FAIR VALUE MEASUREMENTS (Financial Instruments Measured at Fair Value) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets
Fixed maturities	$ 12,616,241	$ 11,435,347
Equity securities	429,903	381,633
Other investments	779,200	787,787
Short-term investments	12,539	144,040
Derivative instruments (refer to Note 5)	1,343	8,237
Total Assets	13,839,226	12,757,044
Liabilities
Derivative instruments (refer to Note 5)	17,601	14,522
Cash settled awards (refer to Note 8)	19,060	20,648
Total Liabilities	36,661	35,170
U.S. government and agency
Assets
Fixed maturities	2,133,956	1,515,697
Non-U.S. government
Assets
Fixed maturities	537,632	493,016
Corporate debt
Assets
Fixed maturities	5,077,095	4,876,921
Agency RMBS
Assets
Fixed maturities	1,632,768	1,643,308
CMBS
Assets
Fixed maturities	1,368,890	1,092,530
Non-Agency RMBS
Assets
Fixed maturities	58,952	40,687
ABS
Assets
Fixed maturities	1,600,535	1,637,603
Municipals
Assets
Fixed maturities	206,413	135,585
Common stocks
Assets
Equity securities	184	527
Exchange-traded funds
Assets
Equity securities	273,722	236,839
Bond mutual funds
Assets
Equity securities	155,997	144,267
Hedge funds
Assets
Other investments	196,696	194,598
Direct lending funds
Assets
Other investments	275,619	274,478
Private equity funds
Assets
Other investments	67,210	64,566
Real estate funds
Assets
Other investments	130,209	84,202
Other privately held investments
Assets
Other investments	30,719	44,518
CLO-Equities
Assets
Other investments	15,454	21,271
Overseas deposits
Assets
Other investments	63,293	104,154
Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed maturities	2,081,388	1,480,466
Equity securities	273,906	237,366
Other investments	0	0
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	2,355,294	1,717,832
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	2,081,388	1,480,466
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	184	527
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds
Assets
Equity securities	273,722	236,839
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) | Other privately held investments
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) | Overseas deposits
Assets
Other investments	0	0
Significant other observable inputs (Level 2)
Assets
Fixed maturities	10,481,036	9,868,202
Equity securities	155,997	144,267
Other investments	63,293	104,154
Short-term investments	12,539	144,040
Derivative instruments (refer to Note 5)	1,343	8,237
Total Assets	10,714,208	10,268,900
Liabilities
Derivative instruments (refer to Note 5)	7,465	4,223
Cash settled awards (refer to Note 8)	19,060	20,648
Total Liabilities	26,525	24,871
Significant other observable inputs (Level 2) | U.S. government and agency
Assets
Fixed maturities	52,568	35,231
Significant other observable inputs (Level 2) | Non-U.S. government
Assets
Fixed maturities	537,632	493,016
Significant other observable inputs (Level 2) | Corporate debt
Assets
Fixed maturities	5,038,544	4,827,909
Significant other observable inputs (Level 2) | Agency RMBS
Assets
Fixed maturities	1,632,768	1,643,308
Significant other observable inputs (Level 2) | CMBS
Assets
Fixed maturities	1,354,116	1,073,396
Significant other observable inputs (Level 2) | Non-Agency RMBS
Assets
Fixed maturities	58,952	40,687
Significant other observable inputs (Level 2) | ABS
Assets
Fixed maturities	1,600,043	1,619,070
Significant other observable inputs (Level 2) | Municipals
Assets
Fixed maturities	206,413	135,585
Significant other observable inputs (Level 2) | Common stocks
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	155,997	144,267
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) | Other privately held investments
Assets
Other investments	0	0
Significant other observable inputs (Level 2) | CLO-Equities
Assets
Other investments	0	0
Significant other observable inputs (Level 2) | Overseas deposits
Assets
Other investments	63,293	104,154
Significant unobservable inputs (Level 3)
Assets
Fixed maturities	53,817	86,679
Equity securities	0	0
Other investments	46,173	65,789
Short-term investments	0	0
Derivative instruments (refer to Note 5)	0	0
Total Assets	99,990	152,468
Liabilities
Derivative instruments (refer to Note 5)	10,136	10,299
Cash settled awards (refer to Note 8)	0	0
Total Liabilities	10,136	10,299
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	38,551	49,012
Significant unobservable inputs (Level 3) | Agency RMBS
Assets
Fixed maturities	0	0
Significant unobservable inputs (Level 3) | CMBS
Assets
Fixed maturities	14,774	19,134
Significant unobservable inputs (Level 3) | Non-Agency RMBS
Assets
Fixed maturities	0	0
Significant unobservable inputs (Level 3) | ABS
Assets
Fixed maturities	492	18,533
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) | 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) | Other privately held investments
Assets
Other investments	30,719	44,518
Significant unobservable inputs (Level 3) | CLO-Equities
Assets
Other investments	15,454	21,271
Significant unobservable inputs (Level 3) | Overseas deposits
Assets
Other investments	0	0
Fair value based on NAV practical expedient
Assets
Other investments	669,734	617,844
Fair value based on NAV practical expedient | Hedge funds
Assets
Other investments	196,696	194,598
Fair value based on NAV practical expedient | Direct lending funds
Assets
Other investments	275,619	274,478
Fair value based on NAV practical expedient | Private equity funds
Assets
Other investments	67,210	64,566
Fair value based on NAV practical expedient | Real estate funds
Assets
Other investments	$ 130,209	$ 84,202