FAIR VALUE MEASUREMENTS (Financial Instruments Measured at Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets
Fixed maturities, available for sale	$ 11,898,300	$ 12,041,799
Equity securities	588,196	518,445
Other investments	865,238	829,156
Short-term investments	112,862	161,897
Derivative instruments (refer to Note 5)	3,140	18,875
Total Assets	13,467,736	13,570,172
Liabilities
Derivative instruments (refer to Note 5)	18,382	11,486
Cash settled awards (refer to Note 8)	5,804	13,273
Total Liabilities	24,186	24,759
U.S. government and agency
Assets
Fixed maturities, available for sale	2,551,940	1,918,699
Non-U.S. government
Assets
Fixed maturities, available for sale	680,572	671,273
Corporate debt
Assets
Fixed maturities, available for sale	4,499,394	4,655,951
Agency RMBS
Assets
Fixed maturities, available for sale	1,044,556	1,286,209
CMBS
Assets
Fixed maturities, available for sale	1,144,592	1,353,587
Non-agency RMBS
Assets
Fixed maturities, available for sale	207,489	140,104
ABS
Assets
Fixed maturities, available for sale	1,499,639	1,720,078
Municipals
Assets
Fixed maturities, available for sale	270,118	295,898
Common stocks
Assets
Equity securities	952	10,942
Preferred shares
Assets
Equity securities	251	8,068
Exchange-traded funds
Assets
Equity securities	309,985	221,718
Bond mutual funds
Assets
Equity securities	277,008	277,717
Hedge funds
Assets
Other investments	112,748	146,720
Direct lending funds
Assets
Other investments	288,404	272,131
Private equity funds
Assets
Other investments	190,465	124,706
Real estate funds
Assets
Other investments	201,833	164,250
CLO-Equities
Assets
Other investments	7,002	6,173
Other privately held investments
Assets
Other investments	64,786	70,011
Overseas deposits
Assets
Other investments	0	45,165
Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities, available for sale	2,504,245	1,861,240
Equity securities	311,188	240,728
Other investments	0	0
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	2,815,433	2,101,968
Liabilities
Derivative instruments (refer to Note 5)	0	0
Cash settled awards (refer to Note 8)	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	2,504,245	1,861,240
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	952	10,942
Quoted prices in active markets for identical assets (Level 1) | Preferred shares
Assets
Equity securities	251	8,068
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds
Assets
Equity securities	309,985	221,718
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) | Hedge 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
Quoted prices in active markets for identical assets (Level 1) | Overseas deposits
Assets
Other investments	0	0
Significant other observable inputs (Level 2)
Assets
Fixed maturities, available for sale	9,358,769	10,165,650
Equity securities	277,008	277,717
Other investments	0	45,165
Short-term investments	112,862	161,897
Derivative instruments (refer to Note 5)	3,140	18,875
Total Assets	9,751,779	10,669,304
Liabilities
Derivative instruments (refer to Note 5)	10,265	2,364
Cash settled awards (refer to Note 8)	5,804	13,273
Total Liabilities	16,069	15,637
Significant other observable inputs (Level 2) | U.S. government and agency
Assets
Fixed maturities, available for sale	47,695	57,459
Significant other observable inputs (Level 2) | Non-U.S. government
Assets
Fixed maturities, available for sale	680,572	671,273
Significant other observable inputs (Level 2) | Corporate debt
Assets
Fixed maturities, available for sale	4,476,668	4,653,447
Significant other observable inputs (Level 2) | Agency RMBS
Assets
Fixed maturities, available for sale	1,044,556	1,286,209
Significant other observable inputs (Level 2) | CMBS
Assets
Fixed maturities, available for sale	1,144,592	1,351,847
Significant other observable inputs (Level 2) | Non-agency RMBS
Assets
Fixed maturities, available for sale	207,489	140,104
Significant other observable inputs (Level 2) | ABS
Assets
Fixed maturities, available for sale	1,487,079	1,709,413
Significant other observable inputs (Level 2) | Municipals
Assets
Fixed maturities, available for sale	270,118	295,898
Significant other observable inputs (Level 2) | Common stocks
Assets
Equity securities	0	0
Significant other observable inputs (Level 2) | Preferred shares
Assets
Equity securities	0	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	277,008	277,717
Significant other observable inputs (Level 2) | Hedge 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 other observable inputs (Level 2) | Overseas deposits
Assets
Other investments	0	45,165
Significant unobservable inputs (Level 3)
Assets
Fixed maturities, available for sale	35,286	14,909
Equity securities	0	0
Other investments	71,788	76,184
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	107,074	91,093
Liabilities
Derivative instruments (refer to Note 5)	8,117	9,122
Cash settled awards (refer to Note 8)	0	0
Total Liabilities	8,117	9,122
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	22,726	2,504
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	1,740
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	12,560	10,665
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) | Hedge 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	7,002	6,173
Significant unobservable inputs (Level 3) | Other privately held investments
Assets
Other investments	64,786	70,011
Significant unobservable inputs (Level 3) | Overseas deposits
Assets
Other investments	0	0
Fair value based on NAV practical expedient
Assets
Other investments	793,450	707,807
Fair value based on NAV practical expedient | Hedge funds
Assets
Other investments	112,748	146,720
Fair value based on NAV practical expedient | Direct lending funds
Assets
Other investments	288,404	272,131
Fair value based on NAV practical expedient | Private equity funds
Assets
Other investments	190,465	124,706
Fair value based on NAV practical expedient | Real estate funds
Assets
Other investments	$ 201,833	$ 164,250