v3.25.2
Investments - Schedule of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Dec. 31, 2024
Jun. 30, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 17,365,354 $ 17,155,012  
Percentage of total 100.00% 100.00%  
U.S. Government direct, guaranteed, and government-sponsored enterprises      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 380,035 $ 358,960  
States, municipalities, and political subdivisions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 2,692,777 2,786,804  
Foreign governments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 29,606 28,031  
Corporates      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 14,187,466 13,861,261  
Corporates | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 7,401,462 7,182,256  
Corporates | Financial      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 4,725,619 4,675,930  
Corporates | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 2,060,385 2,003,075  
Collateralized debt obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 42,866  
Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 75,470 77,090  
Quoted Prices in Active Markets for Identical Assets (Level 1)      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 0 $ 0  
Percentage of total 0.00% 0.00%  
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Government direct, guaranteed, and government-sponsored enterprises      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 0 $ 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | States, municipalities, and political subdivisions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign governments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates | Financial      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporates | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized debt obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Significant Other Observable Inputs (Level 2)      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 16,914,364 $ 16,680,898  
Percentage of total 97.00% 97.00%  
Significant Other Observable Inputs (Level 2) | U.S. Government direct, guaranteed, and government-sponsored enterprises      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 380,035 $ 358,960  
Significant Other Observable Inputs (Level 2) | States, municipalities, and political subdivisions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 2,692,777 2,786,804  
Significant Other Observable Inputs (Level 2) | Foreign governments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 29,606 28,031  
Significant Other Observable Inputs (Level 2) | Corporates      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 13,769,069 13,441,196  
Significant Other Observable Inputs (Level 2) | Corporates | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 7,222,920 6,998,900  
Significant Other Observable Inputs (Level 2) | Corporates | Financial      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 4,603,643 4,551,737  
Significant Other Observable Inputs (Level 2) | Corporates | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 1,942,506 1,890,559  
Significant Other Observable Inputs (Level 2) | Collateralized debt obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Significant Other Observable Inputs (Level 2) | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 42,877 65,907  
Significant Unobservable Inputs (Level 3)      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 450,990 $ 474,114  
Percentage of total 3.00% 3.00% 3.00%
Significant Unobservable Inputs (Level 3) | U.S. Government direct, guaranteed, and government-sponsored enterprises      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 0 $ 0  
Significant Unobservable Inputs (Level 3) | States, municipalities, and political subdivisions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Significant Unobservable Inputs (Level 3) | Foreign governments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Significant Unobservable Inputs (Level 3) | Corporates      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 418,397 420,065  
Percentage of total 3.00%   3.00%
Significant Unobservable Inputs (Level 3) | Corporates | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 178,542 183,356  
Significant Unobservable Inputs (Level 3) | Corporates | Financial      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 121,976 124,193  
Significant Unobservable Inputs (Level 3) | Corporates | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 117,879 112,516  
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 0 42,866  
Percentage of total 0.00%   0.00%
Significant Unobservable Inputs (Level 3) | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 32,593 $ 11,183