v3.26.1
Fair Value Measurements - Schedule of Quantitative Disclosures of Fair Value Assets (Details) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities $ 9,909,753 $ 9,457,176
Equity securities 413,106 384,416
Short-term investments 378,879 648,542
U.S. government and government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 159,693 163,207
Foreign government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 16,170 10,002
Obligations of states and political subdivisions    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 542,687 549,957
Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 3,641,976 3,404,164
CLO and other ABS    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 2,615,374 2,550,286
Residential mortgage-backed securities ("RMBS")    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 2,284,699 2,075,933
Commercial mortgage-backed securities ("CMBS")    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 649,154 703,627
Fair Value Measured at Net Asset Value Per Share    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 296,300 274,800
Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 9,909,753 9,457,176
Equity securities 413,106 384,416
Short-term investments 378,879 648,542
Total assets measured at fair value 10,701,738 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 159,693 163,207
Fair Value, Recurring | Foreign government    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 16,170 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 542,687 549,957
Fair Value, Recurring | Corporate securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fixed income securities 3,641,976 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 2,615,374 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 2,284,699 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 649,154 703,627
Fair Value, Recurring | Common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 411,283 382,577
Fair Value, Recurring | Preferred stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 1,823 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 32,801 39,472
Equity securities 115,979 108,964
Short-term investments 358,715 637,751
Total assets measured at fair value 507,495 786,187
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 32,801 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 114,156 107,125
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 1,823 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 9,101,406 8,475,760
Equity securities 858 653
Short-term investments 20,164 10,791
Total assets measured at fair value 9,122,428 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 126,892 123,735
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 16,170 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 535,278 542,548
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 3,265,812 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 2,263,614 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 2,244,820 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 648,820 703,292
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 858 653
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 775,546 941,944
Equity securities 0 0
Short-term investments 0 0
Total assets measured at fair value 775,546 941,944
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 7,409 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 376,164 369,111
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 351,760 565,089
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 39,879 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 334 335
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