FAIR VALUE MEASUREMENTS (Financial Instruments Measured at Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets
Fixed maturities	$ 12,046,415	$ 12,468,205
Equity securities	378,860	474,207
Other investments	768,635	770,923
Short-term investments	34,337	38,471
Derivative instruments (refer to Note 5)	3,415	3,174
Total Assets	13,231,662	13,754,980
Liabilities
Derivative instruments (refer to Note 5)	11,668	13,637
Cash settled awards (refer to Note 8)	7,675	21,731
Total Liabilities	19,343	35,368
U.S. government and agency
Assets
Fixed maturities	2,013,267	2,112,881
Non-U.S. government
Assets
Fixed maturities	608,097	576,592
Corporate debt
Assets
Fixed maturities	4,662,411	4,930,254
Agency RMBS
Assets
Fixed maturities	1,535,073	1,592,584
CMBS
Assets
Fixed maturities	1,377,264	1,365,052
Non-agency RMBS
Assets
Fixed maturities	118,219	84,922
ABS
Assets
Fixed maturities	1,534,564	1,598,693
Municipals
Assets
Fixed maturities	197,520	207,227
Common stocks
Assets
Equity securities	64	193
Preferred shares
Assets
Equity securities	7,431	0
Exchange-traded funds
Assets
Equity securities	177,421	297,325
Bond mutual funds
Assets
Equity securities	193,944	176,689
Hedge funds
Assets
Other investments	165,924	167,790
Direct lending funds
Assets
Other investments	262,802	277,395
Private equity funds
Assets
Other investments	101,485	80,412
Real estate funds
Assets
Other investments	144,003	130,112
CLO-Equities
Assets
Other investments	9,943	14,328
Other privately held investments
Assets
Other investments	37,420	36,934
Overseas deposits
Assets
Other investments	47,058	63,952
Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities	1,929,996	2,053,622
Equity securities	184,916	297,518
Other investments	0	0
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	2,114,912	2,351,140
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	1,929,996	2,053,622
Quoted prices in active markets for identical assets (Level 1) | Non-U.S. government
Assets
Fixed maturities	0	0
Quoted prices in active markets for identical assets (Level 1) | Corporate debt
Assets
Fixed maturities	0	0
Quoted prices in active markets for identical assets (Level 1) | Agency RMBS
Assets
Fixed maturities	0	0
Quoted prices in active markets for identical assets (Level 1) | CMBS
Assets
Fixed maturities	0	0
Quoted prices in active markets for identical assets (Level 1) | Non-agency RMBS
Assets
Fixed maturities	0	0
Quoted prices in active markets for identical assets (Level 1) | ABS
Assets
Fixed maturities	0	0
Quoted prices in active markets for identical assets (Level 1) | Municipals
Assets
Fixed maturities	0	0
Quoted prices in active markets for identical assets (Level 1) | Common stocks
Assets
Equity securities	64	193
Quoted prices in active markets for identical assets (Level 1) | Preferred shares
Assets
Equity securities	7,431	0
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds
Assets
Equity securities	177,421	297,325
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	10,099,684	10,406,562
Equity securities	193,944	176,689
Other investments	47,058	63,952
Short-term investments	34,337	38,471
Derivative instruments (refer to Note 5)	3,415	3,174
Total Assets	10,378,438	10,688,848
Liabilities
Derivative instruments (refer to Note 5)	1,850	3,965
Cash settled awards (refer to Note 8)	7,675	21,731
Total Liabilities	9,525	25,696
Significant other observable inputs (Level 2) | U.S. government and agency
Assets
Fixed maturities	83,271	59,259
Significant other observable inputs (Level 2) | Non-U.S. government
Assets
Fixed maturities	608,097	576,592
Significant other observable inputs (Level 2) | Corporate debt
Assets
Fixed maturities	4,660,113	4,927,957
Significant other observable inputs (Level 2) | Agency RMBS
Assets
Fixed maturities	1,535,073	1,592,584
Significant other observable inputs (Level 2) | CMBS
Assets
Fixed maturities	1,372,830	1,359,817
Significant other observable inputs (Level 2) | Non-agency RMBS
Assets
Fixed maturities	108,657	84,922
Significant other observable inputs (Level 2) | ABS
Assets
Fixed maturities	1,534,123	1,598,204
Significant other observable inputs (Level 2) | Municipals
Assets
Fixed maturities	197,520	207,227
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	193,944	176,689
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	47,058	63,952
Significant unobservable inputs (Level 3)
Assets
Fixed maturities	16,735	8,021
Equity securities	0	0
Other investments	47,363	51,262
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	64,098	59,283
Liabilities
Derivative instruments (refer to Note 5)	9,818	9,672
Cash settled awards (refer to Note 8)	0	0
Total Liabilities	9,818	9,672
Significant unobservable inputs (Level 3) | U.S. government and agency
Assets
Fixed maturities	0	0
Significant unobservable inputs (Level 3) | Non-U.S. government
Assets
Fixed maturities	0	0
Significant unobservable inputs (Level 3) | Corporate debt
Assets
Fixed maturities	2,298	2,297
Significant unobservable inputs (Level 3) | Agency RMBS
Assets
Fixed maturities	0	0
Significant unobservable inputs (Level 3) | CMBS
Assets
Fixed maturities	4,434	5,235
Significant unobservable inputs (Level 3) | Non-agency RMBS
Assets
Fixed maturities	9,562	0
Significant unobservable inputs (Level 3) | ABS
Assets
Fixed maturities	441	489
Significant unobservable inputs (Level 3) | Municipals
Assets
Fixed maturities	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	9,943	14,328
Significant unobservable inputs (Level 3) | Other privately held investments
Assets
Other investments	37,420	36,934
Significant unobservable inputs (Level 3) | Overseas deposits
Assets
Other investments	0	0
Fair value based on NAV practical expedient
Assets
Other investments	674,214	655,709
Fair value based on NAV practical expedient | Hedge funds
Assets
Other investments	165,924	167,790
Fair value based on NAV practical expedient | Direct lending funds
Assets
Other investments	262,802	277,395
Fair value based on NAV practical expedient | Private equity funds
Assets
Other investments	101,485	80,412
Fair value based on NAV practical expedient | Real estate funds
Assets
Other investments	$ 144,003	$ 130,112