RELATED PARTY TRANSACTIONS (Details) - USD ($)				12 Months Ended
	Feb. 03, 2025	Sep. 22, 2023	Jun. 29, 2021	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Related Party Transaction [Line Items]
Other investments				$ 930,278,000	$ 949,413,000
Equity method investments				206,994,000	174,634,000
Net premiums earned				5,306,235,000	5,083,781,000	$ 5,160,326,000
Acquisition costs				1,070,551,000	1,000,945,000	1,022,017,000
Net losses and loss expenses				3,158,487,000	3,393,102,000	3,242,410,000
Reinsurance recoverable on unpaid claims				6,840,897,000	6,323,083,000	5,831,172,000	$ 5,017,611,000
Reinsurance recoverable on paid losses and loss expenses				546,287,000	575,847,000
Ceded premiums				3,248,537,000	3,254,200,000	2,951,539,000
Ceded losses and loss expenses				2,039,000,000	1,754,000,000	1,754,000,000
Reinsurance recoverable on unpaid and paid losses and loss expenses				7,400,000,000	6,900,000,000
Insurance and reinsurance balances payable				1,713,798,000	1,792,719,000
Loan advance				199,045,000	349,842,000	105,822,000
Loan balance receivable				247,775,000	305,222,000
T-VIII PubOpps LP ("T8") | Subsequent event
Related Party Transaction [Line Items]
Total shares (in shares)	2,234,636
Total cost	$ 200,000,000
Related Party
Related Party Transaction [Line Items]
Other investments				448,000,000	424,000,000
Related Party | SKY Harbor Capital Management, LLC
Related Party Transaction [Line Items]
Income earned				3,000,000	3,000,000	2,000,000
Related Party | Freedom
Related Party Transaction [Line Items]
Income earned				0	1,000,000	2,000,000
Other investments				3,000,000	17,000,000
Related Party | Stone Point Trident Fund VIII
Related Party Transaction [Line Items]
Income earned				4,000,000	2,000,000	2,000,000
Other investments				92,000,000	87,000,000
Related Party | Stone Point Trident VIII Co-Invest
Related Party Transaction [Line Items]
Other investments				30,000,000	26,000,000
Related Party | Trident IX L.P. ("Trident IX")
Related Party Transaction [Line Items]
Income earned				3,000,000	1,000,000	1,000,000
Other investments				39,000,000	24,000,000
Related Party | Rialto Real Estate IV-Property
Related Party Transaction [Line Items]
Other investments				48,000,000	43,000,000
Related Party | Rialto Real Estate Fund IV-Property
Related Party Transaction [Line Items]
Income earned				2,000,000	2,000,000	2,000,000
Other investments				23,000,000	19,000,000
Related Party | Stone Point Credit Corporation
Related Party Transaction [Line Items]
Income earned				500,000	500,000	300,000
Other investments				18,000,000	18,000,000
Related Party | Sound Point Capital Bond
Related Party Transaction [Line Items]
Other investments					5,000,000
Related Party | Gordon Brothers
Related Party Transaction [Line Items]
Income earned					100,000	100,000
Other investments					9,000,000
Related Party | Eagle Point Credit Management, LLC
Related Party Transaction [Line Items]
Other investments				7,000,000	7,000,000
Related Party | Aspida Holdings, Ltd
Related Party Transaction [Line Items]
Income earned				0	0
Other investments				6,000,000	6,000,000
Related Party | Monarch Point Re
Related Party Transaction [Line Items]
Income earned				1,000,000	100,000
Equity method investments				43,000,000	22,000,000
Net premiums earned		$ 119,000,000
Acquisition costs		33,000,000
Net losses and loss expenses		$ 7,000,000
Reinsurance recoverable on unpaid claims					76,000,000
Reinsurance recoverable on paid losses and loss expenses					4,000,000
Ceded premiums				323,000,000	287,000,000
Ceded losses and loss expenses				214,000,000	37,000,000
Reinsurance recoverable on unpaid and paid losses and loss expenses				246,000,000	37,000,000
Insurance and reinsurance balances payable				226,000,000	166,000,000
Related Party | Harrington Re
Related Party Transaction [Line Items]
Ceded premiums				218,000,000	298,000,000	324,000,000
Ceded losses and loss expenses				197,000,000	229,000,000	234,000,000
Reinsurance recoverable on unpaid and paid losses and loss expenses				884,000,000	933,000,000
Insurance and reinsurance balances payable				181,000,000	219,000,000
Related Party | Stone Point Credit, LLC.
Related Party Transaction [Line Items]
Other commitment				60,000,000
Related Party | Stone Point Trident X LP
Related Party Transaction [Line Items]
Other commitment				25,000,000
Stone Pointe Credit Corporation Bonds | Related Party | Stone Point Credit Corporation
Related Party Transaction [Line Items]
Other investments				18,000,000	18,000,000
Stone Point Credit Corporation Bonds, Earned Income | Related Party | Stone Point Credit Corporation
Related Party Transaction [Line Items]
Income earned				1,000,000	1,000,000	$ 600,000
Eagle Point Credit Management Loan, Earned Income | Related Party | Eagle Point Credit Management, LLC
Related Party Transaction [Line Items]
Income earned				700,000	500,000
Loan Agreement With Monarch Point Re | Related Party | Monarch Point Re
Related Party Transaction [Line Items]
Other commitment				0	16,000,000
Loan advance				253,000,000	297,000,000
Non-cash repayment of advance				236,000,000	72,000,000
Loan balance receivable				243,000,000	225,000,000
Interest received in advance				$ 7,000,000	$ 9,000,000
Loan Agreement With Monarch Point Re | Related Party | Monarch Point Re | Minimum
Related Party Transaction [Line Items]
Related party transaction floating rate (as a percent)				4.70%	5.70%
Loan Agreement With Monarch Point Re | Related Party | Monarch Point Re | Maximum
Related Party Transaction [Line Items]
Related party transaction floating rate (as a percent)				5.50%	5.90%
Investment In Related Party | Senior Unsecured Notes Due 2031
Related Party Transaction [Line Items]
Other investments			$ 10,000,000
Related party transaction floating rate (as a percent)			7.25%
Stone Point Group | Related Party | Pantheon
Related Party Transaction [Line Items]
Percentage ownership by related parties				14.50%
Stone Point Trident V Funds | Enstar Group Limited
Related Party Transaction [Line Items]
Indirectly held ownership interest				9.50%