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Fair Value Measurements - Quantitative Disclosures of Fair Value Assets (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
AFS fixed income securities | $ 7,583,504 | $ 7,499,197
Equity securities | 194,320 | 187,155
Short-term investments | 247,883 | 309,317
Fair Value, Recurring |  |
AFS fixed income securities | 7,583,504 | 7,499,197
Equity securities | 194,320 | 187,155
Short-term investments | 247,883 | 309,317
Total assets measured at fair value | 8,025,707 | 7,995,669
U.S. government and government agencies |  |
AFS fixed income securities | 141,826 | 205,035
U.S. government and government agencies | Fair Value, Recurring |  |
AFS fixed income securities | 141,826 | 205,035
Foreign government |  |
AFS fixed income securities | 9,229 | 9,803
Foreign government | Fair Value, Recurring |  |
AFS fixed income securities | 9,229 | 9,803
Obligations of states and political subdivisions |  |
AFS fixed income securities | 538,986 | 585,965
Obligations of states and political subdivisions | Fair Value, Recurring |  |
AFS fixed income securities | 538,986 | 585,965
Corporate securities |  |
AFS fixed income securities | 2,795,050 | 2,711,239
Corporate securities | Fair Value, Recurring |  |
AFS fixed income securities | 2,795,050 | 2,711,239
CLO and other ABS |  |
AFS fixed income securities | 1,897,085 | 1,834,827
CLO and other ABS | Fair Value, Recurring |  |
AFS fixed income securities | 1,897,085 | 1,834,827
RMBS |  |
AFS fixed income securities | 1,511,959 | 1,477,483
RMBS | Fair Value, Recurring |  |
AFS fixed income securities | 1,511,959 | 1,477,483
CMBS |  |
AFS fixed income securities | 689,369 | 674,845
CMBS | Fair Value, Recurring |  |
AFS fixed income securities | 689,369 | 674,845
Common stock: | Fair Value, Recurring |  |
Equity securities | 192,510 | 185,339
Preferred stock | Fair Value, Recurring |  |
Equity securities | 1,810 | 1,816
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Fair Value, Recurring |  |
AFS fixed income securities | 33,530 | 34,056
Equity securities | 38,143 | 22,398
Short-term investments | 246,904 | 308,512
Total assets measured at fair value | 318,577 | 364,966
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | U.S. government and government agencies | Fair Value, Recurring |  |
AFS fixed income securities | 33,530 | 34,056
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Foreign government | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Obligations of states and political subdivisions | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Corporate securities | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | CLO and other ABS | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | RMBS | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | CMBS | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Common stock: | Fair Value, Recurring |  |
Equity securities | 36,333 | 20,582
Quoted Prices in  Active Markets for Identical Assets/ Liabilities (Level 1) | Preferred stock | Fair Value, Recurring |  |
Equity securities | 1,810 | 1,816
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring |  |
AFS fixed income securities | 6,994,279 | 6,914,306
Equity securities | 0 | 0
Short-term investments | 979 | 805
Total assets measured at fair value | 6,995,258 | 6,915,111
Significant Other Observable Inputs (Level 2) | U.S. government and government agencies | Fair Value, Recurring |  |
AFS fixed income securities | 108,296 | 170,979
Significant Other Observable Inputs (Level 2) | Foreign government | Fair Value, Recurring |  |
AFS fixed income securities | 9,229 | 9,803
Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions | Fair Value, Recurring |  |
AFS fixed income securities | 531,239 | 578,131
Significant Other Observable Inputs (Level 2) | Corporate securities | Fair Value, Recurring |  |
AFS fixed income securities | 2,507,554 | 2,413,907
Significant Other Observable Inputs (Level 2) | CLO and other ABS | Fair Value, Recurring |  |
AFS fixed income securities | 1,636,988 | 1,589,514
Significant Other Observable Inputs (Level 2) | RMBS | Fair Value, Recurring |  |
AFS fixed income securities | 1,511,959 | 1,477,483
Significant Other Observable Inputs (Level 2) | CMBS | Fair Value, Recurring |  |
AFS fixed income securities | 689,014 | 674,489
Significant Other Observable Inputs (Level 2) | Common stock: | Fair Value, Recurring |  |
Equity securities | 0 | 0
Significant Other Observable Inputs (Level 2) | Preferred stock | Fair Value, Recurring |  |
Equity securities | 0 | 0
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring |  |
AFS fixed income securities | 555,695 | 550,835
Equity securities | 1,067 | 854
Short-term investments | 0 | 0
Total assets measured at fair value | 556,762 | 551,689
Significant Unobservable Inputs (Level 3) | U.S. government and government agencies | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Significant Unobservable Inputs (Level 3) | Foreign government | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Significant Unobservable Inputs (Level 3) | Obligations of states and political subdivisions | Fair Value, Recurring |  |
AFS fixed income securities | 7,747 | 7,834
Significant Unobservable Inputs (Level 3) | Corporate securities | Fair Value, Recurring |  |
AFS fixed income securities | 287,496 | 297,332
Significant Unobservable Inputs (Level 3) | CLO and other ABS | Fair Value, Recurring |  |
AFS fixed income securities | 260,097 | 245,313
Significant Unobservable Inputs (Level 3) | RMBS | Fair Value, Recurring |  |
AFS fixed income securities | 0 | 0
Significant Unobservable Inputs (Level 3) | CMBS | Fair Value, Recurring |  |
AFS fixed income securities | 355 | 356
Significant Unobservable Inputs (Level 3) | Common stock: | Fair Value, Recurring |  |
Equity securities | 1,067 | 854
Significant Unobservable Inputs (Level 3) | Preferred stock | Fair Value, Recurring |  |
Equity securities | 0 | 0
Fair Value Measured at Net Asset Value Per Share |  |
Equity securities | $ 155,100 | $ 163,900

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