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, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Within one year, Amortized Cost	$ 61,469	$ 27,160
After one year through five years, Amortized Cost	90,936	144,142
After five years through ten years, Amortized Cost	56,622	47,175
After ten years, Amortized Cost	168,186	177,722
Available-for-sale, Amortized Cost	401,063	425,698
Within one year, Fair Value	62,203	27,194
After one year through five years, Fair Value	96,406	146,469
After five years through ten years, Fair Value	60,484	49,419
After ten years, Fair Value	182,834	188,597
Available-for-sale, Fair Value	426,237	441,146
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Without single maturity date, Amortized Cost	23,850	29,499
Available-for-sale, Amortized Cost	23,850	29,499
Without single maturity date, Fair value	24,310	29,467
Available-for-sale, Fair Value	$ 24,310	$ 29,467