FAIR VALUE MEASUREMENTS - Financial Instruments Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Assets
Fixed maturities, available for sale	$ 12,137,475	$ 12,152,753
Equity securities	619,275	579,274
Other investments	938,922	930,278
Short-term investments	51,726	223,666
Derivative instruments (refer to Note 5)	419	9,439
Total Assets	13,747,817	13,895,410
Liabilities
Derivative instruments (refer to Note 5)	16,258	3,100
Total Liabilities	16,258	3,100
U.S. government and agency
Assets
Fixed maturities, available for sale	2,387,175	2,802,986
Non-U.S. government
Assets
Fixed maturities, available for sale	790,476	729,939
Corporate debt
Assets
Fixed maturities, available for sale	4,811,831	4,842,190
Agency RMBS
Assets
Fixed maturities, available for sale	1,740,980	1,184,845
CMBS
Assets
Fixed maturities, available for sale	823,582	819,608
Non-agency RMBS
Assets
Fixed maturities, available for sale	186,151	122,536
ABS
Assets
Fixed maturities, available for sale	1,333,679	1,539,832
Municipals
Assets
Fixed maturities, available for sale	63,601	110,817
Common stocks
Assets
Equity securities	2,710	2,638
Preferred stocks
Assets
Equity securities	12,064	5,867
Exchange-traded funds
Assets
Equity securities	337,709	314,042
Bond mutual funds
Assets
Equity securities	266,792	256,727
Multi-strategy funds
Assets
Other investments	15,290	24,919
Direct lending funds
Assets
Other investments	164,979	171,048
Private equity funds
Assets
Other investments	332,835	320,690
Real estate funds
Assets
Other investments	291,173	291,640
Other privately held investments
Assets
Other investments	134,645	121,981
Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities, available for sale	2,347,567	2,767,315
Equity securities	346,455	316,683
Other investments	0	0
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	2,694,022	3,083,998
Liabilities
Derivative instruments (refer to Note 5)	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	2,347,567	2,767,315
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	2,710	2,638
Quoted prices in active markets for identical assets (Level 1) | Preferred stocks
Assets
Equity securities	6,036	3
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds
Assets
Equity securities	337,709	314,042
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) | Other privately held investments
Assets
Other investments	0	0
Significant other observable inputs (Level 2)
Assets
Fixed maturities, available for sale	9,624,943	9,238,215
Equity securities	272,820	262,591
Other investments	0	0
Short-term investments	51,726	223,666
Derivative instruments (refer to Note 5)	419	9,439
Total Assets	9,949,908	9,733,911
Liabilities
Derivative instruments (refer to Note 5)	16,258	3,100
Total Liabilities	16,258	3,100
Significant other observable inputs (Level 2) | U.S. government and agency
Assets
Fixed maturities, available for sale	39,608	35,671
Significant other observable inputs (Level 2) | Non-U.S. government
Assets
Fixed maturities, available for sale	790,476	729,939
Significant other observable inputs (Level 2) | Corporate debt
Assets
Fixed maturities, available for sale	4,680,358	4,715,799
Significant other observable inputs (Level 2) | Agency RMBS
Assets
Fixed maturities, available for sale	1,740,980	1,184,845
Significant other observable inputs (Level 2) | CMBS
Assets
Fixed maturities, available for sale	823,582	819,608
Significant other observable inputs (Level 2) | Non-agency RMBS
Assets
Fixed maturities, available for sale	186,151	122,536
Significant other observable inputs (Level 2) | ABS
Assets
Fixed maturities, available for sale	1,300,187	1,519,000
Significant other observable inputs (Level 2) | Municipals
Assets
Fixed maturities, available for sale	63,601	110,817
Significant other observable inputs (Level 2) | Common stocks
Assets
Equity securities	0	0
Significant other observable inputs (Level 2) | Preferred stocks
Assets
Equity securities	6,028	5,864
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	266,792	256,727
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) | Other privately held investments
Assets
Other investments	0	0
Significant unobservable inputs (Level 3)
Assets
Fixed maturities, available for sale	164,965	147,223
Equity securities	0	0
Other investments	94,347	92,230
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	259,312	239,453
Liabilities
Derivative instruments (refer to Note 5)	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	131,473	126,391
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	33,492	20,832
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 stocks
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) | Other privately held investments
Assets
Other investments	94,347	92,230
Fair value based on NAV practical expedient
Assets
Other investments	844,575	838,048
Total Assets	844,575	838,048
Fair value based on NAV practical expedient | Multi-strategy funds
Assets
Other investments	15,290	24,919
Fair value based on NAV practical expedient | Direct lending funds
Assets
Other investments	164,979	171,048
Fair value based on NAV practical expedient | Private equity funds
Assets
Other investments	332,835	320,690
Fair value based on NAV practical expedient | Real estate funds
Assets
Other investments	291,173	291,640
Fair value based on NAV practical expedient | Other privately held investments
Assets
Other investments	$ 40,298	$ 29,751