Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 41,342	$ 41,878
After one year through five years, Carrying Amount	157,097	165,216
After five years through ten years, Carrying Amount	122,498	121,739
After ten years, Carrying Amount	155,536	159,093
Held-to-maturity, Carrying Amount, Totals	480,142	491,688
Within one year, Fair Value	41,136	41,652
After one year through five years, Fair Value	152,035	159,006
After five years through ten years, Fair Value	116,231	112,665
After ten years, Fair Value	150,840	151,178
Held-to-maturity, Fair Value, Totals	463,814	468,144
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	3,606	3,696
Held-to-maturity, Carrying Amount, Totals	3,606	3,696
Without single maturity, Fair Value	3,510	3,576
Held-to-maturity, Fair Value, Totals	3,510	3,576
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	63	66
Held-to-maturity, Carrying Amount, Totals	63	66
Without single maturity, Fair Value	62	67
Held-to-maturity, Fair Value, Totals	$ 62	$ 67