Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 70,859	$ 54,316
After one year through five years, Carrying Amount	174,157	195,706
After five years through ten years, Carrying Amount	100,543	107,347
After ten years, Carrying Amount	198,945	220,345
Held-to-maturity, Carrying Amount, Totals	549,231	585,130
Within one year, Fair Value	71,455	54,794
After one year through five years, Fair Value	180,376	204,289
After five years through ten years, Fair Value	104,851	113,643
After ten years, Fair Value	214,060	240,860
Held-to-maturity, Fair Value, Totals	575,850	621,654
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	4,623	7,261
Without single maturity, Fair Value	5,000	7,906
Held-to-maturity, Fair Value, Totals	5,000	7,906
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	104	155
Without single maturity, Fair Value	108	162
Held-to-maturity, Fair Value, Totals	$ 108	$ 162