Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 57,626	$ 54,316
After one year through five years, Carrying Amount	169,880	195,706
After five years through ten years, Carrying Amount	105,046	107,347
After ten years, Carrying Amount	203,500	220,345
Held-to-maturity, Carrying Amount, Totals	541,224	585,130
Within one year, Fair Value	58,183	54,794
After one year through five years, Fair Value	176,260	204,289
After five years through ten years, Fair Value	110,513	113,643
After ten years, Fair Value	218,838	240,860
Held-to-maturity, Fair Value, Totals	569,445	621,654
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	5,060	7,261
Held-to-maturity, Carrying Amount, Totals	5,060	7,261
Without single maturity, Fair Value	5,534	7,906
Held-to-maturity, Fair Value, Totals	5,534	7,906
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	112	155
Held-to-maturity, Carrying Amount, Totals	112	155
Without single maturity, Fair Value	117	162
Held-to-maturity, Fair Value, Totals	$ 117	$ 162