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, 2023	Dec. 31, 2022
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 58,790	$ 41,878
After one year through five years, Carrying Amount	144,736	165,216
After five years through ten years, Carrying Amount	114,028	121,739
After ten years, Carrying Amount	166,165	159,093
Held-to-maturity, Carrying Amount, Totals	487,166	491,688
Within one year, Fair Value	57,976	41,652
After one year through five years, Fair Value	135,910	159,006
After five years through ten years, Fair Value	103,007	112,665
After ten years, Fair Value	151,928	151,178
Held-to-maturity, Fair Value, Totals	452,019	468,144
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	3,403	3,696
Held-to-maturity, Carrying Amount, Totals	3,403	3,696
Without single maturity, Fair Value	3,154	3,576
Held-to-maturity, Fair Value, Totals	3,154	3,576
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	44	66
Held-to-maturity, Carrying Amount, Totals	44	66
Without single maturity, Fair Value	44	67
Held-to-maturity, Fair Value, Totals	$ 44	$ 67