FAIR VALUE MEASUREMENTS - Fair value on a recurring basis (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	$ 12,313,200	$ 12,041,799
Equity securities	655,675	518,445
Other investments	947,982	829,156
Short-term investments	31,063	161,897
Derivative instruments	3,116	18,875
Total Assets	13,951,036	13,570,172
Derivative instruments	20,617	11,486
Cash-settled awards	9,091	13,273
Total Liabilities	29,708	24,759
U.S. government and agency
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	2,682,448	1,918,699
Non-U.S. government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	795,178	671,273
Corporate debt
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	4,495,312	4,655,951
Agency RMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,074,589	1,286,209
CMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,248,191	1,353,587
Non-agency RMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	186,164	140,104
ABS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,622,480	1,720,078
Municipals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	208,838	295,898
Common stocks
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	1,364	10,942
Preferred Shares
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	179	8,068
Exchange-traded funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	336,815	221,718
Bond mutual funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	317,317	277,717
Hedge funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	59,488	146,720
Direct lending funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	289,867	272,131
Private equity funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	249,974	124,706
Real estate funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	238,222	164,250
CLO-Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	5,910	6,173
Other privately held investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	104,521	70,011
Overseas deposits
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	45,165
Quoted prices in active markets for identical assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	2,632,541	1,861,240
Equity securities	338,358	240,728
Other investments	0	0
Short-term investments	0	0
Derivative instruments	0	0
Total Assets	2,970,899	2,101,968
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
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	2,632,541	1,861,240
Quoted prices in active markets for identical assets (Level 1) | Non-U.S. government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Corporate debt
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Agency RMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | CMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Non-agency RMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | ABS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Municipals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Quoted prices in active markets for identical assets (Level 1) | Common stocks
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	1,364	10,942
Quoted prices in active markets for identical assets (Level 1) | Preferred Shares
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	179	8,068
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	336,815	221,718
Quoted prices in active markets for identical assets (Level 1) | Bond mutual funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Quoted prices in active markets for identical assets (Level 1) | Hedge funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Direct lending funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Private equity funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Real estate funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | CLO-Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Other privately held investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Overseas deposits
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant other observable inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	9,637,765	10,165,650
Equity securities	317,317	277,717
Other investments	0	45,165
Short-term investments	31,063	161,897
Derivative instruments	3,116	18,875
Total Assets	9,989,261	10,669,304
Derivative instruments	14,987	2,364
Cash-settled awards	9,091	13,273
Total Liabilities	24,078	15,637
Significant other observable inputs (Level 2) | U.S. government and agency
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	49,907	57,459
Significant other observable inputs (Level 2) | Non-U.S. government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	795,178	671,273
Significant other observable inputs (Level 2) | Corporate debt
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	4,452,418	4,653,447
Significant other observable inputs (Level 2) | Agency RMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,074,589	1,286,209
Significant other observable inputs (Level 2) | CMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,248,191	1,351,847
Significant other observable inputs (Level 2) | Non-agency RMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	186,164	140,104
Significant other observable inputs (Level 2) | ABS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	1,622,480	1,709,413
Significant other observable inputs (Level 2) | Municipals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	208,838	295,898
Significant other observable inputs (Level 2) | Common stocks
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Significant other observable inputs (Level 2) | Preferred Shares
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Significant other observable inputs (Level 2) | Exchange-traded funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Significant other observable inputs (Level 2) | Bond mutual funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	317,317	277,717
Significant other observable inputs (Level 2) | Hedge funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant other observable inputs (Level 2) | Direct lending funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant other observable inputs (Level 2) | Private equity funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant other observable inputs (Level 2) | Real estate funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant other observable inputs (Level 2) | CLO-Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant other observable inputs (Level 2) | Other privately held investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant other observable inputs (Level 2) | Overseas deposits
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	45,165
Significant unobservable inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	42,894	14,909
Equity securities	0	0
Other investments	110,431	76,184
Short-term investments	0	0
Derivative instruments	0	0
Total Assets	153,325	91,093
Derivative instruments	5,630	9,122
Cash-settled awards	0	0
Total Liabilities	5,630	9,122
Significant unobservable inputs (Level 3) | U.S. government and agency
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | Non-U.S. government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | Corporate debt
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	42,894	2,504
Significant unobservable inputs (Level 3) | Agency RMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | CMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	1,740
Significant unobservable inputs (Level 3) | Non-agency RMBS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | ABS
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	10,665
Significant unobservable inputs (Level 3) | Municipals
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities, available for sale	0	0
Significant unobservable inputs (Level 3) | Common stocks
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Significant unobservable inputs (Level 3) | Preferred Shares
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Significant unobservable inputs (Level 3) | Exchange-traded funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Significant unobservable inputs (Level 3) | Bond mutual funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Significant unobservable inputs (Level 3) | Hedge funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant unobservable inputs (Level 3) | Direct lending funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant unobservable inputs (Level 3) | Private equity funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant unobservable inputs (Level 3) | Real estate funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Significant unobservable inputs (Level 3) | CLO-Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	5,910	6,173
Significant unobservable inputs (Level 3) | Other privately held investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	104,521	70,011
Significant unobservable inputs (Level 3) | Overseas deposits
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Fair value based on NAV practical expedient
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	837,551	707,807
Total Assets	837,551	707,807
Fair value based on NAV practical expedient | Hedge funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	59,488	146,720
Fair value based on NAV practical expedient | Direct lending funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	289,867	272,131
Fair value based on NAV practical expedient | Private equity funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	249,974	124,706
Fair value based on NAV practical expedient | Real estate funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	238,222	164,250
Fair value based on NAV practical expedient | CLO-Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Fair value based on NAV practical expedient | Other privately held investments
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Fair value based on NAV practical expedient | Overseas deposits
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	$ 0	$ 0