v3.26.3
Fair Value Measurements - Schedule of Quantitative Disclosures of Fair Value Assets (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities $ 9,810,058 $ 9,457,176
Equity securities 359,134 384,416
Short-term investments 391,710 648,542
U.S. government and government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 182,116 163,207
Foreign government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 15,363 10,002
Obligations of states and political subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 491,915 549,957
Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 3,753,825 3,404,164
CLO and other ABS    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 2,505,402 2,550,286
Residential mortgage-backed securities (“RMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 2,253,193 2,075,933
Commercial mortgage-backed securities (“CMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 608,244 703,627
Fair Value Measured at Net Asset Value Per Share    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 253,700 274,800
Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 9,810,058 9,457,176
Equity securities 359,134 384,416
Short-term investments 391,710 648,542
Total assets measured at fair value 10,560,902 10,490,134
Fair Value, Recurring | U.S. government and government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 182,116 163,207
Fair Value, Recurring | Foreign government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 15,363 10,002
Fair Value, Recurring | Obligations of states and political subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 491,915 549,957
Fair Value, Recurring | Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 3,753,825 3,404,164
Fair Value, Recurring | CLO and other ABS    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 2,505,402 2,550,286
Fair Value, Recurring | Residential mortgage-backed securities (“RMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 2,253,193 2,075,933
Fair Value, Recurring | Commercial mortgage-backed securities (“CMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 608,244 703,627
Fair Value, Recurring | Common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 357,530 382,577
Fair Value, Recurring | Preferred stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 1,604 1,839
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 31,760 39,472
Equity securities 104,623 108,964
Short-term investments 381,645 637,751
Total assets measured at fair value 518,028 786,187
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | U.S. government and government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 31,760 39,472
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Foreign government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
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    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 0 0
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 0 0
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | CLO and other ABS    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 0 0
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Residential mortgage-backed securities (“RMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 0 0
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Commercial mortgage-backed securities (“CMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 0 0
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 103,019 107,125
Fair Value, Recurring | Quoted Prices in 
Active Markets for
Identical Assets/
Liabilities (Level 1) | Preferred stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 1,604 1,839
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 8,995,136 8,475,760
Equity securities 772 653
Short-term investments 10,065 10,791
Total assets measured at fair value 9,005,973 8,487,204
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | U.S. government and government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 150,356 123,735
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Foreign government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 15,363 10,002
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 484,610 542,548
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 3,375,427 3,035,053
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | CLO and other ABS    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 2,108,273 1,985,197
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities (“RMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 2,253,193 2,075,933
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Commercial mortgage-backed securities (“CMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 607,914 703,292
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 772 653
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Preferred stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 783,162 941,944
Equity securities 0 0
Short-term investments 0 0
Total assets measured at fair value 783,162 941,944
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | U.S. government and government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 0 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Foreign government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 0 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Obligations of states and political subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 7,305 7,409
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 378,398 369,111
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | CLO and other ABS    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 397,129 565,089
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities (“RMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 0 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Commercial mortgage-backed securities (“CMBS”)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 330 335
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 0 0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Preferred stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities $ 0 $ 0