FAIR VALUE MEASUREMENTS (Financial Instruments Measured at Fair Value) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets
Fixed maturities	$ 12,609,241	$ 12,468,205
Equity securities	417,886	474,207
Other investments	760,206	770,923
Short-term investments	69,996	38,471
Derivative instruments (refer to Note 5)	7,267	3,174
Total Assets	13,864,596	13,754,980
Liabilities
Derivative instruments (refer to Note 5)	11,026	13,637
Cash settled awards (refer to Note 8)	10,160	21,731
Total Liabilities	21,186	35,368
U.S. government and agency
Assets
Fixed maturities	1,991,466	2,112,881
Non-U.S. government
Assets
Fixed maturities	673,801	576,592
Corporate debt
Assets
Fixed maturities	4,796,731	4,930,254
Agency RMBS
Assets
Fixed maturities	1,677,106	1,592,584
CMBS
Assets
Fixed maturities	1,380,738	1,365,052
Non-agency RMBS
Assets
Fixed maturities	133,530	84,922
ABS
Assets
Fixed maturities	1,672,175	1,598,693
Municipals
Assets
Fixed maturities	283,694	207,227
Common stocks
Assets
Equity securities	9,295	193
Preferred shares
Assets
Equity securities	7,754	0
Exchange-traded funds
Assets
Equity securities	192,202	297,325
Bond mutual funds
Assets
Equity securities	208,635	176,689
Hedge funds
Assets
Other investments	136,795	167,790
Direct lending funds
Assets
Other investments	269,783	277,395
Private equity funds
Assets
Other investments	106,701	80,412
Real estate funds
Assets
Other investments	153,163	130,112
CLO-Equities
Assets
Other investments	9,189	14,328
Other privately held investments
Assets
Other investments	39,493	36,934
Overseas deposits
Assets
Other investments	45,082	63,952
Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities	1,911,014	2,053,622
Equity securities	209,251	297,518
Other investments	0	0
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	2,120,265	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	1,911,014	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	9,295	193
Quoted prices in active markets for identical assets (Level 1) | Preferred shares
Assets
Equity securities	7,754	0
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds
Assets
Equity securities	192,202	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	10,673,063	10,406,562
Equity securities	208,635	176,689
Other investments	45,082	63,952
Short-term investments	69,996	38,471
Derivative instruments (refer to Note 5)	7,267	3,174
Total Assets	11,004,043	10,688,848
Liabilities
Derivative instruments (refer to Note 5)	1,578	3,965
Cash settled awards (refer to Note 8)	10,160	21,731
Total Liabilities	11,738	25,696
Significant other observable inputs (Level 2) | U.S. government and agency
Assets
Fixed maturities	80,452	59,259
Significant other observable inputs (Level 2) | Non-U.S. government
Assets
Fixed maturities	673,801	576,592
Significant other observable inputs (Level 2) | Corporate debt
Assets
Fixed maturities	4,794,332	4,927,957
Significant other observable inputs (Level 2) | Agency RMBS
Assets
Fixed maturities	1,677,106	1,592,584
Significant other observable inputs (Level 2) | CMBS
Assets
Fixed maturities	1,378,115	1,359,817
Significant other observable inputs (Level 2) | Non-agency RMBS
Assets
Fixed maturities	123,898	84,922
Significant other observable inputs (Level 2) | ABS
Assets
Fixed maturities	1,661,665	1,598,204
Significant other observable inputs (Level 2) | Municipals
Assets
Fixed maturities	283,694	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	208,635	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	45,082	63,952
Significant unobservable inputs (Level 3)
Assets
Fixed maturities	25,164	8,021
Equity securities	0	0
Other investments	48,682	51,262
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	73,846	59,283
Liabilities
Derivative instruments (refer to Note 5)	9,448	9,672
Cash settled awards (refer to Note 8)	0	0
Total Liabilities	9,448	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	2,399	2,297
Significant unobservable inputs (Level 3) | Agency RMBS
Assets
Fixed maturities	0	0
Significant unobservable inputs (Level 3) | CMBS
Assets
Fixed maturities	2,623	5,235
Significant unobservable inputs (Level 3) | Non-agency RMBS
Assets
Fixed maturities	9,632	0
Significant unobservable inputs (Level 3) | ABS
Assets
Fixed maturities	10,510	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	9,189	14,328
Significant unobservable inputs (Level 3) | Other privately held investments
Assets
Other investments	39,493	36,934
Significant unobservable inputs (Level 3) | Overseas deposits
Assets
Other investments	0	0
Fair value based on NAV practical expedient
Assets
Other investments	666,442	655,709
Fair value based on NAV practical expedient | Hedge funds
Assets
Other investments	136,795	167,790
Fair value based on NAV practical expedient | Direct lending funds
Assets
Other investments	269,783	277,395
Fair value based on NAV practical expedient | Private equity funds
Assets
Other investments	106,701	80,412
Fair value based on NAV practical expedient | Real estate funds
Assets
Other investments	$ 153,163	$ 130,112