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, 2022	Dec. 31, 2021
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 66,348	$ 70,859
After one year through five years, Carrying Amount	172,853	174,157
After five years through ten years, Carrying Amount	111,775	100,543
After ten years, Carrying Amount	186,134	198,945
Held-to-maturity, Carrying Amount, Totals	541,297	549,231
Within one year, Fair Value	66,309	71,455
After one year through five years, Fair Value	170,260	180,376
After five years through ten years, Fair Value	105,557	104,851
After ten years, Fair Value	179,272	214,060
Held-to-maturity, Fair Value, Totals	525,637	575,850
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	4,102	4,623
Held-to-maturity, Carrying Amount, Totals	4,102	4,623
Without single maturity, Fair Value	4,151	5,000
Held-to-maturity, Fair Value, Totals	4,151	5,000
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	85	104
Held-to-maturity, Carrying Amount, Totals	85	104
Without single maturity, Fair Value	88	108
Held-to-maturity, Fair Value, Totals	$ 88	$ 108