reportable Segments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2022 USD ($) reportableSegment	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | reportableSegment	2
Segment Information [Line Items]
Gross premiums written	$ 8,214,595	$ 7,685,984	$ 6,826,938
Net premiums written	5,263,056	4,926,624	4,336,409
Net premiums earned	5,160,326	4,709,850	4,371,309
Other insurance related income (loss)	13,073	23,295	(8,089)
Net losses and loss expenses	(3,242,410)	(3,008,783)	(3,281,252)
Acquisition costs	(1,022,017)	(921,834)	(929,517)
Underwriting-related general and administrative expenses	(680,343)	(663,304)	(579,790)
Net investment income	418,829	454,301	349,601
Net Investment Gain (Loss)	(456,789)	134,279	129,133
Foreign exchange losses	157,945	(315)	(81,069)
Interest expense and financing costs	(63,146)	(62,302)	(75,049)
Reorganization expenses	(31,426)	0	(7,881)
Amortization of value of business acquired	0	(3,854)	(5,139)
Amortization of intangible assets	(10,917)	(12,424)	(11,390)
Income (loss) before income taxes and interest in income (loss) of equity method investments	243,125	648,909	(129,133)
Income tax expense	(22,037)	(62,384)	12,321
Interest in income (loss) of equity method investments	1,995	32,084	(3,612)
Net income (loss)	223,083	618,609	(120,424)
Preferred share dividends	30,250	30,250	30,250
Net income (loss) available (attributable) to common shareholders	192,833	588,359	(150,674)
Goodwill and intangible assets	298,601	309,518	324,288	$ 341,545
Operating Segments
Segment Information [Line Items]
Gross premiums written	8,214,595	7,685,984	6,826,938
Net premiums written	5,263,056	4,926,624	4,336,409
Net premiums earned	5,160,326	4,709,850	4,371,309
Other insurance related income (loss)	13,073	23,295	(8,089)
Net losses and loss expenses	(3,242,410)	(3,008,783)	(3,281,252)
Acquisition costs	(1,022,017)	(921,834)	(929,517)
Underwriting-related general and administrative expenses	(550,289)	(536,834)	(477,968)
Underwriting income (loss)	$ 358,683	$ 265,694	$ (325,517)
Net losses and loss expenses ratio	62.80%	63.90%	75.10%
Acquisition cost ratio	19.80%	19.60%	21.30%
General and administrative expense ratio	13.20%	14.00%	13.20%
Combined ratio	95.80%	97.50%	109.60%
Goodwill and intangible assets	$ 298,601	$ 309,518	$ 320,434
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	5,585,581	4,863,232	4,018,399
Net premiums written	3,377,906	2,894,885	2,357,501
Net premiums earned	3,134,155	2,651,339	2,299,038
Other insurance related income (loss)	559	1,662	2,647
Net losses and loss expenses	(1,785,854)	(1,514,998)	(1,697,014)
Acquisition costs	(577,838)	(484,344)	(461,533)
Underwriting-related general and administrative expenses	(443,704)	(429,282)	(378,839)
Underwriting income (loss)	$ 327,318	$ 224,377	$ (235,701)
Net losses and loss expenses ratio	57.00%	57.10%	73.80%
Acquisition cost ratio	18.40%	18.30%	20.10%
General and administrative expense ratio	14.20%	16.20%	16.50%
Combined ratio	89.60%	91.60%	110.40%
Goodwill and intangible assets	$ 298,601	$ 309,518	$ 320,434
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	2,629,014	2,822,752	2,808,539
Net premiums written	1,885,150	2,031,739	1,978,908
Net premiums earned	2,026,171	2,058,511	2,072,271
Other insurance related income (loss)	12,514	21,633	(10,736)
Net losses and loss expenses	(1,456,556)	(1,493,785)	(1,584,238)
Acquisition costs	(444,179)	(437,490)	(467,984)
Underwriting-related general and administrative expenses	(106,585)	(107,552)	(99,129)
Underwriting income (loss)	$ 31,365	$ 41,317	$ (89,816)
Net losses and loss expenses ratio	71.90%	72.60%	76.40%
Acquisition cost ratio	21.90%	21.30%	22.60%
General and administrative expense ratio	5.30%	5.10%	4.80%
Combined ratio	99.10%	99.00%	103.80%
Goodwill and intangible assets	$ 0	$ 0	$ 0
Significant Reconciling Items
Segment Information [Line Items]
Net investment income	418,829	454,301	349,601
Net Investment Gain (Loss)	(456,789)	134,279	129,133
Foreign exchange losses	157,945	(315)	(81,069)
Interest expense and financing costs	(63,146)	(62,302)	(75,049)
Reorganization expenses	(31,426)	0	(7,881)
Amortization of value of business acquired	0	(3,854)	(5,139)
Amortization of intangible assets	(10,917)	(12,424)	(11,390)
Corporate
Segment Information [Line Items]
Underwriting-related general and administrative expenses	$ (130,054)	$ (126,470)	$ (101,822)