v3.26.1
Investments - Schedule of Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2026
Dec. 31, 2025
Mar. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 17,579,376 $ 17,589,342  
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 $ 389,151 $ 383,853  
States, municipalities, and political subdivisions      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 2,884,630 2,880,626  
Foreign governments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 38,835 39,546  
Corporates      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 14,151,002 14,171,518  
Corporates | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 7,297,399 7,317,686  
Corporates | Financial      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 4,746,717 4,782,326  
Corporates | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 2,106,886 2,071,506  
Corporates | Other corporate sectors      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 7,297,399    
Collateralized debt obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 0 0  
Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 115,758 113,799  
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  
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) | Corporates | Other corporate sectors      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 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 $ 17,166,272 $ 17,194,117  
Percentage of total 98.00% 98.00%  
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 $ 389,151 $ 383,853  
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 2,881,653 2,880,626  
Significant Other Observable Inputs (Level 2) | Foreign governments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 38,835 39,546  
Significant Other Observable Inputs (Level 2) | Corporates      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 13,840,499 13,862,194  
Significant Other Observable Inputs (Level 2) | Corporates | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value   7,232,179  
Significant Other Observable Inputs (Level 2) | Corporates | Financial      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 4,627,718 4,661,175  
Significant Other Observable Inputs (Level 2) | Corporates | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 1,995,488 1,968,840  
Significant Other Observable Inputs (Level 2) | Corporates | Other corporate sectors      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 7,217,293    
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 16,134 27,898  
Significant Unobservable Inputs (Level 3)      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 413,104 $ 395,225  
Percentage of total 2.00% 2.00% 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 $ 2,977 0  
Percentage of total 0.00%   0.00%
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 $ 310,503 309,324  
Percentage of total 2.00%   3.00%
Significant Unobservable Inputs (Level 3) | Corporates | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value   85,507  
Significant Unobservable Inputs (Level 3) | Corporates | Financial      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 118,999 121,151  
Significant Unobservable Inputs (Level 3) | Corporates | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 111,398 102,666  
Significant Unobservable Inputs (Level 3) | Corporates | Other corporate sectors      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value 80,106    
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value $ 0 0  
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 $ 99,624 $ 85,901