SEGMENT INFORMATION - Summary of Underwriting Results of Reportable Segments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Segment Information [Line Items]
Gross premiums written	$ 9,005,888	$ 8,356,525	$ 8,214,595
Net premiums written	5,757,351	5,102,325	5,263,056
Net premiums earned	5,306,235	5,083,781	5,160,326
Other insurance related income	30,721	22,495	13,073
Current accident year net losses and loss expenses	(3,182,810)	(2,981,220)	(3,267,943)
Net favorable prior year reserve development	24,323	(411,882)	25,533
Acquisition costs	(1,070,551)	(1,000,945)	(1,022,017)
Underwriting-related general and administrative expenses	(666,202)	(684,446)	(680,343)
Net investment income	759,229	611,742	418,829
Net investment gains (losses)	(138,534)	(74,630)	(456,789)
Foreign exchange gains	50,822	(58,115)	157,945
Interest expense and financing costs	(67,766)	(68,421)	(63,146)
Reorganization expenses	(26,312)	(28,997)	(31,426)
Amortization of intangible assets	(10,917)	(10,917)	(10,917)
Income before income taxes and interest in income of equity method investments	1,008,238	398,445	243,125
Income tax benefit	55,595	(26,316)	(22,037)
Interest in income of equity method investments	17,953	4,163	1,995
Net income	1,081,786	376,292	223,083
Preferred share dividends	30,250	30,250	30,250
Net income available to common shareholders	1,051,536	346,042	192,833
Goodwill and intangible assets	242,465	287,685	298,602	$ 309,519
Insurance
Segment Information [Line Items]
Net favorable prior year reserve development	16,209	(176,353)	16,350
Reinsurance
Segment Information [Line Items]
Net favorable prior year reserve development	8,114	(235,529)	9,183
Operating Segments
Segment Information [Line Items]
Gross premiums written	9,005,888	8,356,525	8,214,595
Net premiums written	5,757,351	5,102,325	5,263,056
Net premiums earned	5,306,235	5,083,781	5,160,326
Other insurance related income	30,721	22,495	13,073
Current accident year net losses and loss expenses	(3,182,810)	(2,981,220)	(3,267,943)
Acquisition costs	(1,070,551)	(1,000,945)	(1,022,017)
Underwriting-related general and administrative expenses	(536,442)	(551,467)	(550,289)
Underwriting income	$ 571,476	$ 160,762	$ 358,683
Current accident year loss ratio	60.00%	58.60%	63.30%
Prior year reserve development ratio	(0.50%)	8.10%	(0.50%)
Net losses and loss expenses ratio	59.50%	66.70%	62.80%
Acquisition cost ratio	20.20%	19.70%	19.80%
General and administrative expense ratio	12.60%	13.50%	13.20%
Combined ratio	92.30%	99.90%	95.80%
Goodwill and intangible assets	$ 242,465	$ 287,684	$ 298,601
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	6,615,584	6,140,764	5,585,581
Net premiums written	4,250,545	3,758,720	3,377,906
Net premiums earned	3,926,036	3,461,700	3,134,155
Other insurance related income	94	(198)	559
Current accident year net losses and loss expenses	(2,261,629)	(1,903,648)	(1,802,204)
Acquisition costs	(766,915)	(648,463)	(577,838)
Underwriting-related general and administrative expenses	(485,929)	(472,094)	(443,704)
Underwriting income	$ 427,866	$ 260,944	$ 327,318
Current accident year loss ratio	57.60%	55.00%	57.50%
Prior year reserve development ratio	(0.40%)	5.10%	(0.50%)
Net losses and loss expenses ratio	57.20%	60.10%	57.00%
Acquisition cost ratio	19.50%	18.70%	18.40%
General and administrative expense ratio	12.40%	13.70%	14.20%
Combined ratio	89.10%	92.50%	89.60%
Goodwill and intangible assets	$ 242,465	$ 287,684	$ 298,601
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	2,390,304	2,215,761	2,629,014
Net premiums written	1,506,806	1,343,605	1,885,150
Net premiums earned	1,380,199	1,622,081	2,026,171
Other insurance related income	30,627	22,693	12,514
Current accident year net losses and loss expenses	(921,181)	(1,077,572)	(1,465,739)
Acquisition costs	(303,636)	(352,482)	(444,179)
Underwriting-related general and administrative expenses	(50,513)	(79,373)	(106,585)
Underwriting income	$ 143,610	$ (100,182)	$ 31,365
Current accident year loss ratio	66.70%	66.40%	72.30%
Prior year reserve development ratio	(0.50%)	14.60%	(0.40%)
Net losses and loss expenses ratio	66.20%	81.00%	71.90%
Acquisition cost ratio	22.00%	21.70%	21.90%
General and administrative expense ratio	3.60%	4.90%	5.30%
Combined ratio	91.80%	107.60%	99.10%
Goodwill and intangible assets	$ 0	$ 0	$ 0
Significant Reconciling Items
Segment Information [Line Items]
Net investment income	759,229	611,742	418,829
Net investment gains (losses)	(138,534)	(74,630)	(456,789)
Foreign exchange gains	50,822	(58,115)	157,945
Interest expense and financing costs	(67,766)	(68,421)	(63,146)
Reorganization expenses	(26,312)	(28,997)	(31,426)
Amortization of intangible assets	(10,917)	(10,917)	(10,917)
Corporate
Segment Information [Line Items]
Underwriting-related general and administrative expenses	$ (129,760)	$ (132,979)	$ (130,054)