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, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 54,316	$ 49,967
After one year through five years, Carrying Amount	195,706	198,025
After five years through ten years, Carrying Amount	107,347	110,460
After ten years, Carrying Amount	220,345	229,782
Held-to-maturity, Carrying Amount, Totals	585,130	599,421
Within one year, Fair Value	54,794	50,348
After one year through five years, Fair Value	204,289	202,109
After five years through ten years, Fair Value	113,643	113,877
After ten years, Fair Value	240,860	243,279
Held-to-maturity, Fair Value, Totals	621,654	621,343
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	7,261	10,967
Held-to-maturity, Carrying Amount, Totals	7,261
Without single maturity, Fair Value	7,906	11,511
Held-to-maturity, Fair Value, Totals	7,906	11,511
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	155	220
Held-to-maturity, Carrying Amount, Totals	155
Without single maturity, Fair Value	162	219
Held-to-maturity, Fair Value, Totals	$ 162	$ 219