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Fair Value Measurements - Quantitative Disclosures of Fair Value Assets (Details) - USD ($) $ in Thousands | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | ---
AFS fixed income securities | $ 8,605,175 | $ 8,127,334
Equity securities | 266,471 | 213,601
Short-term investments | 631,090 | 509,318
U.S. government and government agencies |  |
AFS fixed income securities | 134,475 | 120,155
Foreign government |  |
AFS fixed income securities | 10,388 | 9,302
Obligations of states and political subdivisions |  |
AFS fixed income securities | 432,950 | 451,230
Corporate securities |  |
AFS fixed income securities | 3,237,221 | 3,068,180
CLO and other ABS |  |
AFS fixed income securities | 2,197,185 | 2,033,149
RMBS |  |
AFS fixed income securities | 1,852,079 | 1,692,358
CMBS |  |
AFS fixed income securities | 740,877 | 752,960
Fair Value Measured at Net Asset Value Per Share |  |
Equity securities | 196,200 | 169,500
Fair Value, Recurring |  |
AFS fixed income securities | 8,605,175 | 8,127,334
Equity securities | 266,471 | 213,601
Short-term investments | 631,090 | 509,318
Total assets measured at fair value | 9,502,736 | 8,850,253
Fair Value, Recurring | U.S. government and government agencies |  |
AFS fixed income securities | 134,475 | 120,155
Fair Value, Recurring | Foreign government |  |
AFS fixed income securities | 10,388 | 9,302
Fair Value, Recurring | Obligations of states and political subdivisions |  |
AFS fixed income securities | 432,950 | 451,230
Fair Value, Recurring | Corporate securities |  |
AFS fixed income securities | 3,237,221 | 3,068,180
Fair Value, Recurring | CLO and other ABS |  |
AFS fixed income securities | 2,197,185 | 2,033,149
Fair Value, Recurring | RMBS |  |
AFS fixed income securities | 1,852,079 | 1,692,358
Fair Value, Recurring | CMBS |  |
AFS fixed income securities | 740,877 | 752,960
Fair Value, Recurring | Common Stock |  |
Equity securities | 264,650 | 211,767
Fair Value, Recurring | Preferred stock |  |
Equity securities | 1,821 | 1,834
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) |  |
AFS fixed income securities | 38,061 | 35,518
Equity securities | 69,423 | 43,279
Short-term investments | 630,818 | 474,225
Total assets measured at fair value | 738,302 | 553,022
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | U.S. government and government agencies |  |
AFS fixed income securities | 38,061 | 35,518
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Foreign government |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Obligations of states and political subdivisions |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Corporate securities |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | CLO and other ABS |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | RMBS |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | CMBS |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Common Stock |  |
Equity securities | 67,602 | 41,445
Fair Value, Recurring | Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Preferred stock |  |
Equity securities | 1,821 | 1,834
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) |  |
AFS fixed income securities | 7,905,444 | 7,473,377
Equity securities | 0 | 0
Short-term investments | 272 | 35,093
Total assets measured at fair value | 7,905,716 | 7,508,470
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | U.S. government and government agencies |  |
AFS fixed income securities | 96,414 | 84,637
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Foreign government |  |
AFS fixed income securities | 10,388 | 9,302
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions |  |
AFS fixed income securities | 425,344 | 443,804
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Corporate securities |  |
AFS fixed income securities | 2,996,304 | 2,825,501
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | CLO and other ABS |  |
AFS fixed income securities | 1,784,383 | 1,665,155
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | RMBS |  |
AFS fixed income securities | 1,852,079 | 1,692,358
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | CMBS |  |
AFS fixed income securities | 740,532 | 752,620
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Common Stock |  |
Equity securities | 0 | 0
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Preferred stock |  |
Equity securities | 0 | 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) |  |
AFS fixed income securities | 661,670 | 618,439
Equity securities | 841 | 808
Short-term investments | 0 | 0
Total assets measured at fair value | 662,511 | 619,247
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | U.S. government and government agencies |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Foreign government |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Obligations of states and political subdivisions |  |
AFS fixed income securities | 7,606 | 7,426
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Corporate securities |  |
AFS fixed income securities | 240,917 | 242,679
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | CLO and other ABS |  |
AFS fixed income securities | 412,802 | 367,994
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | RMBS |  |
AFS fixed income securities | 0 | 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | CMBS |  |
AFS fixed income securities | 345 | 340
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Common Stock |  |
Equity securities | 841 | 808
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Preferred stock |  |
Equity securities | $ 0 | $ 0

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