Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Within one year, Amortized Cost	$ 45,964	$ 27,160
After one year through five years, Amortized Cost	117,341	144,142
After five years through ten years, Amortized Cost	40,164	47,175
After ten years, Amortized Cost	164,802	177,722
Available-for-sale, Amortized Cost	401,671	425,698
Within one year, Fair Value	46,148	27,194
After one year through five years, Fair Value	120,232	146,469
After five years through ten years, Fair Value	42,920	49,419
After ten years, Fair Value	175,523	188,597
Available-for-sale, Fair Value	419,036	441,146
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Without single maturity date, Amortized Cost	33,400	29,499
Available-for-sale, Amortized Cost	33,400	29,499
Without single maturity date, Fair value	34,213	29,467
Available-for-sale, Fair Value	$ 34,213	$ 29,467