Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Within one year, Amortized Cost	$ 65,386	$ 27,160
After one year through five years, Amortized Cost	89,127	144,142
After five years through ten years, Amortized Cost	45,235	47,175
After ten years, Amortized Cost	163,788	177,722
Available-for-sale, Amortized Cost	392,117	425,698
Within one year, Fair Value	66,197	27,194
After one year through five years, Fair Value	93,818	146,469
After five years through ten years, Fair Value	49,460	49,419
After ten years, Fair Value	177,609	188,597
Available-for-sale, Fair Value	416,240	441,146
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Without single maturity date, Amortized Cost	28,581	29,499
Available-for-sale, Amortized Cost	28,581	29,499
Without single maturity date, Fair value	29,156	29,467
Available-for-sale, Fair Value	$ 29,156	$ 29,467