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Investments (Available-For-Sale Fixed Maturities) (Details) - USD ($)
$ in Thousands
Mar. 31, 2024
Dec. 31, 2023
Mar. 31, 2023
Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost $ 3,813,857 $ 3,758,720    
Allowance for Expected Credit Losses 1,137 555 $ 424 $ 427
Gross Unrealized Gains 7,343 10,122    
Gross Unrealized Losses 275,926 274,690    
Estimated Fair Value 3,544,137 3,493,597    
U.S. Treasury obligations        
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost 258,462 259,834    
Allowance for Expected Credit Losses 0 0    
Gross Unrealized Gains 24 165    
Gross Unrealized Losses 17,597 16,474    
Estimated Fair Value 240,889 243,525    
U.S. Government-sponsored enterprise obligations        
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost 19,957 19,752    
Allowance for Expected Credit Losses 0 0    
Gross Unrealized Gains 0 2    
Gross Unrealized Losses 1,030 1,030    
Estimated Fair Value 18,927 18,724    
State and municipal bonds        
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost 502,549 482,367    
Allowance for Expected Credit Losses 0 0    
Gross Unrealized Gains 1,736 1,885    
Gross Unrealized Losses 30,020 29,871    
Estimated Fair Value 474,265 454,381    
Corporate debt        
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost 1,849,162 1,883,308    
Allowance for Expected Credit Losses 740 0    
Gross Unrealized Gains 2,421 4,025    
Gross Unrealized Losses 136,791 136,759    
Estimated Fair Value 1,714,052 1,750,574    
Residential mortgage-backed securities        
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost 507,312 481,267    
Allowance for Expected Credit Losses 207 211 $ 227 $ 229
Gross Unrealized Gains 2,020 2,876    
Gross Unrealized Losses 59,073 53,795    
Estimated Fair Value 450,052 430,137    
Agency commercial mortgage-backed securities        
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost 8,964 9,369    
Allowance for Expected Credit Losses 0 0    
Gross Unrealized Gains 0 5    
Gross Unrealized Losses 1,014 987    
Estimated Fair Value 7,950 8,387    
Other commercial mortgage-backed securities        
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost 248,756 210,469    
Allowance for Expected Credit Losses 0 151    
Gross Unrealized Gains 221 60    
Gross Unrealized Losses 19,948 20,904    
Estimated Fair Value 229,029 189,474    
Other asset-backed securities        
Debt Securities, Available-for-Sale [Line Items]        
Amortized Cost 418,695 412,354    
Allowance for Expected Credit Losses 190 193    
Gross Unrealized Gains 921 1,104    
Gross Unrealized Losses 10,453 14,870    
Estimated Fair Value $ 408,973 $ 398,395