FAIR VALUE MEASUREMENTS - Financial Instruments Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Assets
Fixed maturities, available for sale	$ 11,723,368	$ 11,326,894
Equity securities	556,262	485,253
Other investments	954,571	996,751
Short-term investments	115,959	70,310
Derivative instruments (refer to Note 5)	4,466	37,682
Total Assets	13,354,626	12,916,890
Liabilities
Derivative instruments	9,666	703
Cash-settled awards	0	4,792
Total Liabilities	9,666	5,495
U.S. government and agency
Assets
Fixed maturities, available for sale	2,860,633	2,639,330
Non-U.S. government
Assets
Fixed maturities, available for sale	670,255	562,029
Corporate debt
Assets
Fixed maturities, available for sale	4,229,863	4,255,556
Agency RMBS
Assets
Fixed maturities, available for sale	1,541,505	1,202,785
CMBS
Assets
Fixed maturities, available for sale	865,051	947,778
Non-agency RMBS
Assets
Fixed maturities, available for sale	137,804	133,534
ABS
Assets
Fixed maturities, available for sale	1,266,868	1,429,527
Municipals
Assets
Fixed maturities, available for sale	151,389	156,355
Common stocks
Assets
Equity securities	3,050	7,473
Preferred stocks
Assets
Equity securities	5,086	72
Exchange-traded funds
Assets
Equity securities	273,794	269,806
Bond mutual funds
Assets
Equity securities	274,332	207,902
Multi-strategy funds
Assets
Other investments	25,465	32,616
Direct lending funds
Assets
Other investments	229,235	258,626
Private equity funds
Assets
Other investments	283,838	265,836
Real estate funds
Assets
Other investments	307,177	298,499
CLO-Equities
Assets
Other investments	4,684	5,016
Other privately held investments
Assets
Other investments	104,172	136,158
Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities, available for sale	2,829,541	2,600,636
Equity securities	276,847	277,351
Other investments	0	0
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	3,106,388	2,877,987
Liabilities
Derivative instruments	0	0
Cash-settled awards	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	2,829,541	2,600,636
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	3,050	7,473
Quoted prices in active markets for identical assets (Level 1) | Preferred stocks
Assets
Equity securities	3	72
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds
Assets
Equity securities	273,794	269,806
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	8,760,359	8,607,154
Equity securities	279,415	207,902
Other investments	0	0
Short-term investments	115,959	70,310
Derivative instruments (refer to Note 5)	4,466	37,682
Total Assets	9,160,199	8,923,048
Liabilities
Derivative instruments	9,666	703
Cash-settled awards	0	4,792
Total Liabilities	9,666	5,495
Significant other observable inputs (Level 2) | U.S. government and agency
Assets
Fixed maturities, available for sale	31,092	38,694
Significant other observable inputs (Level 2) | Non-U.S. government
Assets
Fixed maturities, available for sale	670,255	562,029
Significant other observable inputs (Level 2) | Corporate debt
Assets
Fixed maturities, available for sale	4,096,395	4,136,452
Significant other observable inputs (Level 2) | Agency RMBS
Assets
Fixed maturities, available for sale	1,541,505	1,202,785
Significant other observable inputs (Level 2) | CMBS
Assets
Fixed maturities, available for sale	865,051	947,778
Significant other observable inputs (Level 2) | Non-agency RMBS
Assets
Fixed maturities, available for sale	137,804	133,534
Significant other observable inputs (Level 2) | ABS
Assets
Fixed maturities, available for sale	1,266,868	1,429,527
Significant other observable inputs (Level 2) | Municipals
Assets
Fixed maturities, available for sale	151,389	156,355
Significant other observable inputs (Level 2) | Common stocks
Assets
Equity securities	0	0
Significant other observable inputs (Level 2) | Preferred stocks
Assets
Equity securities	5,083	0
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	274,332	207,902
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	133,468	119,104
Equity securities	0	0
Other investments	94,635	141,174
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	228,103	260,278
Liabilities
Derivative instruments	0	0
Cash-settled awards	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	133,468	119,104
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	0	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 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) | CLO-Equities
Assets
Other investments	4,684	5,016
Significant unobservable inputs (Level 3) | Other privately held investments
Assets
Other investments	89,951	136,158
Fair value based on NAV practical expedient
Assets
Other investments	859,936	855,577
Total Assets	859,936	855,577
Fair value based on NAV practical expedient | Multi-strategy funds
Assets
Other investments	25,465	32,616
Fair value based on NAV practical expedient | Direct lending funds
Assets
Other investments	229,235	258,626
Fair value based on NAV practical expedient | Private equity funds
Assets
Other investments	283,838	265,836
Fair value based on NAV practical expedient | Real estate funds
Assets
Other investments	307,177	$ 298,499
Fair value based on NAV practical expedient | Other privately held investments
Assets
Other investments	$ 14,221