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FAIR VALUE MEASUREMENTS - Fair value on a recurring basis (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | $ 11,435,347 | $ 12,622,006
Equity securities | 381,633 | 635,511
Other investments | 787,787 | 1,009,373
Short-term investments | 144,040 | 83,661
Derivative instruments | 8,237 | 5,125
Insurance-linked securities |  | 25,090
Total Assets | 12,757,044 | 14,380,766
Net derivative assets | 14,522 | 14,386
Cash settled awards | 20,648 | 21,535
Total Liabilities | 35,170 | 35,921
U.S. government and agency |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,515,697 | 1,712,469
Non-U.S. government |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 493,016 | 806,299
Corporate debt |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 4,876,921 | 5,297,866
Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,643,308 | 2,395,152
CMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,092,530 | 777,728
Non-Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 40,687 | 46,831
ABS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,637,603 | 1,436,281
Municipals |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 135,585 | 149,380
Common stocks |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 527 | 25,658
Exchange-traded funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 236,839 | 427,633
Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 144,267 | 182,220
Hedge funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 194,598 | 363,811
Direct lending funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 274,478 | 250,681
Private equity funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 64,566 | 68,812
Real estate funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 84,202 | 50,009
Other privately held investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 44,518 | 46,430
CLO-Equities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 21,271 | 31,413
Overseas deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 104,154 | 198,217
Quoted prices in active markets for identical assets (Level 1) |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,480,466 | 1,658,622
Equity securities | 237,366 | 453,291
Other investments | 0 | 0
Short-term investments | 0 | 0
Derivative instruments | 0 | 0
Insurance-linked securities |  | 0
Total Assets | 1,717,832 | 2,111,913
Net derivative assets | 0 | 0
Cash settled awards | 0 | 0
Total Liabilities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | U.S. government and agency |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,480,466 | 1,658,622
Quoted prices in active markets for identical assets (Level 1) | Non-U.S. government |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Corporate debt |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | CMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Non-Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | ABS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Municipals |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Common stocks |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 527 | 25,658
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 236,839 | 427,633
Quoted prices in active markets for identical assets (Level 1) | Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Hedge funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Direct lending funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Private equity funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Real estate funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Other privately held investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | CLO-Equities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Overseas deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant other observable inputs (Level 2) |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 9,868,202 | 10,910,487
Equity securities | 144,267 | 182,220
Other investments | 104,154 | 198,217
Short-term investments | 144,040 | 83,661
Derivative instruments | 8,237 | 5,125
Insurance-linked securities |  | 0
Total Assets | 10,268,900 | 11,379,710
Net derivative assets | 4,223 | 2,876
Cash settled awards | 20,648 | 21,535
Total Liabilities | 24,871 | 24,411
Significant other observable inputs (Level 2) | U.S. government and agency |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 35,231 | 53,847
Significant other observable inputs (Level 2) | Non-U.S. government |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 493,016 | 806,299
Significant other observable inputs (Level 2) | Corporate debt |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 4,827,909 | 5,244,969
Significant other observable inputs (Level 2) | Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,643,308 | 2,395,152
Significant other observable inputs (Level 2) | CMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,073,396 | 777,728
Significant other observable inputs (Level 2) | Non-Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 40,687 | 46,831
Significant other observable inputs (Level 2) | ABS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,619,070 | 1,436,281
Significant other observable inputs (Level 2) | Municipals |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 135,585 | 149,380
Significant other observable inputs (Level 2) | Common stocks |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant other observable inputs (Level 2) | Exchange-traded funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant other observable inputs (Level 2) | Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 144,267 | 182,220
Significant other observable inputs (Level 2) | Hedge funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant other observable inputs (Level 2) | Direct lending funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant other observable inputs (Level 2) | Private equity funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant other observable inputs (Level 2) | Real estate funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant other observable inputs (Level 2) | Other privately held investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant other observable inputs (Level 2) | CLO-Equities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant other observable inputs (Level 2) | Overseas deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 104,154 | 198,217
Significant unobservable inputs (Level 3) |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 86,679 | 52,897
Equity securities | 0 | 0
Other investments | 65,789 | 77,843
Short-term investments | 0 | 0
Derivative instruments | 0 | 0
Insurance-linked securities |  | 25,090
Total Assets | 152,468 | 155,830
Net derivative assets | 10,299 | 11,510
Cash settled awards | 0 | 0
Total Liabilities | 10,299 | 11,510
Significant unobservable inputs (Level 3) | U.S. government and agency |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Significant unobservable inputs (Level 3) | Non-U.S. government |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Significant unobservable inputs (Level 3) | Corporate debt |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 49,012 | 52,897
Significant unobservable inputs (Level 3) | Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Significant unobservable inputs (Level 3) | CMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 19,134 | 0
Significant unobservable inputs (Level 3) | Non-Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Significant unobservable inputs (Level 3) | ABS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 18,533 | 0
Significant unobservable inputs (Level 3) | Municipals |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 0 | 0
Significant unobservable inputs (Level 3) | Common stocks |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant unobservable inputs (Level 3) | Exchange-traded funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant unobservable inputs (Level 3) | Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 0 | 0
Significant unobservable inputs (Level 3) | Hedge funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant unobservable inputs (Level 3) | Direct lending funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant unobservable inputs (Level 3) | Private equity funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant unobservable inputs (Level 3) | Real estate funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Significant unobservable inputs (Level 3) | Other privately held investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 44,518 | 46,430
Significant unobservable inputs (Level 3) | CLO-Equities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 21,271 | 31,413
Significant unobservable inputs (Level 3) | Overseas deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Fair value based on NAV practical expedient |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 617,844 | 733,313
Total Assets | 617,844 | 733,313
Fair value based on NAV practical expedient | Hedge funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 194,598 | 363,811
Fair value based on NAV practical expedient | Direct lending funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 274,478 | 250,681
Fair value based on NAV practical expedient | Private equity funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 64,566 | 68,812
Fair value based on NAV practical expedient | Real estate funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 84,202 | 50,009
Fair value based on NAV practical expedient | Other privately held investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Fair value based on NAV practical expedient | CLO-Equities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 0 | 0
Fair value based on NAV practical expedient | Overseas deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | $ 0 | $ 0

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