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Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Schedule Of Held To Maturity Securities [Line Items]    
Within one year, Carrying Amount $ 27,683 $ 28,620
After one year through five years, Carrying Amount 73,237 76,161
After five years through ten years, Carrying Amount 111,110 119,321
After ten years, Carrying Amount 109,504 123,573
Held-to-maturity, Carrying Amount, Totals 323,658 350,087
Within one year, Fair Value 27,520 28,561
After one year through five years, Fair Value 69,653 74,506
After five years through ten years, Fair Value 102,300 116,970
After ten years, Fair Value 98,433 122,182
Held-to-maturity, Fair Value, Totals 299,849 344,576
U.S. Agency-Based Mortgage-Backed Securities [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Without single maturity, Carrying Amount 2,115 2,403
Held-to-maturity, Carrying Amount, Totals 2,115 2,403
Without single maturity, Fair Value 1,934 2,348
Held-to-maturity, Fair Value, Totals 1,934 2,348
Asset-Backed Securities [Member]    
Schedule Of Held To Maturity Securities [Line Items]    
Without single maturity, Carrying Amount 9 9
Held-to-maturity, Carrying Amount, Totals 9 9
Without single maturity, Fair Value 9 9
Held-to-maturity, Fair Value, Totals $ 9 $ 9