SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) segment	Jun. 30, 2024 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Information [Line Items]
Net premiums earned	$ 1,393,431	$ 1,304,478	$ 2,734,251	$ 2,562,519
Other insurance related income (loss)	8,662	8,526	12,240	16,867
Current accident year net losses and loss expenses			(1,625,845)	(1,494,659)
Net favorable prior year reserve development	20,229	0	38,166	0
Acquisition costs	(275,897)	(265,091)	(540,477)	(519,345)
Underwriting-related general and administrative expenses	(161,078)	(148,441)	(320,241)	(311,813)
Net investment income	187,297	190,975	395,009	358,358
Net investment gains (losses)	43,468	(53,479)	13,462	(62,687)
Foreign exchange (losses) gains	(94,885)	7,384	(151,920)	30,936
Interest expense and financing costs	(16,586)	(17,010)	(33,158)	(34,157)
Reorganization expenses	0	(14,014)	0	(26,312)
Amortization of intangible assets	(2,396)	(2,729)	(5,125)	(5,458)
Income before income taxes and interest in income (loss) of equity method investments	280,262	244,611	516,362	514,249
Income tax (expense) benefit	(56,199)	(40,547)	(100,521)	84,107
Interest in income (loss) of equity method investments	(705)	7,900	1,586	9,069
Net income	223,358	211,964	417,427	607,425
Preferred share dividends	7,563	7,563	15,125	15,125
Net income available to common shareholders	215,795	204,401	402,302	592,300
Insurance
Segment Information [Line Items]
Net favorable prior year reserve development	15,216	0	29,194	0
Reinsurance
Segment Information [Line Items]
Net favorable prior year reserve development	5,013	0	8,972	0
Operating Segments
Segment Information [Line Items]
Gross premiums written	2,515,971	2,440,236	5,310,622	5,094,663
Net premiums written	1,635,434	1,573,744	3,385,473	3,295,817
Net premiums earned	1,393,431	1,304,478	2,734,251	2,562,519
Other insurance related income (loss)	8,662	8,526	12,240	16,867
Current accident year net losses and loss expenses	(821,983)	(765,988)	(1,625,845)	(1,494,659)
Net favorable prior year reserve development	20,229	0	38,166	0
Acquisition costs	(275,897)	(265,091)	(540,477)	(519,345)
Underwriting-related general and administrative expenses	(135,241)	(120,768)	(265,679)	(258,561)
Underwriting income	$ 189,201	$ 161,157	$ 352,656	$ 306,821
Current accident year loss ratio (as a percent)	59.00%	58.70%	59.50%	58.30%
Prior year reserve development ratio (as a percent)	(1.50%)	0.00%	(1.40%)	0.00%
Net losses and loss expenses ratio (as a percent)	57.50%	58.70%	58.10%	58.30%
Acquisition cost ratio (as a percent)	19.80%	20.30%	19.80%	20.30%
General and administrative expense ratio (as a percent)	11.60%	11.40%	11.60%	12.20%
Combined ratio (as a percent)	88.90%	90.40%	89.50%	90.80%
Goodwill and intangible assets	$ 237,340	$ 282,227	$ 237,340	$ 282,227
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	1,932,435	1,814,066	3,588,337	3,388,571
Net premiums written	1,290,510	1,194,197	2,335,090	2,216,551
Net premiums earned	1,032,961	958,212	2,043,047	1,876,159
Other insurance related income (loss)	6	(61)	162	(39)
Current accident year net losses and loss expenses	(576,986)	(542,591)	(1,153,052)	(1,039,455)
Net favorable prior year reserve development	15,216	0	29,194	0
Acquisition costs	(194,912)	(188,026)	(388,933)	(364,055)
Underwriting-related general and administrative expenses	(124,646)	(111,894)	(244,238)	(233,981)
Underwriting income	$ 151,639	$ 115,640	$ 286,180	$ 238,629
Current accident year loss ratio (as a percent)	55.90%	56.60%	56.40%	55.40%
Prior year reserve development ratio (as a percent)	(1.50%)	0.00%	(1.40%)	0.00%
Net losses and loss expenses ratio (as a percent)	54.40%	56.60%	55.00%	55.40%
Acquisition cost ratio (as a percent)	18.90%	19.60%	19.00%	19.40%
General and administrative expense ratio (as a percent)	12.00%	11.70%	12.00%	12.50%
Combined ratio (as a percent)	85.30%	87.90%	86.00%	87.30%
Goodwill and intangible assets	$ 237,340	$ 282,227	$ 237,340	$ 282,227
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	583,536	626,170	1,722,285	1,706,092
Net premiums written	344,924	379,547	1,050,383	1,079,266
Net premiums earned	360,470	346,266	691,204	686,360
Other insurance related income (loss)	8,656	8,587	12,078	16,906
Current accident year net losses and loss expenses	(244,997)	(223,397)	(472,793)	(455,204)
Net favorable prior year reserve development	5,013	0	8,972	0
Acquisition costs	(80,985)	(77,065)	(151,544)	(155,290)
Underwriting-related general and administrative expenses	(10,595)	(8,874)	(21,441)	(24,580)
Underwriting income	$ 37,562	$ 45,517	$ 66,476	$ 68,192
Current accident year loss ratio (as a percent)	68.00%	64.50%	68.40%	66.30%
Prior year reserve development ratio (as a percent)	(1.40%)	0.00%	(1.30%)	0.00%
Net losses and loss expenses ratio (as a percent)	66.60%	64.50%	67.10%	66.30%
Acquisition cost ratio (as a percent)	22.50%	22.30%	21.90%	22.60%
General and administrative expense ratio (as a percent)	2.90%	2.50%	3.10%	3.60%
Combined ratio (as a percent)	92.00%	89.30%	92.10%	92.50%
Goodwill and intangible assets	$ 0	$ 0	$ 0	$ 0
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	187,297	190,975	395,009	358,358
Net investment gains (losses)	43,468	(53,479)	13,462	(62,687)
Foreign exchange (losses) gains	(94,885)	7,384	(151,920)	30,936
Interest expense and financing costs	(16,586)	(17,010)	(33,158)	(34,157)
Reorganization expenses	0	(14,014)	0	(26,312)
Amortization of intangible assets	(2,396)	(2,729)	(5,125)	(5,458)
Corporate expenses
Segment Information [Line Items]
Underwriting-related general and administrative expenses	$ (25,837)	$ (27,673)	$ (54,562)	$ (53,252)