Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Within one year, Amortized Cost	$ 29,845	$ 28,290
After one year through five years, Amortized Cost	105,650	68,876
After five years through ten years, Amortized Cost	83,150	102,296
After ten years, Amortized Cost	107,777	133,147
Available-for-sale, Amortized Cost	331,798	338,593
Within one year, Fair Value	29,610	27,814
After one year through five years, Fair Value	98,566	65,406
After five years through ten years, Fair Value	75,968	95,366
After ten years, Fair Value	97,232	127,089
Available-for-sale, Fair Value	306,040	321,121
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Without single maturity date, Amortized Cost	5,376	5,984
Without single maturity date, Fair value	$ 4,664	$ 5,446