Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 55,462	$ 54,316
After one year through five years, Carrying Amount	199,837	195,706
After five years through ten years, Carrying Amount	101,316	107,347
After ten years, Carrying Amount	211,996	220,345
Held-to-maturity, Carrying Amount, Totals	575,108	585,130
Within one year, Fair Value	55,896	54,794
After one year through five years, Fair Value	207,924	204,289
After five years through ten years, Fair Value	104,986	113,643
After ten years, Fair Value	227,843	240,860
Held-to-maturity, Fair Value, Totals	603,720	621,654
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	6,357	7,261
Held-to-maturity, Carrying Amount, Totals	6,357	7,261
Without single maturity, Fair Value	6,925	7,906
Held-to-maturity, Fair Value, Totals	6,925	7,906
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	140	155
Held-to-maturity, Carrying Amount, Totals	140	155
Without single maturity, Fair Value	146	162
Held-to-maturity, Fair Value, Totals	$ 146	$ 162