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, 2025	Dec. 31, 2024
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 37,236	$ 49,303
After one year through five years, Carrying Amount	83,176	93,087
After five years through ten years, Carrying Amount	115,415	115,307
After ten years, Carrying Amount	133,692	152,570
Held-to-maturity, Carrying Amount, Totals	372,018	413,061
Within one year, Fair Value	37,130	48,831
After one year through five years, Fair Value	81,264	89,418
After five years through ten years, Fair Value	112,555	109,812
After ten years, Fair Value	131,006	149,004
Held-to-maturity, Fair Value, Totals	364,386	399,721
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	2,490	2,781
Held-to-maturity, Carrying Amount, Totals	2,490	2,781
Without single maturity, Fair Value	2,422	2,643
Held-to-maturity, Fair Value, Totals	2,422	2,643
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	9	13
Held-to-maturity, Carrying Amount, Totals	9	13
Without single maturity, Fair Value	9	13
Held-to-maturity, Fair Value, Totals	$ 9	$ 13