Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 64,898	$ 64,129
After one year through five years, Carrying Amount	126,220	136,854
After five years through ten years, Carrying Amount	116,791	114,990
After ten years, Carrying Amount	164,201	164,270
Held-to-maturity, Carrying Amount, Totals	475,322	483,575
Within one year, Fair Value	64,232	63,703
After one year through five years, Fair Value	120,555	131,396
After five years through ten years, Fair Value	111,569	110,814
After ten years, Fair Value	162,244	163,944
Held-to-maturity, Fair Value, Totals	461,681	473,092
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	3,183	3,297
Held-to-maturity, Carrying Amount, Totals	3,183	3,297
Without single maturity, Fair Value	3,052	3,199
Held-to-maturity, Fair Value, Totals	3,052	3,199
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	29	35
Held-to-maturity, Carrying Amount, Totals	29	35
Without single maturity, Fair Value	29	36
Held-to-maturity, Fair Value, Totals	$ 29	$ 36