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FAIR VALUE MEASUREMENTS - Summary of Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Assets |  |
Fixed maturities, available for sale | $ 12,152,753 | $ 12,234,742
Equity securities | 579,274 | 588,511
Other investments | 930,278 | 949,413
Short-term investments | 223,666 | 17,216
Derivative instruments | 9,439 | 4,424
Total Assets | 13,895,410 | 13,794,306
Liabilities |  |
Derivative instruments | 3,100 | 10,165
Total Liabilities | 3,100 | 10,165
U.S. government and agency |  |
Assets |  |
Fixed maturities, available for sale | 2,802,986 | 3,007,528
Non-U.S. government |  |
Assets |  |
Fixed maturities, available for sale | 729,939 | 723,959
Corporate debt |  |
Assets |  |
Fixed maturities, available for sale | 4,842,190 | 4,474,172
Agency RMBS |  |
Assets |  |
Fixed maturities, available for sale | 1,184,845 | 1,634,661
CMBS |  |
Assets |  |
Fixed maturities, available for sale | 819,608 | 839,696
Non-agency RMBS |  |
Assets |  |
Fixed maturities, available for sale | 122,536 | 153,396
ABS |  |
Assets |  |
Fixed maturities, available for sale | 1,539,832 | 1,242,971
Municipals |  |
Assets |  |
Fixed maturities, available for sale | 110,817 | 158,359
Common stocks |  |
Assets |  |
Equity securities | 2,638 | 2,546
Preferred shares |  |
Assets |  |
Equity securities | 5,867 | 5,601
Exchange-traded funds |  |
Assets |  |
Equity securities | 314,042 | 287,275
Bond mutual funds |  |
Assets |  |
Equity securities | 256,727 | 293,089
Multi-strategy funds |  |
Assets |  |
Other investments | 24,919 | 24,619
Direct lending funds |  |
Assets |  |
Other investments | 171,048 | 192,270
Private equity funds |  |
Assets |  |
Other investments | 320,690 | 301,712
Real estate funds |  |
Assets |  |
Other investments | 291,640 | 317,325
CLO-Equities |  |
Assets |  |
Other investments | 0 | 5,300
Other privately held investments |  |
Assets |  |
Other investments | 121,981 | 108,187
Quoted prices in active markets for identical assets (Level 1) |  |
Assets |  |
Fixed maturities, available for sale | 2,767,315 | 2,989,612
Equity securities | 316,683 | 289,822
Other investments | 0 | 0
Short-term investments | 0 | 0
Derivative instruments | 0 | 0
Total Assets | 3,083,998 | 3,279,434
Liabilities |  |
Derivative instruments | 0 | 0
Total Liabilities | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | U.S. government and agency |  |
Assets |  |
Fixed maturities, available for sale | 2,767,315 | 2,989,612
Quoted prices in active markets for identical assets (Level 1) | Non-U.S. government |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Corporate debt |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Agency RMBS |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | CMBS |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Non-agency RMBS |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | ABS |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Municipals |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Quoted prices in active markets for identical assets (Level 1) | Common stocks |  |
Assets |  |
Equity securities | 2,638 | 2,546
Quoted prices in active markets for identical assets (Level 1) | Preferred shares |  |
Assets |  |
Equity securities | 3 | 1
Quoted prices in active markets for identical assets (Level 1) | Exchange-traded funds |  |
Assets |  |
Equity securities | 314,042 | 287,275
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) | Multi-strategy 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
Significant other observable inputs (Level 2) |  |
Assets |  |
Fixed maturities, available for sale | 9,238,215 | 9,109,377
Equity securities | 262,591 | 298,689
Other investments | 0 | 0
Short-term investments | 223,666 | 17,216
Derivative instruments | 9,439 | 4,424
Total Assets | 9,733,911 | 9,429,706
Liabilities |  |
Derivative instruments | 3,100 | 10,165
Total Liabilities | 3,100 | 10,165
Significant other observable inputs (Level 2) | U.S. government and agency |  |
Assets |  |
Fixed maturities, available for sale | 35,671 | 17,916
Significant other observable inputs (Level 2) | Non-U.S. government |  |
Assets |  |
Fixed maturities, available for sale | 729,939 | 723,959
