Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Within one year, Amortized Cost	$ 69,177	$ 27,160
After one year through five years, Amortized Cost	80,593	144,142
After five years through ten years, Amortized Cost	53,835	47,175
After ten years, Amortized Cost	165,406	177,722
Available-for-sale, Amortized Cost	387,665	425,698
Within one year, Fair Value	69,938	27,194
After one year through five years, Fair Value	85,829	146,469
After five years through ten years, Fair Value	57,829	49,419
After ten years, Fair Value	181,631	188,597
Available-for-sale, Fair Value	414,279	441,146
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Without single maturity date, Amortized Cost	18,654	29,499
Available-for-sale, Amortized Cost	18,654	29,499
Without single maturity date, Fair value	19,052	29,467
Available-for-sale, Fair Value	$ 19,052	$ 29,467