FAIR VALUE MEASUREMENTS (DETAILS) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	$ 11,397,114	$ 11,719,749
Equity securities	638,744	597,998
Other investments	830,219	816,756
Short-term investments	127,461	34,406
Derivative instruments	16,897	6,467
Derivative instruments	15,576	18,629
Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	11,397,114	11,719,749
Equity securities	638,744	597,998
Other investments	830,219	816,756
Short-term investments	127,461	34,406
Derivative instruments	16,897	6,467
Insurance-linked securities	25,023	24,925
Total Assets	13,035,458	13,200,301
Derivative instruments	15,576	18,629
Cash settled awards	48,432	33,215
Total Liabilities	64,008	51,844
US Government and Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	1,656,069	1,651,949
US Government and Agency [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	1,656,069	1,651,949
Non-US Government [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	565,834	739,005
Non-US Government [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	565,834	739,005
Corporate Debt [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	4,600,743	4,362,769
Corporate Debt [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	4,600,743	4,362,769
Agency RMBS [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	2,465,135	2,249,236
Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	2,465,135	2,249,236
CMBS [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	666,237	1,083,298
CMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	666,237	1,083,298
Non-Agency RMBS [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	56,921	101,008
Non-Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	56,921	101,008
ABS [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	1,222,214	1,371,270
ABS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	1,222,214	1,371,270
Municipals [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	163,961	161,214
Municipals [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	163,961	161,214
Common Stock [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	78	0
Common Stock [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	78	0
Exchange-Traded Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	514,707	473,973
Exchange-Traded Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	514,707	473,973
Bond Mutual Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	123,959	124,025
Bond Mutual Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	123,959	124,025
Hedge Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	498,150	656,773
Direct Lending Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	134,650	90,120
Private Equity Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	81,223	0
Real Estate Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	13,354	4,929
Other Privately Held Investments [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	42,142	0
CLO-Equities [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	60,700	64,934
Level 1 [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	1,583,106	1,632,355
Equity securities	514,785	473,973
Other investments	0	0
Short-term investments	0	0
Derivative instruments	0	0
Insurance-linked securities	0	0
Total Assets	2,097,891	2,106,328
Derivative instruments	0	0
Cash settled awards	0	0
Total Liabilities	0	0
Level 1 [Member] | US Government and Agency [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	1,583,106	1,632,355
Level 1 [Member] | Non-US Government [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 1 [Member] | Corporate Debt [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 1 [Member] | Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 1 [Member] | CMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 1 [Member] | Non-Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 1 [Member] | ABS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 1 [Member] | Municipals [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 1 [Member] | Common Stock [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	78	0
Level 1 [Member] | Exchange-Traded Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	514,707	473,973
Level 1 [Member] | Bond Mutual Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Level 1 [Member] | Hedge Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 1 [Member] | Direct Lending Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 1 [Member] | Private Equity Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 1 [Member] | Real Estate Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 1 [Member] | Other Privately Held Investments [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 1 [Member] | CLO-Equities [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 2 [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	9,717,608	10,037,954
Equity securities	123,959	124,025
Other investments	0	0
Short-term investments	127,461	34,406
Derivative instruments	14,365	2,072
Insurance-linked securities	0	0
Total Assets	9,983,393	10,198,457
Derivative instruments	9,076	7,692
Cash settled awards	48,432	33,215
Total Liabilities	57,508	40,907
Level 2 [Member] | US Government and Agency [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	72,963	19,594
Level 2 [Member] | Non-US Government [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	565,834	739,005
Level 2 [Member] | Corporate Debt [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	4,524,868	4,324,251
Level 2 [Member] | Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	2,465,135	2,249,236
Level 2 [Member] | CMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	663,176	1,072,376
Level 2 [Member] | Non-Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	56,921	101,008
Level 2 [Member] | ABS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	1,204,750	1,371,270
Level 2 [Member] | Municipals [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	163,961	161,214
Level 2 [Member] | Common Stock [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Level 2 [Member] | Exchange-Traded Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Level 2 [Member] | Bond Mutual Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	123,959	124,025
Level 2 [Member] | Hedge Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 2 [Member] | Direct Lending Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 2 [Member] | Private Equity Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 2 [Member] | Real Estate Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 2 [Member] | Other Privately Held Investments [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 2 [Member] | CLO-Equities [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 3 [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	96,400	49,440
Equity securities	0	0
Other investments	102,842	27,257
Short-term investments	0	0
Derivative instruments	2,532	4,395
Insurance-linked securities	25,023	24,925
Total Assets	226,797	106,017
Derivative instruments	6,500	10,937
Cash settled awards	0	0
Total Liabilities	6,500	10,937
Level 3 [Member] | US Government and Agency [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 3 [Member] | Non-US Government [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 3 [Member] | Corporate Debt [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	75,875	38,518
Level 3 [Member] | Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 3 [Member] | CMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	3,061	10,922
Level 3 [Member] | Non-Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 3 [Member] | ABS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	17,464	0
Level 3 [Member] | Municipals [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Level 3 [Member] | Common Stock [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Level 3 [Member] | Exchange-Traded Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Level 3 [Member] | Bond Mutual Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Level 3 [Member] | Hedge Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 3 [Member] | Direct Lending Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 3 [Member] | Private Equity Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 3 [Member] | Real Estate Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Level 3 [Member] | Other Privately Held Investments [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	42,142	0
Level 3 [Member] | CLO-Equities [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	60,700	27,257
Fair value based on NAV practical expedient [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Equity securities	0	0
Other investments	727,377	789,499
Short-term investments	0	0
Derivative instruments	0	0
Insurance-linked securities	0	0
Total Assets	727,377	789,499
Derivative instruments	0	0
Cash settled awards	0	0
Total Liabilities	0	0
Fair value based on NAV practical expedient [Member] | US Government and Agency [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Fair value based on NAV practical expedient [Member] | Non-US Government [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Fair value based on NAV practical expedient [Member] | Corporate Debt [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Fair value based on NAV practical expedient [Member] | Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Fair value based on NAV practical expedient [Member] | CMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Fair value based on NAV practical expedient [Member] | Non-Agency RMBS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Fair value based on NAV practical expedient [Member] | ABS [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Fair value based on NAV practical expedient [Member] | Municipals [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fixed maturities	0	0
Fair value based on NAV practical expedient [Member] | Common Stock [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Fair value based on NAV practical expedient [Member] | Exchange-Traded Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Fair value based on NAV practical expedient [Member] | Bond Mutual Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity securities	0	0
Fair value based on NAV practical expedient [Member] | Hedge Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	498,150	656,773
Fair value based on NAV practical expedient [Member] | Direct Lending Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	134,650	90,120
Fair value based on NAV practical expedient [Member] | Private Equity Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	81,223	0
Fair value based on NAV practical expedient [Member] | Real Estate Funds [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	13,354	4,929
Fair value based on NAV practical expedient [Member] | Other Privately Held Investments [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	0	0
Fair value based on NAV practical expedient [Member] | CLO-Equities [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other investments	$ 0	$ 37,677