Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 52,814	$ 54,316
After one year through five years, Carrying Amount	188,923	195,706
After five years through ten years, Carrying Amount	99,220	107,347
After ten years, Carrying Amount	211,703	220,345
Held-to-maturity, Carrying Amount, Totals	558,431	585,130
Within one year, Fair Value	53,202	54,794
After one year through five years, Fair Value	196,297	204,289
After five years through ten years, Fair Value	104,090	113,643
After ten years, Fair Value	229,729	240,860
Held-to-maturity, Fair Value, Totals	589,601	621,654
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	5,646	7,261
Held-to-maturity, Carrying Amount, Totals	5,646	7,261
Without single maturity, Fair Value	6,152	7,906
Held-to-maturity, Fair Value, Totals	6,152	7,906
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	125	155
Held-to-maturity, Carrying Amount, Totals	125	155
Without single maturity, Fair Value	131	162
Held-to-maturity, Fair Value, Totals	$ 131	$ 162