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, 2022	Dec. 31, 2021
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 49,585	$ 70,859
After one year through five years, Carrying Amount	170,447	174,157
After five years through ten years, Carrying Amount	113,055	100,543
After ten years, Carrying Amount	177,636	198,945
Held-to-maturity, Carrying Amount, Totals	514,610	549,231
Within one year, Fair Value	49,329	71,455
After one year through five years, Fair Value	163,167	180,376
After five years through ten years, Fair Value	101,641	104,851
After ten years, Fair Value	162,038	214,060
Held-to-maturity, Fair Value, Totals	479,911	575,850
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	3,812	4,623
Held-to-maturity, Carrying Amount, Totals	3,812	4,623
Without single maturity, Fair Value	3,659	5,000
Held-to-maturity, Fair Value, Totals	3,659	5,000
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	75	104
Held-to-maturity, Carrying Amount, Totals	75	104
Without single maturity, Fair Value	77	108
Held-to-maturity, Fair Value, Totals	$ 77	$ 108