FAIR VALUE MEASUREMENTS - Summary of Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Assets
Fixed maturities, available for sale	$ 12,152,753	$ 12,234,742
Equity securities	579,274	588,511
Other investments	930,278	949,413
Short-term investments	223,666	17,216
Derivative instruments	9,439	4,424
Total Assets	13,895,410	13,794,306
Liabilities
Derivative instruments	3,100	10,165
Total Liabilities	3,100	10,165
U.S. government and agency
Assets
Fixed maturities, available for sale	2,802,986	3,007,528
Non-U.S. government
Assets
Fixed maturities, available for sale	729,939	723,959
Corporate debt
Assets
Fixed maturities, available for sale	4,842,190	4,474,172
Agency RMBS
Assets
Fixed maturities, available for sale	1,184,845	1,634,661
CMBS
Assets
Fixed maturities, available for sale	819,608	839,696
Non-agency RMBS
Assets
Fixed maturities, available for sale	122,536	153,396
ABS
Assets
Fixed maturities, available for sale	1,539,832	1,242,971
Municipals
Assets
Fixed maturities, available for sale	110,817	158,359
Common stocks
Assets
Equity securities	2,638	2,546
Preferred shares
Assets
Equity securities	5,867	5,601
Exchange-traded funds
Assets
Equity securities	314,042	287,275
Bond mutual funds
Assets
Equity securities	256,727	293,089
Multi-strategy funds
Assets
Other investments	24,919	24,619
Direct lending funds
Assets
Other investments	171,048	192,270
Private equity funds
Assets
Other investments	320,690	301,712
Real estate funds
Assets
Other investments	291,640	317,325
CLO-Equities
Assets
Other investments	0	5,300
Other privately held investments
Assets
Other investments	121,981	108,187
Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities, available for sale	2,767,315	2,989,612
Equity securities	316,683	289,822
Other investments	0	0
Short-term investments	0	0
Derivative instruments	0	0
Total Assets	3,083,998	3,279,434
Liabilities
Derivative instruments	0	0
Total Liabilities	0	0
Quoted prices in active markets for identical assets (Level 1) | U.S. government and agency
Assets
Fixed maturities, available for sale	2,767,315	2,989,612
Quoted prices in active markets for identical assets (Level 1) | Non-U.S. government
Assets
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Corporate debt
Assets
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Agency RMBS
Assets
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | CMBS
Assets
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Non-agency RMBS
Assets
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | ABS
Assets
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Municipals
Assets
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Common stocks
Assets
Equity securities	2,638	2,546
Quoted prices in active markets for identical assets (Level 1) | Preferred shares
Assets
Equity securities	3	1
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds
Assets
Equity securities	314,042	287,275
Quoted prices in active markets for identical assets (Level 1) | Bond mutual funds
Assets
Equity securities	0	0
Quoted prices in active markets for identical assets (Level 1) | Multi-strategy funds
Assets
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Direct lending funds
Assets
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Private equity funds
Assets
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Real estate funds
Assets
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | CLO-Equities
Assets
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Other privately held investments
Assets
Other investments	0	0
Significant other observable inputs (Level 2)
Assets
Fixed maturities, available for sale	9,238,215	9,109,377
Equity securities	262,591	298,689
Other investments	0	0
Short-term investments	223,666	17,216
Derivative instruments	9,439	4,424
Total Assets	9,733,911	9,429,706
Liabilities
Derivative instruments	3,100	10,165
Total Liabilities	3,100	10,165
Significant other observable inputs (Level 2) | U.S. government and agency
Assets
Fixed maturities, available for sale	35,671	17,916
Significant other observable inputs (Level 2) | Non-U.S. government
Assets
Fixed maturities, available for sale	729,939	723,959
Significant other observable inputs (Level 2) | Corporate debt
Assets
Fixed maturities, available for sale	4,715,799	4,338,419
