SEGMENT INFORMATION - Segment results (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($) reportableSegment	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | reportableSegment	2
Segment Information [Line Items]
Gross premiums written	$ 7,685,984	$ 6,826,938	$ 6,898,858
Net premiums written	4,926,624	4,336,409	4,489,615
Net premiums earned	4,709,850	4,371,309	4,587,178
Other insurance related income	23,295	(8,089)	16,444
Net losses and loss expenses	(3,008,783)	(3,281,252)	(3,044,798)
Acquisition costs	(921,834)	(929,517)	(1,024,582)
Underwriting-related general and administrative expenses	(663,304)	(579,790)	(634,831)
Net investment income	454,301	349,601	478,572
Net investment gains	134,279	129,133	91,233
Foreign exchange losses	(315)	(81,069)	12,041
Interest expense and financing costs	(62,302)	(75,049)	(68,107)
Reorganization expenses	0	(7,881)	(37,384)
Amortization of value of business acquired	(3,854)	(5,139)	(26,722)
Amortization of intangible assets	(12,424)	(11,390)	(11,597)
Income before income taxes and interest in income of equity method investments	648,909	(129,133)	337,447
Income tax expense	(62,384)	12,321	(23,692)
Interest in income of equity method investments	32,084	(3,612)	9,718
Net income (loss)	618,609	(120,424)	323,473
Preferred share dividends	30,250	30,250	41,112
Net income (loss) available (attributable) to common shareholders	588,359	(150,674)	282,361
Goodwill and intangible assets	309,518
Operating Segments
Segment Information [Line Items]
Gross premiums written	7,685,984	6,826,938	6,898,858
Net premiums written	4,926,624	4,336,409	4,489,615
Net premiums earned	4,709,850	4,371,309	4,587,178
Other insurance related income	23,295	(8,089)	16,444
Net losses and loss expenses	(3,008,783)	(3,281,252)	(3,044,798)
Acquisition costs	(921,834)	(929,517)	(1,024,582)
Underwriting-related general and administrative expenses	(536,834)	(477,968)	(505,735)
Underwriting income (loss)	$ 265,694	$ (325,517)	$ 28,507
Net losses and loss expenses ratio	63.90%	75.10%	66.40%
Acquisition cost ratio	19.60%	21.30%	22.30%
General and administrative expense ratio	14.00%	13.20%	13.90%
Combined ratio	97.50%	109.60%	102.60%
Goodwill and intangible assets	$ 309,518	$ 320,434	$ 332,553
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	4,863,232	4,018,399	3,675,931
Net premiums written	2,894,885	2,357,501	2,209,155
Net premiums earned	2,651,339	2,299,038	2,190,084
Other insurance related income	1,662	2,647	2,858
Net losses and loss expenses	(1,514,998)	(1,697,014)	(1,278,679)
Acquisition costs	(484,344)	(461,533)	(468,281)
Underwriting-related general and administrative expenses	(429,282)	(378,839)	(401,963)
Underwriting income (loss)	$ 224,377	$ (235,701)	$ 44,019
Net losses and loss expenses ratio	57.10%	73.80%	58.40%
Acquisition cost ratio	18.30%	20.10%	21.40%
General and administrative expense ratio	16.20%	16.50%	18.30%
Combined ratio	91.60%	110.40%	98.10%
Goodwill and intangible assets	$ 309,518	$ 320,434	$ 332,553
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	2,822,752	2,808,539	3,222,927
Net premiums written	2,031,739	1,978,908	2,280,460
Net premiums earned	2,058,511	2,072,271	2,397,094
Other insurance related income	21,633	(10,736)	13,586
Net losses and loss expenses	(1,493,785)	(1,584,238)	(1,766,119)
Acquisition costs	(437,490)	(467,984)	(556,301)
Underwriting-related general and administrative expenses	(107,552)	(99,129)	(103,772)
Underwriting income (loss)	$ 41,317	$ (89,816)	$ (15,512)
Net losses and loss expenses ratio	72.60%	76.40%	73.70%
Acquisition cost ratio	21.30%	22.60%	23.20%
General and administrative expense ratio	5.10%	4.80%	4.30%
Combined ratio	99.00%	103.80%	101.20%
Goodwill and intangible assets	$ 0	$ 0	$ 0
Significant Reconciling Items
Segment Information [Line Items]
Net investment income	454,301	349,601	478,572
Net investment gains	134,279	129,133	91,233
Foreign exchange losses	(315)	(81,069)	12,041
Interest expense and financing costs	(62,302)	(75,049)	(68,107)
Reorganization expenses	0	(7,881)	(37,384)
Amortization of value of business acquired	(3,854)	(5,139)	(26,722)
Amortization of intangible assets	(12,424)	(11,390)	(11,597)
Corporate
Segment Information [Line Items]
Underwriting-related general and administrative expenses	$ (126,470)	$ (101,822)	$ (129,096)