Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 64,129	$ 41,878
After one year through five years, Carrying Amount	136,854	165,216
After five years through ten years, Carrying Amount	114,990	121,739
After ten years, Carrying Amount	164,270	159,093
Held-to-maturity, Carrying Amount, Totals	483,575	491,688
Within one year, Fair Value	63,703	41,652
After one year through five years, Fair Value	131,396	159,006
After five years through ten years, Fair Value	110,814	112,665
After ten years, Fair Value	163,944	151,178
Held-to-maturity, Fair Value, Totals	473,092	468,144
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	3,297	3,696
Held-to-maturity, Carrying Amount, Totals	3,297	3,696
Without single maturity, Fair Value	3,199	3,576
Held-to-maturity, Fair Value, Totals	3,199	3,576
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	35	66
Held-to-maturity, Carrying Amount, Totals	35	66
Without single maturity, Fair Value	36	67
Held-to-maturity, Fair Value, Totals	$ 36	$ 67