SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2025 USD ($) segment	Mar. 31, 2024 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Segment Information [Line Items]
Net premiums earned	$ 1,340,820	$ 1,258,041
Other insurance related income	3,578	8,340
Current accident year net losses and loss expenses	(803,862)	(728,671)
Net favorable prior year reserve development	17,937	0
Acquisition costs	(264,581)	(254,254)
Underwriting-related general and administrative expenses	(159,163)	(163,373)
Net investment income	207,713	167,383
Foreign exchange (losses) gains	(57,034)	23,552
Interest expense and financing costs	(16,572)	(17,147)
Reorganization expenses	0	(12,299)
Amortization of intangible assets	(2,729)	(2,729)
Income before income taxes and interest in income of equity method investments	236,102	269,636
Income tax (expense) benefit	(44,322)	124,654
Interest in income of equity method investments	2,291	1,169
Net income	194,071	395,459
Preferred share dividends	7,563	7,563
Net income available to common shareholders	186,508	387,896
Insurance
Segment Information [Line Items]
Net favorable prior year reserve development	13,978	0
Reinsurance
Segment Information [Line Items]
Net favorable prior year reserve development	3,959	0
Operating Segments
Segment Information [Line Items]
Gross premiums written	2,794,652	2,654,427
Net premiums written	1,750,039	1,722,073
Net premiums earned	1,340,820	1,258,041
Other insurance related income	3,578	8,340
Current accident year net losses and loss expenses	(803,862)	(728,671)
Net favorable prior year reserve development	17,937	0
Acquisition costs	(264,581)	(254,254)
Underwriting-related general and administrative expenses	(130,438)	(137,793)
Underwriting income	$ 163,454	$ 145,663
Current accident year loss ratio (as a percent)	60.00%	57.90%
Prior year reserve development ratio (as a percent)	(1.40%)	0.00%
Net losses and loss expenses ratio (as a percent)	58.60%	57.90%
Acquisition cost ratio (as a percent)	19.70%	20.20%
General and administrative expense ratio (as a percent)	11.90%	13.00%
Combined ratio (as a percent)	90.20%	91.10%
Goodwill and intangible assets	$ 239,736	$ 284,956
Operating Segments | Insurance
Segment Information [Line Items]
Gross premiums written	1,655,903	1,574,505
Net premiums written	1,044,580	1,022,354
Net premiums earned	1,010,086	917,946
Other insurance related income	156	21
Current accident year net losses and loss expenses	(576,066)	(496,864)
Net favorable prior year reserve development	13,978	0
Acquisition costs	(194,021)	(176,029)
Underwriting-related general and administrative expenses	(119,592)	(122,087)
Underwriting income	$ 134,541	$ 122,987
Current accident year loss ratio (as a percent)	57.00%	54.10%
Prior year reserve development ratio (as a percent)	(1.40%)	0.00%
Net losses and loss expenses ratio (as a percent)	55.60%	54.10%
Acquisition cost ratio (as a percent)	19.20%	19.20%
General and administrative expense ratio (as a percent)	11.90%	13.30%
Combined ratio (as a percent)	86.70%	86.60%
Goodwill and intangible assets	$ 239,736	$ 284,956
Operating Segments | Reinsurance
Segment Information [Line Items]
Gross premiums written	1,138,749	1,079,922
Net premiums written	705,459	699,719
Net premiums earned	330,734	340,095
Other insurance related income	3,422	8,319
Current accident year net losses and loss expenses	(227,796)	(231,807)
Net favorable prior year reserve development	3,959	0
Acquisition costs	(70,560)	(78,225)
Underwriting-related general and administrative expenses	(10,846)	(15,706)
Underwriting income	$ 28,913	$ 22,676
Current accident year loss ratio (as a percent)	68.90%	68.20%
Prior year reserve development ratio (as a percent)	(1.20%)	0.00%
Net losses and loss expenses ratio (as a percent)	67.70%	68.20%
Acquisition cost ratio (as a percent)	21.30%	23.00%
General and administrative expense ratio (as a percent)	3.30%	4.60%
Combined ratio (as a percent)	92.30%	95.80%
Goodwill and intangible assets	$ 0	$ 0
Segment Reconciling Items
Segment Information [Line Items]
Net investment income	207,713	167,383
Net investment gains (losses)	(30,005)	(9,207)
Foreign exchange (losses) gains	(57,034)	23,552
Interest expense and financing costs	(16,572)	(17,147)
Reorganization expenses	0	(12,299)
Amortization of intangible assets	(2,729)	(2,729)
Corporate expenses
Segment Information [Line Items]
Underwriting-related general and administrative expenses	$ (28,725)	$ (25,580)