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, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Within one year, Amortized Cost	$ 32,152	$ 69,177
After one year through five years, Amortized Cost	88,065	80,593
After five years through ten years, Amortized Cost	56,405	53,835
After ten years, Amortized Cost	136,754	165,406
Available-for-sale, Amortized Cost	322,863	387,665
Within one year, Fair Value	32,718	69,938
After one year through five years, Fair Value	92,012	85,829
After five years through ten years, Fair Value	58,625	57,829
After ten years, Fair Value	148,768	181,631
Available-for-sale, Fair Value	341,838	414,279
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Without single maturity date, Amortized Cost	9,487	18,654
Available-for-sale, Amortized Cost	9,487	18,654
Without single maturity date, Fair value	9,715	19,052
Available-for-sale, Fair Value	$ 9,715	$ 19,052