Significant other observable inputs (Level 2) | Corporate debt |  |
Assets |  |
Fixed maturities, available for sale | 4,715,799 | 4,338,419
Significant other observable inputs (Level 2) | Agency RMBS |  |
Assets |  |
Fixed maturities, available for sale | 1,184,845 | 1,634,661
Significant other observable inputs (Level 2) | CMBS |  |
Assets |  |
Fixed maturities, available for sale | 819,608 | 839,696
Significant other observable inputs (Level 2) | Non-agency RMBS |  |
Assets |  |
Fixed maturities, available for sale | 122,536 | 153,396
Significant other observable inputs (Level 2) | ABS |  |
Assets |  |
Fixed maturities, available for sale | 1,519,000 | 1,242,971
Significant other observable inputs (Level 2) | Municipals |  |
Assets |  |
Fixed maturities, available for sale | 110,817 | 158,359
Significant other observable inputs (Level 2) | Common stocks |  |
Assets |  |
Equity securities | 0 | 0
Significant other observable inputs (Level 2) | Preferred shares |  |
Assets |  |
Equity securities | 5,864 | 5,600
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 | 256,727 | 293,089
Significant other observable inputs (Level 2) | Multi-strategy 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 unobservable inputs (Level 3) |  |
Assets |  |
Fixed maturities, available for sale | 147,223 | 135,753
Equity securities | 0 | 0
Other investments | 92,230 | 92,589
Short-term investments | 0 | 0
Derivative instruments | 0 | 0
Total Assets | 239,453 | 228,342
Liabilities |  |
Derivative instruments | 0 | 0
Total Liabilities | 0 | 0
Significant unobservable inputs (Level 3) | U.S. government and agency |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Significant unobservable inputs (Level 3) | Non-U.S. government |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Significant unobservable inputs (Level 3) | Corporate debt |  |
Assets |  |
Fixed maturities, available for sale | 126,391 | 135,753
Significant unobservable inputs (Level 3) | Agency RMBS |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Significant unobservable inputs (Level 3) | CMBS |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Significant unobservable inputs (Level 3) | Non-agency RMBS |  |
Assets |  |
Fixed maturities, available for sale | 0 | 0
Significant unobservable inputs (Level 3) | ABS |  |
Assets |  |
Fixed maturities, available for sale | 20,832 | 0
Significant unobservable inputs (Level 3) | Municipals |  |
Assets |  |
Fixed maturities, available for sale | 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) | Multi-strategy 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 | 0 | 5,300
Significant unobservable inputs (Level 3) | Other privately held investments |  |
Assets |  |
Other investments | 92,230 | 87,289
Fair value based on NAV practical expedient |  |
Assets |  |
Other investments | 838,048 | 856,824
Total Assets | 838,048 | 856,824
Fair value based on NAV practical expedient | Multi-strategy funds |  |
Assets |  |
Other investments | 24,919 | 24,619
Fair value based on NAV practical expedient | Direct lending funds |  |
Assets |  |
Other investments | 171,048 | 192,270
Fair value based on NAV practical expedient | Private equity funds |  |
Assets |  |
Other investments | 320,690 | 301,712
Fair value based on NAV practical expedient | Real estate funds |  |
Assets |  |
Other investments | 291,640 | 317,325
Fair value based on NAV practical expedient | CLO-Equities |  |
Assets |  |
Other investments | 0 | 0
Fair value based on NAV practical expedient | Other privately held investments |  |
Assets |  |
Other investments | $ 29,751 | $ 20,898

X
- 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/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 107 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-107 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 103 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-103 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 100 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-100 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 35 -Paragraph 54B -Publisher FASB -URI https://asc.fasb.org/1943274/2147482134/820-10-35-54B Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-6A + Details
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