Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 66,971	$ 64,129
After one year through five years, Carrying Amount	116,482	136,854
After five years through ten years, Carrying Amount	119,289	114,990
After ten years, Carrying Amount	159,022	164,270
Held-to-maturity, Carrying Amount, Totals	464,834	483,575
Within one year, Fair Value	66,374	63,703
After one year through five years, Fair Value	110,911	131,396
After five years through ten years, Fair Value	112,628	110,814
After ten years, Fair Value	155,780	163,944
Held-to-maturity, Fair Value, Totals	448,608	473,092
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	3,047	3,297
Held-to-maturity, Carrying Amount, Totals	3,047	3,297
Without single maturity, Fair Value	2,892	3,199
Held-to-maturity, Fair Value, Totals	2,892	3,199
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	23	35
Held-to-maturity, Carrying Amount, Totals	23	35
Without single maturity, Fair Value	23	36
Held-to-maturity, Fair Value, Totals	$ 23	$ 36