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, 2023	Dec. 31, 2022
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 41,420	$ 41,878
After one year through five years, Carrying Amount	158,329	165,216
After five years through ten years, Carrying Amount	125,395	121,739
After ten years, Carrying Amount	147,706	159,093
Held-to-maturity, Carrying Amount, Totals	476,401	491,688
Within one year, Fair Value	41,148	41,652
After one year through five years, Fair Value	151,287	159,006
After five years through ten years, Fair Value	117,523	112,665
After ten years, Fair Value	142,240	151,178
Held-to-maturity, Fair Value, Totals	455,613	468,144
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	3,499	3,696
Held-to-maturity, Carrying Amount, Totals	3,499	3,696
Without single maturity, Fair Value	3,364	3,576
Held-to-maturity, Fair Value, Totals	3,364	3,576
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	52	66
Held-to-maturity, Carrying Amount, Totals	52	66
Without single maturity, Fair Value	51	67
Held-to-maturity, Fair Value, Totals	$ 51	$ 67