Significant other observable inputs (Level 2) | Agency RMBS
Assets
Fixed maturities, available for sale	1,184,845	1,634,661
Significant other observable inputs (Level 2) | CMBS
Assets
Fixed maturities, available for sale	819,608	839,696
Significant other observable inputs (Level 2) | Non-agency RMBS
Assets
Fixed maturities, available for sale	122,536	153,396
Significant other observable inputs (Level 2) | ABS
Assets
Fixed maturities, available for sale	1,519,000	1,242,971
Significant other observable inputs (Level 2) | Municipals
Assets
Fixed maturities, available for sale	110,817	158,359
Significant other observable inputs (Level 2) | Common stocks
Assets
Equity securities	0	0
Significant other observable inputs (Level 2) | Preferred shares
Assets
Equity securities	5,864	5,600
Significant other observable inputs (Level 2) | Exchange-traded funds
Assets
Equity securities	0	0
Significant other observable inputs (Level 2) | Bond mutual funds
Assets
Equity securities	256,727	293,089
Significant other observable inputs (Level 2) | Multi-strategy funds
Assets
Other investments	0	0
Significant other observable inputs (Level 2) | Direct lending funds
Assets
Other investments	0	0
Significant other observable inputs (Level 2) | Private equity funds
Assets
Other investments	0	0
Significant other observable inputs (Level 2) | Real estate funds
Assets
Other investments	0	0
Significant other observable inputs (Level 2) | CLO-Equities
Assets
Other investments	0	0
Significant other observable inputs (Level 2) | Other privately held investments
Assets
Other investments	0	0
Significant unobservable inputs (Level 3)
Assets
Fixed maturities, available for sale	147,223	135,753
Equity securities	0	0
Other investments	92,230	92,589
Short-term investments	0	0
Derivative instruments	0	0
Total Assets	239,453	228,342
Liabilities
Derivative instruments	0	0
Total Liabilities	0	0
Significant unobservable inputs (Level 3) | U.S. government and agency
Assets
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | Non-U.S. government
Assets
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | Corporate debt
Assets
Fixed maturities, available for sale	126,391	135,753
Significant unobservable inputs (Level 3) | Agency RMBS
Assets
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | CMBS
Assets
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | Non-agency RMBS
Assets
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | ABS
Assets
Fixed maturities, available for sale	20,832	0
Significant unobservable inputs (Level 3) | Municipals
Assets
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | Common stocks
Assets
Equity securities	0	0
Significant unobservable inputs (Level 3) | Preferred shares
Assets
Equity securities	0	0
Significant unobservable inputs (Level 3) | Exchange-traded funds
Assets
Equity securities	0	0
Significant unobservable inputs (Level 3) | Bond mutual funds
Assets
Equity securities	0	0
Significant unobservable inputs (Level 3) | Multi-strategy funds
Assets
Other investments	0	0
Significant unobservable inputs (Level 3) | Direct lending funds
Assets
Other investments	0	0
Significant unobservable inputs (Level 3) | Private equity funds
Assets
Other investments	0	0
Significant unobservable inputs (Level 3) | Real estate funds
Assets
Other investments	0	0
Significant unobservable inputs (Level 3) | CLO-Equities
Assets
Other investments	0	5,300
Significant unobservable inputs (Level 3) | Other privately held investments
Assets
Other investments	92,230	87,289
Fair value based on NAV practical expedient
Assets
Other investments	838,048	856,824
Total Assets	838,048	856,824
Fair value based on NAV practical expedient | Multi-strategy funds
Assets
Other investments	24,919	24,619
Fair value based on NAV practical expedient | Direct lending funds
Assets
Other investments	171,048	192,270
Fair value based on NAV practical expedient | Private equity funds
Assets
Other investments	320,690	301,712
Fair value based on NAV practical expedient | Real estate funds
Assets
Other investments	291,640	317,325
Fair value based on NAV practical expedient | CLO-Equities
Assets
Other investments	0	0
Fair value based on NAV practical expedient | Other privately held investments
Assets
Other investments	$ 29,751	$ 20,898