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FAIR VALUE MEASUREMENTS - Fair value on a recurring basis (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | $ 12,468,205 | $ 11,435,347
Equity securities | 474,207 | 381,633
Other investments | 770,923 | 787,787
Short-term investments | 38,471 | 144,040
Derivative instruments | 3,174 | 8,237
Total Assets | 13,754,980 | 12,757,044
Derivative instruments | 13,637 | 14,522
Cash settled awards | 21,731 | 20,648
Total Liabilities | 35,368 | 35,170
U.S. government and agency |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 2,112,881 | 1,515,697
Non-U.S. government |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 576,592 | 493,016
Corporate debt |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 4,930,254 | 4,876,921
Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,592,584 | 1,643,308
CMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,365,052 | 1,092,530
Non-Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 84,922 | 40,687
ABS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,598,693 | 1,637,603
Municipals |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 207,227 | 135,585
Common stocks |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 193 | 527
Exchange-traded funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 297,325 | 236,839
Bond mutual funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Equity securities | 176,689 | 144,267
Hedge funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 167,790 | 194,598
Direct lending funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 277,395 | 274,478
Private equity funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 80,412 | 64,566
Real estate funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 130,112 | 84,202
Other privately held investments |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 36,934 | 44,518
CLO-Equities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 14,328 | 21,271
Overseas deposits |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 63,952 | 104,154
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 | 2,053,622 | 1,480,466
Equity securities | 297,518 | 237,366
Other investments | 0 | 0
Short-term investments | 0 | 0
Derivative instruments | 0 | 0
Total Assets | 2,351,140 | 1,717,832
Derivative instruments | 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 | 2,053,622 | 1,480,466
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 | 193 | 527
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 | 297,325 | 236,839
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 | 10,406,562 | 9,868,202
Equity securities | 176,689 | 144,267
Other investments | 63,952 | 104,154
Short-term investments | 38,471 | 144,040
Derivative instruments | 3,174 | 8,237
Total Assets | 10,688,848 | 10,268,900
Derivative instruments | 3,965 | 4,223
Cash settled awards | 21,731 | 20,648
Total Liabilities | 25,696 | 24,871
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 | 59,259 | 35,231
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 | 576,592 | 493,016
Significant other observable inputs (Level 2) | Corporate debt |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 4,927,957 | 4,827,909
Significant other observable inputs (Level 2) | Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,592,584 | 1,643,308
Significant other observable inputs (Level 2) | CMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,359,817 | 1,073,396
Significant other observable inputs (Level 2) | Non-Agency RMBS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 84,922 | 40,687
Significant other observable inputs (Level 2) | ABS |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 1,598,204 | 1,619,070
Significant other observable inputs (Level 2) | Municipals |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 207,227 | 135,585
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 | 176,689 | 144,267
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 | 63,952 | 104,154
Significant unobservable inputs (Level 3) |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Fixed maturities | 8,021 | 86,679
Equity securities | 0 | 0
Other investments | 51,262 | 65,789
Short-term investments | 0 | 0
Derivative instruments | 0 | 0
Total Assets | 59,283 | 152,468
Derivative instruments | 9,672 | 10,299
Cash settled awards | 0 | 0
Total Liabilities | 9,672 | 10,299
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 | 2,297 | 49,012
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 | 5,235 | 19,134
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 | 489 | 18,533
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 | 36,934 | 44,518
Significant unobservable inputs (Level 3) | CLO-Equities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 14,328 | 21,271
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 | 655,709 | 617,844
Total Assets | 655,709 | 617,844
Fair value based on NAV practical expedient | Hedge funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Other investments | 167,790 | 194,598
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 | 277,395 | 274,478
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 | 80,412 | 64,566
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 | 130,112 | 84,202
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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- Definition Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 35 -Paragraph 54B -URI http://asc.fasb.org/extlink&oid=117332851&loc=SL7495116-110257 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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